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INVESTIGATION OF PORTABLE ORHANDHELD DEVICES FOR DETECTING

CONTAMINANTS IN LPG

FINAL REPORT(Revision 2)

SwRI Project No. 08-10524PERC Docket No. 11296

Prepared for:

Propane Education and Research Council (PERC)1140 Connecticut Ave., NW, Suite 1075

Washington DC 20036

MARCH 2005

This report must be reproduced in full,

unless SwRI approves a summary or

abridgement

INVESTIGATION OF PORTABLE ORHANDHELD DEVICES FOR DETECTING

CONTAMINANTS IN LPG

FINAL REPORT(Revision 2)

SwRI Project No. 08-10524PERC Docket No. 11296

Prepared for:

Propane Education and Research Council (PERC)1140 Connecticut Ave., NW, Suite 1075

Washington DC 20036

Prepared by:

Scott A. Hutzler, Research ScientistJames E. Johnson, Principal Engineer

Southwest Research Institute

6220 Culebra RoadSan Antonio, TX 78238

MARCH 2005

Approved:

Edwin C. Owens, DirectorFuels and Lubricants Technology DepartmentFuels and Lubricants Research Division

SwRI Project No. 08-10524March 2005

Investigation of Portable or Hand-Held Devices Page ifor Detecting Contaminants in LPG

Table of Contents

Section Page Number

EXECUTIVE SUMMARY ............................................................................................................................ 1

1.0 BACKGROUND............................................................................................................................... 1

2.0 OBJECTIVES ................................................................................................................................... 1

3.0 PART I - LITERATURE REVIEW .................................................................................................. 2

3.1 Technical Approach and Summary of Findings............................................................................ 2

3.2 SENSOR CONCEPTS AND RECOMMENDATIONS............................................................... 4

3.3 SENSOR TECHNOLOGY - CONCLUDING REMARKS ....................................................... 11

4.0 PART II - INSTRUMENT EVALUATION ................................................................................... 13

4.1 Technical Approach .................................................................................................................... 13

4.2 Summary of Results .................................................................................................................... 14

4.3 Recommendations....................................................................................................................... 14

APPENDIX A .............................................................................................................................................. 15

5.0 LPG QUALITY............................................................................................................................... 16

5.1 LPG Composition ....................................................................................................................... 16

5.2 LPG Contaminants...................................................................................................................... 16

6.0 INTERVIEWS ................................................................................................................................ 18

7.0 LPG SENSOR SPECIFICATION................................................................................................... 19

7.1 Objective..................................................................................................................................... 19

7.2 Subsystem Definition.................................................................................................................. 20

7.2.1 Packaging ........................................................................................................................... 21

7.2.2 Power ................................................................................................................................. 21

7.2.3 Sample Handling and Preparation...................................................................................... 21

7.2.4 Sensor System .................................................................................................................... 21

7.2.5 Control System................................................................................................................... 22

7.2.6 Operator Interface .............................................................................................................. 22

7.2.7 Data Interface ..................................................................................................................... 22

7.3 Summary Characteristics ............................................................................................................ 22

8.0 SAMPLE CONDITIONING TECHNIQUES ................................................................................. 25

8.1 Molecular Sieves (Zeolite).......................................................................................................... 26

8.2 Activated Alumina ...................................................................................................................... 27

8.3 Activated Carbon ........................................................................................................................ 28

8.3.1 Molecular Sieves (Carbon)................................................................................................. 28

8.4 Silica Gel..................................................................................................................................... 29


Investigation of Portable or Hand-Held Devices Page iifor Detecting Contaminants in LPG

Filter/Separators................................................................................................................................... 29

8.6 Particulate Filters ........................................................................................................................ 31

8.7 Sample Conditioning - Application to Portable LPG Sensors .................................................... 31

9.0 LPG SAMPLING............................................................................................................................ 32

9.1 The Sampling Environment ........................................................................................................ 32

9.2 Common Methods in Use For Sampling..................................................................................... 33

9.3 Sampling Considerations for Portable Systems .......................................................................... 35

10.0 SENSOR TECHNOLOGIES...................................................................................................... 36

10.1 Catalytic Bead ........................................................................................................................ 38

10.2 Electrochemical Sensors......................................................................................................... 39

10.2.1 Interfering Gases ................................................................................................................ 40

10.2.2 Blocking Mechanisms ........................................................................................................ 41

10.2.3 Poisoning............................................................................................................................ 42

10.2.4 Pressure Effects .................................................................................................................. 42

10.2.5 Humidity ............................................................................................................................ 42

10.2.6 Sensor Life ......................................................................................................................... 43

10.2.7 Advantages/Disadvantages................................................................................................. 43

10.3 Metal Oxide Semiconductor (MOS)....................................................................................... 43


10.4 Bulk Acoustic Wave (BAW) Sensors..................................................................................... 45


10.5 Surface Acoustic Wave (SAW) Sensors................................................................................. 46


10.6 Metal Oxide Field-Effect Transistor (MOSFET) ................................................................... 47

10.7 Conducting Organic Polymers (COP) .................................................................................... 48


10.8 Chemoresistors ....................................................................................................................... 49

10.9 Spectroscopic.......................................................................................................................... 50

10.9.1 Spectroscopic Instrument Technologies............................................................................. 52

10.9.2 Mid-Infrared Spectroscopy................................................................................................. 53

10.9.3 Near-Infrared Spectroscopy ............................................................................................... 54

10.9.4 Laser Spectroscopy ............................................................................................................ 55

10.10 Dielectric Measurement.......................................................................................................... 57

10.11 Gas Chromatography.............................................................................................................. 58

10.11.1 Advantages/Disadvantages ............................................................................................ 58

10.12 Ionization Detectors................................................................................................................ 59

10.13 Gas Detection Tubes............................................................................................................... 62


Investigation of Portable or Hand-Held Devices Page iiifor Detecting Contaminants in LPG

10.13.1 Gas Sampling Methods .................................................................................................. 63

10.13.2 Reaction Principles ........................................................................................................ 64

10.13.3 Temperature Effects....................................................................................................... 65

10.13.4 Correcting Tube Results ................................................................................................ 65

10.13.5 Storage of Gas Detector Tubes ...................................................................................... 66

10.14 Coriolis Measurements ........................................................................................................... 66

10.15 Interface Detection ................................................................................................................. 67

10.15.1 Ultrasonic....................................................................................................................... 68

10.15.2 Radar.............................................................................................................................. 70

10.15.3 Capacitance.................................................................................................................... 71

10.15.4 Conductance................................................................................................................... 72

10.15.5 Field Effect .................................................................................................................... 72

10.16 Particle Counters..................................................................................................................... 73

10.16.1 Light-Scattering Particle Counters................................................................................. 74

10.16.2 Light-Blocking Particle Counters .................................................................................. 74

10.16.3 Advantages/Disadvantages ............................................................................................ 75

10.17 Summary Notes for Sensor Technologies .............................................................................. 75

10.17.1 Gas Monitoring Systems................................................................................................ 75

10.17.2 Chemical Array Sensors ................................................................................................ 76

10.17.3 Spectroscopic Sensors.................................................................................................... 76

10.17.4 Dielectric Measurement ................................................................................................. 77

10.17.5 Gas Chromatography ..................................................................................................... 77

10.17.6 Photoionization Detector (PID) ..................................................................................... 77

10.17.7 Flame Ionization Detector (FID).................................................................................... 78

10.17.8 Gas Detection Tubes ...................................................................................................... 78

10.17.9 Coriolis Measurement.................................................................................................... 78

10.17.10 Particle Counters............................................................................................................ 79

11.0 SENSOR TRADE-OFF STUDY ................................................................................................ 79

11.1 Weighting Factors................................................................................................................... 82

11.2 Trade-off Summary ................................................................................................................ 82

APPENDIX B............................................................................................................................................... 85

12.0 BACKGROUND ............................................................................................................................. 86

13.0 MATERIALS.............................................................................................................................. 87

14.0 INSTRUMENTATION .............................................................................................................. 89

14.1 Sampling System .................................................................................................................... 91

14.2 Purge Cycle ............................................................................................................................ 91

14.3 Chemical Sensors ................................................................................................................... 92


Investigation of Portable or Hand-Held Devices Page ivfor Detecting Contaminants in LPG

14.4 Signal Processing and Data Analysis...................................................................................... 92

14.5 Outlier Diagnostics ................................................................................................................. 93

14.5.1 Principal Component Analysis (PCA)................................................................................ 93

14.6 Algorithms.............................................................................................................................. 94

14.6.1 KNN................................................................................................................................... 94

14.6.2 K-Means............................................................................................................................. 95

14.6.3 Canonical Discriminant Analysis (CDA)........................................................................... 95

14.7 Sensor Selection ..................................................................................................................... 95

14.8 Sensor Conditioning ............................................................................................................... 95

14.9 Sampling Considerations ........................................................................................................ 96

14.9.1 Sample temperature............................................................................................................ 96

14.9.2 Relative Humidity .............................................................................................................. 96

14.9.3 Sampling Sequence ............................................................................................................ 96

14.9.4 Substrate Temperature........................................................................................................ 96

15.0 EXPERIMENTATION AND RESULTS ................................................................................... 97

15.1 Sample Preparation and Sampling Apparatus......................................................................... 97

15.1.1 Neat Compounds................................................................................................................ 98

15.1.2 Hydrocarbon Solutions....................................................................................................... 99

15.1.3 Gas Samples ..................................................................................................................... 100

15.1.4 Instrument Settings........................................................................................................... 101

15.1.5 Understanding the Results................................................................................................ 101

15.2 Experiment #1 - Neat Plasticizers......................................................................................... 102

15.3 Experiment #2 – Plasticizers Revisited ................................................................................ 110

15.4 Experiment #3 - Contaminant Comparison .......................................................................... 122

15.5 Experiment #4 - HC Solutions.............................................................................................. 130

15.6 Experiment #5 - HC Solutions with Dry Air Bubbler .......................................................... 134

15.7 Experiment #6 - HC Solutions and Differential Measurements ........................................... 138

15.8 Experiment #7 - Propane Trials ............................................................................................ 142

15.9 Experiment #8 - Propane Comparison.................................................................................. 146

16.0 REFERENCES ......................................................................................................................... 150

List of Tables

Table Page NumberTable 1. Sensor Trade-Off Table _________________________________________________________ 4

Table 2. GPA 2140-97 LPG Specifications ________________________________________________ 16

Table 3. Summary Table ______________________________________________________________ 23

Table 4. Comparison of MOS Sensors ____________________________________________________ 45


Investigation of Portable or Hand-Held Devices Page vfor Detecting Contaminants in LPG

Table 5. PID/FID Comparison __________________________________________________________ 61

Table 6. Sensor Trade-Off Table ________________________________________________________ 83

Table 7. Test Compounds______________________________________________________________ 87

Table 8. Propane Certificate of Analysis __________________________________________________ 89

Table 9. Cyranose 320 Sensor Specification _______________________________________________ 90

Table 10. Common Instrument Settings __________________________________________________ 101

Table 11. Experiment #1. Instrument Settings ____________________________________________ 103

Table 12. Experiment #1. Training Set __________________________________________________ 104

Table 13. Experiment #1. Interclass Mahalanobis Distance, All Classes ________________________ 106

Table 14. Experiment #1. Interclass Mahalanobis Distance, Classes B and E Removed ____________ 107

Table 15. Experiment #1. Interclass Mahalanobis Distance, Classes B and D Removed ____________ 108

Table 16. Experiment #1. Interclass Mahalanobis Distance, Classes D and E Removed ____________ 109




Table 20. Experiment #2. Training Set, Outliers Removed___________________________________ 115

Table 21. Experiment #2. Interclass Mahalanobis Distance, All Classes, Outliers Removed_________ 117

Table 22. Experiment #2. Training Set, Classes A, B, C, and E, Outliers Removed________________ 118

Table 23. Experiment #2. Interclass Mahalanobis Distance, Classes A, B, C, and E, Outliers Removed 119

Table 24. Experiment #2. Training Set, Classes A, C, D, and E, Outliers Removed _______________ 120

Table 25. Experiment #2. Interclass Mahalanobis Distance, Classes A, C, D, and E Outliers Removed 121




Table 29. Experiment #3. Training Set (Plasticizers, Hexadecane, Water), Outliers Removed._______ 127

Table 30. Experiment #1. Interclass Mahalanobis Distance (Plasticizers, Hexadecane, Water), Outliers

Removed __________________________________________________________________________ 129












Investigation of Portable or Hand-Held Devices Page vifor Detecting Contaminants in LPG






List of Figures

Figure Page NumberFigure 1. Compressed Breathing Air Analysis Kit ........................................................................................ 6

Figure 2. Basic Differential Measurment .................................................................................................... 10

Figure 3. High Pressure Purifier Housing and Element (2" diamater)......................................................... 11

Figure 4. Contaminant Distribution.............................................................................................................. 18

Figure 5. Configuration for a Sampling Container (from ASTM D1265 – 92)............................................ 33

Figure 6. Typical Visual Indicator Sampling System (from GPA Standard 2174-93) ................................ 34

Figure 7. Cylinder Sample Panel shown in three modes of operation......................................................... 35

Figure 8. Catalytic Bead Sensor .................................................................................................................. 38

Figure 9. Electrochemical Sensor ................................................................................................................ 40

Figure 10. Example of a Metal Oxide Semiconductor (MOS) Sensor ........................................................ 44

Figure 11. General Schematic for a Chemoresistor ..................................................................................... 50

Figure 12. AC Bridge Network for Dielectric Sample Cell.......................................................................... 58

Figure 13. Example of a Handheld PID Monitor Capable of ...................................................................... 59

Figure 14. Example Detctor Tube Specification for Ammonia from Sensidyne......................................... 62

Figure 15. Example of a Manual Detctor Tube Pump................................................................................. 63

Figure 16. Depth of Bottom Sludge as a Percent of Full Tank Cross Section............................................. 67

Figure 17. Interface Detection by Attentuation ........................................................................................... 69

Figure 18. Interface Detection by Reflection .............................................................................................. 70

Figure 19. Examples of Tank Radar Antennas ............................................................................................ 71

Figure 20. Guided Wave Radar ................................................................................................................... 71

Figure 21. Basic Capacitor .......................................................................................................................... 72

Figure 22. Field Effect Sensor..................................................................................................................... 73

Figure 23. Cyranose 320.............................................................................................................................. 89

Figure 24. Schematic of the Purge Cycle .................................................................................................... 91

Figure 25. Smellprint Containing 32 sensor Responses .............................................................................. 93

Figure 26. Sample Vial Holder.................................................................................................................... 97

Figure 27. Dry Air Purge............................................................................................................................. 98

Figure 28. Basic Sampling Technique......................................................................................................... 98

Figure 29. Sampling With a Dry Air Bubbler ............................................................................................. 99


Investigation of Portable or Hand-Held Devices Page viifor Detecting Contaminants in LPG

Figure 30. Sampling With a Dry Air Bubbler and Isooctane Purge .......................................................... 100

Figure 31. Sampling from a Gas Cylinder................................................................................................. 100

Figure 32. Experiment #1. PCA Plot, All Classes .................................................................................... 105

Figure 33. Experiment #1. Canonical Plot, All Classes............................................................................ 106

Figure 34. Experiment #1. Canonical Plot, Classes B and E Removed.................................................... 107

Figure 35. Experiment #1. Canonical Plot, Classes B and D Removed ................................................... 108

Figure 36. Experiment #1. Canonical Plot, Classes D and E Removed ................................................... 109



Figure 39. Experiment #2. PCA Plot, All Classes, Outliers Removed ..................................................... 116

Figure 40. Experiment #2. Canonical Plot, All Classes, Outliers Removed............................................. 117

Figure 41. Experiment #2. Canonical Plot, Classes A, B, C, and E, Outliers Removed .......................... 119

Figure 42. Experiment #2. Canonical Plot, Classes A, C, D, and E, Outliers Removed .......................... 121



Figure 45. Experiment #3. PCA Plot (Plasticizers, Hexadecane, Water), Outliers Removed .................. 128

Figure 46. Experiment #3. Canonical Plot (Plasticizers, Hexadecane, Water), Outliers Removed.......... 129











Abbreviations

Å Angstroms (10-10 m)

ASTM American Society for Testing and Materials

ATR Attenuated Total Reflectance

BAW Bulk Acoustic Wave

BEHPhth Bis(2-ethylhexyl) phthalate

BEHSeb Bis(2-ethylhexyl) sebacate

BenButPhth Benzyl butyl phthalate


Investigation of Portable or Hand-Held Devices Page viiifor Detecting Contaminants in LPG

ButylSeb Di-n-butyl sebacate

CDA Canonical Discriminant Analysis

CMS Carbon Molecular Sieves

COP Conducting Organic Polymer

EMF/RFI Electromagnetic Field/Radio Frequency Interference

EV Electron Volts

FID Flame Ionization Detector

FMCW Frequency-Modulated Continuous Wave

FT Fourier Transform

FTIR (FT-IR) Fourier Transform Infrared

GASFET Gallium Arsenide Field Effect Transistor

GC Gas Chromatography

GPA Gas Processors Association

GTD Gas Detection Tubes

HC Hydrocarbon

HITRAN High Resolution Transmission

IP Ionization Potential

KNN K-Nearest Neighbor

LEL Lower Explosive Limit

LP Liquefied Petroleum

LPG Liquefied Petroleum Gas

MIR Mid-Infrared

MOS Metal Oxide Semiconductor

MOSFET Metal-Oxide Semiconductor Field-Effect Transistor

OctPhth Di-n-octyl phthalate

PC Principal Component

PCA Principal Component Analysis

PERC Propane Education and Research Council

PID Photoionization Detector

PPM Parts Per Million

QCM Quartz Crystal Microbalance

QMB Quartz Microbalance

SAW Surface Acoustic Wave

SwRI Southwest Research Institute


Investigation of Portable or Hand-Held Devices Page ixfor Detecting Contaminants in LPG

TDR Time Domain Reflectometry

UV/VIS Ultraviolet/Visible

VOC Volatile Organic Compounds

Naming Conventions

Throughout this document the term LPG (Liquefied Petroleum Gas), will be used repeatedly. In

order to eliminate confusion, the term LPG will refer to those product grades composed primarily

of propane (e.g. HD-5). Where necessary, the actual state of the sample, i.e. gas or liquid, will be

clarified. Specific compounds, such as propane or methane and other hydrocarbons will be

referred to by name. Although butane is considered one of the four major LP-gases, its use is

primarily industrial. Our primary concern in this document is with LPG for domestic and

commercial use and those that are suitable for internal combustion engines, i.e. LPG-based.

Despite this convention, the findings in this report may also be relevant to butane.

Organization of the Report

The report is organized into the following subject areas:

Executive Summary

• Background (Section 1.0)

• Objectives (Section 2.0)

• Part I - Literature Review (Section 3.0)

• Part II - Instrument Evaluation (Section 4.0)

Appendix A (Literature Review) Sections

• LPG Quality (Section 5.0)

• Interviews (Section 6.0)

• LPG Sensor Specification (Section 7.0)

• Sample Conditioning Techniques (Section 8.0)

• LPG Sampling (Section 9.0)

• Sensor Technologies (Section 10.0)

• Sensor Trade-Off Study (Section 11.0)


Investigation of Portable or Hand-Held Devices Page xfor Detecting Contaminants in LPG

Appendix B (Instrument Evaluation) Sections

• Background (Section 12.0)

• Materials (Section 13.0)

• Instrumentation (Section 14.0)

• Experimentation and Results (Section 15.0)

Miscellaneous Sections

• References (Section 16.0)


Investigation of Portable or Hand-Held Devices Page 1for Detecting Contaminants in LPG

EXECUTIVE SUMMARY

1.0 BACKGROUND

The research discussed herein supports the Propane Education and Research Council (PERC)

Strategic Goals primarily in the principal component area of Industrial Productivity; although

spin-off ideas could very well support the New Application component of the PERC Strategic

Goals. In particular, Southwest Research Institute® (SwRI®) is addressing issues within the

PERC critical challenge area of fuel specifications. This program is investigating relevant

technologies needed to ascertain LPG fuel quality.

A simple-to-use device is needed to quickly determine the level of specific contaminants in LPG.

The LPG may harbor contaminants such as water, oily or waxy residues (from storage caverns,

compressors, pipe dopes, gaskets, hoses, heat transfer fluids), ammonia (potentially serious for

promoting copper and brass corrosion), and other corrosion agents that include fluorides,

chlorides, bromides, hydrogen sulfide, and sulfur. The impact of these contaminants on

residential or commercial appliances and equipment may be tolerable to a certain level, but

contaminants in LPG used for fueling vehicles can cause both performance and emission

problems. Our goal is to determine what technologies are viable for rapidly and easily

determining fuel quality, especially for field applications where it is important to sense for

contaminants that have migrated into the fuel.

2.0 OBJECTIVES

Proliferation of the LPG market for industrial use is strongly driven by life-cycle costing issues

(especially fuel prices and equipment that may be related to the fuel), whereas residential users

are concerned about availability (rural areas) and the need for long-life appliances and storage

tanks. For alternatively fueled vehicles and engines, proliferation of the LPG market is strongly

dependent upon an industry specification that clearly defines the required quality of the produced

fuel, especially as related to burning and emission characteristics when consumed in vehicles.

Standards such as GPA 2140 and ASTM D1835 (Standard Specification for Liquefied Petroleum

Gases) could be used as a starting point because they define the fuel properties at the time of

delivery in bulk. It is our goal to use these standards as an initial basis from which to investigate

technologies that would yield a fast response and inexpensive fuel-quality sensors that could

measure important components of the LPG.



This research study consisted of two parts. For Part I, a literature study was undertaken to

explore technologies that could be employed for determining fuel quality and measuring

contaminant levels. For Part II of the study, we investigated a hand-held device (based on

advanced chemical array technology) for its application to LPG fuel quality.

The specific objectives for Part I of this study were as follows:

• To assess relevant LPG specifications to understand fundamental LPG fuel quality issues.

An anticipated result of the initial assessment will be a clear technical definition of what

is needed for a fuel-quality sensor.

• To identify or create sensor concepts that are responsive to a clear technical definition of

fuel-quality measurement needs.

• To integrate relevant LPG fuel-quality issues along with emerging and existing sensor

technologies, then develop a road map for actually developing sensor systems.

The specific objectives for Part II of this study were as follows:

• evaluate a handheld chemical sensor array that is currently on the market

• determine its sensitivity to several contaminants found in LPG

• provide recommendations for further use of the technology

3.0 PART I - LITERATURE REVIEW

3.1 Technical Approach and Summary of Findings

In this investigation, we utilized a comprehensive approach to understand the problem from as

many angles and perspectives as possible. To that end, we focused our investigations on the areas

summarized below. Refer to the indicated section in the appendices for a more detailed

discussion.

• LPG Quality (Section 5.0)

LPG specifications were examined in order to understand the nature of typical LPG

contaminants that might be encountered. Although the specifications were fairly limited,

they did provide composition information and general classification of contaminant

types.



• Interviews (Section 6.0)

We conducted interviews with a couple of local LPG retailers to get their perspective of

the problem and determine if they would be open to using a device for determining LPG

fuel quality. Their response was positive and they would consider buying such an

instrument if it could reduce the potential for supplying contaminated fuel (resulting in

high cleanup costs).

• Sensor Specification (Section 7.0)

A draft sensor specification was generated to help guide future sensor developments.

This should be considered a working document and subject to modification as this

investigation continues.

• Sample Conditioning (Section 8.0)

A brief investigation of sample conditioning/filtration techniques for LPG was

conducted. These techniques are important because they provide a means to alter a

sample in a predictable way. The investigation identified several types of filtration

media, such as molecular sieves and activated carbon, which can be used to selectively

remove certain components of LPG. This could be important for removing interfering

species or for concentrating low concentration species.

• LPG Sampling (Section 9.0)

LPG sampling techniques are well documented in the literature. Our focus in this part of

the study was a discussion of the importance of collecting a representative sample. Since

contaminant species can exist in the liquid phase, gas phase, or both, collecting a

representative sample is critical. Equally as important is designing the sensor system to

facilitate the handling of the sample.

• Sensor Technologies (Section 10.0)

Sensor technologies were the primary focus of our investigation. It became apparent that

finding a truly handheld instrument for this application was unlikely, as they simply do

not exist. Therefore, we expanded our search to include "sensor technologies" that could

potentially be incorporated into a portable device. The range of our search included

sensors to detect specific chemical compounds (e.g. ammonia) to sensors for detecting

bulk contaminants (e.g. tank bottoms).

• Sensor Trade-Off Study (Section 11.0)

A sensor trade-off analysis was undertaken to identify those technologies that show

promise for incorporation into portable or handheld devices for detecting contaminants in

LPG. The technologies were assessed in terms of six major attributes: 1) low operation



complexity, 2) portability, 3) high reliability, 4) detection limit, 5) good sensitivity, and

6) high specificity. When assigning ratings to each attribute, consideration was given to

the ultimate end user - the LPG retailer. While many of these technologies are certainly

applicable to LPG analysis, a suitable instrument package may not be currently available

or ready for direct employment at the retail level. In those cases, we considered only the

technology and its suitability for the intended purpose. The results of the trade-off

analysis are summarized in Table 1. It should be noted that a high score in the trade-off

study simply implies that the particular sensor or technology has desirable attributes

based on our understanding of the application and the criteria that we have defined. In all

cases, further investigation of specific technology is warranted to define their actual

application and use.

Table 1. Sensor Trade-Off Table

Attribute Color Legend: 1-3 (red), 4-7 (yellow), 8-10 (green)

Composite Score Legend: 43-236 (red), 237-353 (yellow), 354-430 (green)Desired Attributes Composite

Low Operational High Detection Good High ScoreComplexity Portability Reliability Limit Sensitivity Specificity sum XiGi

Weighting Factors [Xi] 6 8 6 10 5 8Sensing Technology [grade, Gi]Gas Detection Tubes 9 10 8 9 7 8 371Gas Chromatography 5 7 9 10 10 10 370Particle Counter (bulk solids) 8 5 9 9 9 10 357Ionization - PID 8 10 7 9 9 5 345Spectroscopic Sensors 8 7 8 6 7 8 311Gas Monitoring Sensors 7 9 3 6 9 7 293Dielectric Sensor 7 6 7 8 7 2 263Chemical Array Sensors 6 8 5 5 6 5 250Ionization - FID 2 3 6 9 9 3 231Coriolis Sensor 9 5 8 3 7 2 223

The composite score is based on the following percentage of the total range:

50%20% 30%

(green) (yellow) (red)

3.2 SENSOR CONCEPTS AND RECOMMENDATIONS

During this investigation, we compiled a number of possible contaminants that may end up in

LPG (Section 5.0). Given the varying chemical nature of the contaminants it is unreasonable to

expect a single sensor to be able to detect all of them. Then there is the added complication that

some contaminants exist in the gas phase, some in the liquid phase, and some in both. Some of



the sensors investigated in this report (e.g. spectroscopic and dielectric) are capable of analyzing

both liquids and gases. The sensor cost must be weighed against the risk of delivering

contaminated fuel, the cost to cleanup a contaminated LPG tank, or the damage that the fuel

might cause to a customer's equipment.

Based on the trade-off study, we provide the following general recommendations for pursuing

LPG sensors.

Gas Chromatography (gas and liquid phase)

Gas chromatography might well be considered the "gold standard" because it is widely

applicable to the analysis of both liquids and gases. Although portable GCs are available

they still suffer from the shortcomings of their laboratory grade versions. System

maintenance and frequent calibration is generally required. The need for carrier gas, GC

columns, cost (~$10K+), and the required skill level of the operator make them a poor

candidate for use at the retail level.

Gas Detection Tubes (gas phase)

Gas detection tubes inexpensive (<$1,000 startup), easy to operate, and can provide the

selectivity, sensitivity, and low level detection capability that is needed for this

application. In addition, the tubes yield an approximate quantity of contaminant based on

the progression of the color change in the tube. Few other techniques can provide so

much information on such a wide variety of chemical compounds. Some method

development may be required since proper tube selection is critical. One concern is that

operator subjectivity may affect tube reading. This would primarily affect the

determination of quantity since the color change may lighten as it progresses down the

tube. This technique should at least be applicable to sulfides, mercaptans, ammonia, and

water. The only other concern is operating costs since the tubes may run between $5-10

each.

Gas detection tubes are normally used to sample batches of air for area monitoring.

Obviously, open-air sampling does not apply to LPG analysis. However, a system found

in the literature may be directly applicable to LPG. The system, shown in Figure 1,

consists of a regulator, a flow controller, and a tube holder. The purpose of this simple

system is to analyze cylinders of compressed air for toxic substances. We believe that a



system of this type could be used in a similar manner for analyzing gas phase LPG

samples. If bulk contamination is all that is required, then this system may be used

directly. If quantification is desirable, then modifications would be needed to meter a

precise volume of sample through the tube.

To expand on this idea, some additional automation could be added to the system. For

example, a pump that delivers a specific volume of gas to the tube could be included

(normal handheld units already do this). Also, a manifold capable of holding a selected

series of gas tubes for different compounds may also prove useful. We envision this

sensor being used in a batch-mode where an LPG sample is withdrawn and brought to the

instrument. The instrument would be simple to operate, inexpensive, and provide batch

sampling for LPG received at the retail outlet.

Figure 1. Compressed Breathing Air Analysis Kit

Particle Counting (liquid phase)

Particle counting also ranked high as a sensing technique. This is the only sensor in this

study that is capable of detecting and quantifying solid particulate matter in a flowing

stream and is therefore intended to be used with liquid propane. They are not normally

intended to be portable but they are small enough that they could be incorporated into a



portable system. Benchtop units for particle counting are available but they would need

to be modified for liquid propane. With the proper peripheral hardware, mainly a pump

and flow loop, a cylinder of liquid propane could be interfaced to the sensor for analysis.

The startup cost for a particle counting system may be too high for an LPG retailer (at

least $2,000). However, once installed they can run unattended and operator interaction

is minimal. The primary challenge is in determining what sizes and levels of solid

contaminants are problematic. With some minor maintenance and annual calibration, this

mature sensor technology should provide years of dependable service.

Spectroscopic Techniques (gas and liquid phase)

Spectroscopic techniques in general have potential to become reasonable LPG sensors.

Like many of the other sensors, cost is a concern. A portable, off-the-shelf near-infrared

system will start at around $2000 and that doesn’t include the peripheral hardware to

handle the sampling. There are many possible approaches for utilizing spectroscopic

sensors for LPG. Examples include checking for bulk contamination by continuously

monitoring the LPG in-flow or batch sampling to spot check LPG quality in storage

tanks. If ammonia were a real concern, then a diode laser-based system could surely be

designed to detect it in the LPG vapor phase though the cost of materials would be higher

($5000+). Spectroscopic techniques are used in a variety of ways in the refining and fuel

industry and the military has shown some interest in them for portable fuel

(diesel/aviation) quality analyzers.

Measuring for bulk contamination in liquid or gaseous propane is probably the most

realistic application for the short term. The sensor could be designed to "recognize" the

fingerprint of a normal LPG sample (e.g. HD-5 vs commercial propane). Any deviation

from that spectral fingerprint would cause an immediate response from the sensor. Its

sensitivity would be limited to gross changes in composition. In the future, arrays of

single-wavelength sensors may be available to detect trace quantities of multiple analytes

simultaneously. A tunable diode laser system could also be constructed to handle

perhaps 2-3 analytes per system.



Interface Detection (liquid phase)

Of primary concern is free water in LPG. The specifications for LPG require that it be

sub-saturated, i.e. essentially no water. This is normally not a concern. Since propane

has low density and low viscosity, free water will tend to collect at the bottom of fuel

storage tanks. Some of the sensors that we investigated (e.g. those based on dielectric

measurements) would be able to detect this water if the sensor could be physically

inserted into the bottom of the tank where the water collects. Given the tanks currently in

service, this is likely to be a problem. Surfactants or other chemical compounds that are

partially soluble in LPG (e.g. methanol) can help to suspend water. Generally, with the

right choice of filtration system, free water can be easily removed from a flowing stream.

One possibility would be to include a dielectric-based sensor in the sump of a filter

housing. If a slug of water is encountered it would be filtered out and fall into the sump

where the sensor would detect it and trigger an alarm to stop dispensing. The sump

would need to be drained periodically to prevent false alarms from a slow build-up of

water. Like the particle counters, this technology should be applicable but it is not

intended to be portable.

Although we were unable to find a good example in the literature, it is conceivably

possible to create a non-invasive, non-contact sensor that can detect interfaces through

tank walls. The field-effect sensor is an example of this but it appears to have problems

with metal walls. Additional searching on this subject will undoubtedly reveal related

technologies that can identify liquid/liquid or liquid/solid interfaces in the same way that

a "stud" finder works for locating studs in walls. A report for PERC by The Adept Group

dated 2003 discusses the design and testing of a sonic liquid level gauge that apparently

uses sonic (ultrasonic?) pulses to detect the liquid/gas interface in stationary LPG tanks.

If sensitive enough, this device may be able to sense a liquid/liquid or liquid/solid

interface in the bottom of the tank.

Photoionization Detectors (gas phase)

In general, photoionization detectors are usually incorporated into larger systems to act as

the main detector. For example, PIDs are often used as GC detectors. When used in gas

monitoring systems, the sensor is usually diffusion-based. An LPG system based on a

PID would need to be designed and built. While it is very sensitive, its use in LPG may



be limited. PIDs operate by ionizing the sample, which is then detected at an electrode.

The compounds that ionize are those whose ionization potential is lower than the energy

of the lamp being used. Propane has a large IP so it won't show a significant response

even with high-energy lamps. However, most of the organic contaminants and the

odorant have low IPs so they will all respond simultaneously and are not separable.

Hence the reason that a GC column is used to physically separate the mixture before

detection by a PID.

Although not necessarily analyte specific, one possible application for an ionization

detector could be a measurement of "bulk contamination." Relative to other petroleum

products, LPG is considerably pure hence the contaminants are generally low

concentration. A high-energy lamp would make most of the potential LPG

"contaminants" detectable while the response from propane would be negligible.

Compounds such as methanol, hydrogen sulfide, mercaptans, ammonia, and propene

would be detectable. Although limits are not yet defined for some of these substances, it

is arguable that the total additive response from all of these compounds for a given

sample size should not exceed some limit. For instance, total sulfur, including the

odorant, must be below 123 ppm for HD-5. Propene concentration may be as high as

5%. Although you wouldn't be able to identify the contaminant, this semi-quantitative

measurement might give an indication of overall contamination. A more realistic use of a

PID might be to verify propane and propene composition. Using one or several of the

sample conditioning techniques described in this report, nearly all contaminants and the

odorant could be selectively removed from the LPG leaving behind primarily propane

and propene. With the appropriate lamp, the propene concentration could be determined.

Needless to say, some experimentation to determine feasibility, some method

development, and construction of an instrument would be required.

Differential Measurements (gas or liquid phase)

Differential measurements may be applied to several of the sensing techniques described

in this report. The greatest challenge with this technique is that it would need to be

designed and built for a specific application. A typical differential measurement scheme

is shown in Figure 2. The incoming LPG (gas or liquid) is split into two streams. One

stream passes though without modification and is analyzed by the sensor (bottom leg).

The other stream (upper leg) is processed in some way and is then analyzed by an



identical sensor. The two sensor outputs are compared and the resulting output is the

difference between the two sensors. There are other variations on this approach. In the

case of spectroscopic sensors, the light from a laser or source lamp may be split. Each

beam performs a separate analysis and is detected by a different detector and then

compared. Measurements like this also tend to include a reference path where no sample

is analyzed. This is often done to improve the signal to noise ratio.

SampleProcessing

Propane sensors output

Comparator

Figure 2. Basic Differential Measurment

This technique is mostly used to infer the quality of the fuel based on the known changes

that the processing causes in the sample. For example, if a particular conditioning

technique is known to remove water and the differential measurement shows the

processed sample to be different than the unprocessed sample you might infer that the

sample was contaminated with water. This technique might be used to create a

"standard" sample when one is not available. For instance, if the goal is to compare the

collected sample to a "pure" sample of LPG then the "pure" LPG can be generated. In

this case, you might want to pass the sample through several filters (silica gel for water

removal, activated carbon for organics, etc). With proper processing, the resulting

sample would be essentially free of contaminants. In conclusion, this technique may be

best suited for identifying bulk contamination.

The sample processing section of the instrument does not need to be large. Small

filtration canisters are available which have swappable cartridges (Figure 3). For small

samples, these cartridges should be good for many runs. Depending on the filtration

media, the cartridges may even allow regeneration.



Figure 3. High Pressure Purifier Housing and Element (2" diamater)

3.3 SENSOR TECHNOLOGY - CONCLUDING REMARKS

Based on the sensor characteristics described in this report we can attempt to define the "ideal"

LPG sensor. It should

• be relatively simple to operate,

• be easily transportable,

• have high reliability,

• have low detection limits,

• require few consumables,

• require little maintenance,

• be insensitive to shock and vibration,

• automatically compensate for temperature and pressure fluctuations,

• operate at ambient temperature,

• have a long operational or shelf life,

• be selective to the target compounds

• be sensitive to changes in composition

While no single sensor currently exists that can detect and quantify all possible contaminants in

LPG there are some that show promise for detecting bulk contamination or detecting single

analytes at extremely low concentration. Ultimately, an array of sensors (same or different type)

may be needed if multiple contaminants are to be detected. For the LPG retailer, the cost of a

portable instrument will be a primary concern; however we believe that, in the short term, a

portable system can be acquired for under $1000 that will provide a good level of confidence for

detecting LPG contaminants. Additional investments into sensor research and design and method



development will likely be needed to commercialize many of the sensor technologies discussed in

this report and make them suitable for LPG.

While sensors are generally available with sufficient sensitivity to detect low concentrations of

relevant contaminants, the LPG industry should identify and specify what contaminants (and

what level) are the most detrimental. This information could form the basis of a custody transfer

procedure. Use of such a procedure will eventually drive all operations involved in the transport

of LPG towards better control of contaminant levels.

Regarding specific recommendations, we believe that the following are possible solutions that

merit consideration for further investigation and/or development:

• Investigation of a "Gas Tube Manifold" that utilizes several gas detection tubes

simultaneously - One of the concerns with gas detection tubes is interference from other

chemical species (e.g. mercaptans interfering with sulfide measurements). A sample

conditioning system in conjunction with the manifold (or any instrument for that matter)

might be used to eliminate specific interferents.

• Non-intrusive sensors for interface detection that operate through metal containers - This

capability would allow existing tanks to be retrofit without extensive modification. This type

of sensor could serve many industries besides LPG.

• Identification of specific sample conditioning/filtration systems to isolate specific

contaminants - Isolating a particular contaminant could be advantageous in two ways. First,

the isolated material would be more concentrated and thus easier to analyze. Alternatively,

the isolated contaminant is removed from the main sample reducing its effect on the

measurement of other components. This research could also benefit the LPG industry in

general by increasing the general knowledge of LPG filtration.

• The use of particle counting technology in LPG streams (liquid) - Variations on this

technology might be particularly useful in detecting solid contaminants or free water droplets

in real-time.

• Although not particularly affordable yet, tunable lasers are a class of sensor to keep in mind.

We found these very interesting because in some cases they can be tuned to a particular

frequency that is specific to a single chemical compound (e.g. ammonia). If detecting a

particular contaminant was deemed extremely critical, then this may be a viable option.



To complement each of the above ideas, a better understanding of LPG contaminants and their

effect on specific systems seems warranted.

To conclude our summary of the sensor investigation, we would like to stress that fuel sensors

only constitute one aspect of a successful Fuel Quality Program. A report from PERC claims

that LPG is the only fuel without a filter and this appears to be accurate. To maintain a successful

fuel quality program, the problem must be attacked from several angles. Many of the

contaminants that are believed to be a problem could be efficiently removed with a proper

filtration system. The technology for that filtration system exists today. With a proper filtration

system in place, the sensors discussed in this report would be relegated to periodic spot checks for

product quality assurance.

4.0 PART II - INSTRUMENT EVALUATION

4.1 Technical Approach

The instrument that we investigated is known as the Cyranose 320 (Cyrano Sciences Inc). It

consists of 32 individual thin-film chemiresistors configured into an array. Each individual

detector of the sensor array is a composite material consisting of conductive carbon black blended

throughout a non-conducting polymer. When a composite is exposed to a vapor-phase analyte,

the polymer matrix absorbs the analyte, which causes a concomitant increase in resistance. When

the analyte is removed, the polymer de-gasses causing the film to shrink and return to normal.

The collective output of the array is used to identify an unknown analyte using standard data

analysis techniques.

The unit is trained by measuring vapor "fingerprints" representative of the samples you intend to

analyze. Future measurements are then compared to these patterns to identify (i.e. classify) the

vapor. The unique polymer composite sensors have been shown to respond to a wide range of

organic compounds. Such a device, which is sensitive to a variety of volatile organic compounds,

might be used to "fingerprint" LPG (that meets required specifications) and form a basis for

future comparison to similar fuels. The Cyranose 320 is not necessarily an analytical tool for

detecting and quantifying individual chemical compounds in a complex mixture. Rather, its

strength lies in its ability to fingerprint complex odors and vapors and classify them using pattern

recognition algorithms.



4.2 Summary of Results

The focus of this experimentation was to investigate the sensitivity of the Cyranose 320 to a

variety of chemical compounds that may exist in LPG. Initially, we investigated pure compounds

such as plasticizers, alcohols, and hydrocarbons. The results were generally very good with the

instrument showing good separation between distinct compound classes. From there, we

attempted to detect low concentrations of contaminants in a hydrocarbon matrix to simulate LPG.

The results here were mixed. We found that compounds with high vapor pressures, such as

alcohol, could be easily detected in the headspace above the liquid hydrocarbon. However,

compounds such as plasticizers that have very low vapor pressures could not be reliably detected.

We conducted some limited testing with propane and found essentially identical results. We were

only able to see differences between propane samples that were grossly different (e.g. with and

without odorant). For further details, please see Section 12.0 in Appendix B.

4.3 Recommendations

Generally, we found the Cyranose 320 to be very sensitive and performed as advertised. Its

ability to distinguish chemical compounds with very similar structural features was outstanding.

Apparently, this ability lies in its large, 32-sensor array where each sensor is unique. Although

truly a handheld device, it might be somewhat difficult to implement at the retail level in its

current configuration. Furthermore, the Cyranose 320 is probably cost prohibitive (~$8000) for

most small retail outlets. Its major shortcomings are its inability to provide a quantitative analysis

and the potentially complicated calibration procedures. Calibration would be difficult because it

must be trained to recognize particular classes of samples that one expects to encounter in the

field. In addition, these calibrations would need to be updated periodically as the sensors age and

their responses change. The design of the instrument must also be taken into consideration. The

Cyranose 320 is not designed for high-pressure operation. The sample vapors must be near

atmospheric pressure and at ambient temperature. The primary application of the Cyranose 320

to LPG would be in detecting gross changes in composition, e.g. commercial propane vs. HD-5 or

lack of odorant. Batch sampling would be required and the instrument would be limited to the

analysis of vapor phase samples. As a result, compounds such as plasticizers that primarily exist

in the liquid phase may go undetected.



APPENDIX ALiterature Review



5.0 LPG QUALITY

5.1 LPG Composition

Compared to other combustible fuels, such as diesel and gasoline, LPG is a simple fuel in terms

of composition. Its primary component, propane (C3H8), is a colorless and odorless gas derived

from petroleum sources such as natural gas and crude oil. At atmospheric pressure and

temperature propane is a gas but it is easily liquefied under moderate pressures (e.g. 123.7 psia at

70ºF). LPG is sold as three different grades whose propane concentrations vary from around 50-

90%. LPG specifications are documented in two essentially equivalent standards: GPA Standard

2140 provided by the Gas Processors Association (GPA) and ASTM D 1835 (Table 2).

Table 2. GPA 2140-97 LPG Specifications

Component CommercialPropane

HD-5Propane

CommercialButane/Propane Mix

TestMethod(ASTM)

Composition Propane andpropylene

90% propane(min)

5% propylene(max)

butanes/ butyleneswith

propane/ propylenes

Butane+, liq vol 2.5% max 2.5% max --Pentane+, liq vol -- -- 2.0% max

D-2163

Moisture Content pass pass -- D-2713Residual Matter (mL) 0.05 mL 0.05 mL -- D-2158Total Sulfur ppm(incl. odorant) 185 123 140 D-2784

Cu Strip Corrosion,max No. 1 No. 1 No. 1 D-1838

Vapor Pressure100ºF, psig max38ºC, kPa, max

2081434

2081434

2081434

D-1267

5.2 LPG Contaminants

Because of its simple composition, one might assume that LPG fuel quality issues are rare and

remediation efforts are minimal. However, despite its simplicity users of LPG will encounter fuel

quality issues similar to that of liquid petroleum fuels. The general consensus is that the majority

of LPG leaving the point of production or import meets applicable commercial specifications.

For that matter, most LPG is believed to meet the more stringent HD-5 specification. This is

fortunate because although the commercial grade propane is generally acceptable for residential,



agricultural, and industrial use, motor vehicles that operate on LPG are particularly susceptible to

contamination. New emission regulations for vehicles, forklifts, and industrial and agricultural

engines are requiring more sophisticated fuel systems that are more sensitive to contaminants.

One element of LPG usage that isn't well documented is exactly how far off-specification a fuel

can drift before it begins to create problems in end-use applications.

This places the LPG industry in a quandary - how to handle LPG for those applications that are

sensitive to fuel composition. Producing all LPG to the HD-5 specification may be overkill since

the majority of applications can use commercial grade propane. The alternative is to handle

multiple grades of LPG. This would likely place a burden on the supply chain, requiring separate

storage tanks and driving costs up.

Problems with LPG contamination begin from the moment the LPG leaves the supply point.

Throughout the vast LPG supply chain there are many places that LPG can pick up contamination

including pipelines, rail cars, ships, storage tanks, tanker trucks, and delivery vehicles. The

contamination at each point of transfer is probably very small. After many years of

accumulation, sludge consisting of heavy residuals and water-soluble materials can collect in tank

bottoms if left unchecked.

For the purpose of this document we will define "contamination" as any change in LPG

composition outside of the relevant specification. Therefore, besides obvious contaminants like

water, contamination also includes incorrect proportions of normal LPG components like

propylene. The LPG specifications say very little with regard to contaminants. Indeed, not

enough is known about specific LPG contaminants and their effects to place limits on them.

Most of these are true contaminants - compounds that were not part of the LPG as it was

produced or imported. The typical types of contaminants reportedly found in LPG samples

include

• ammonia,

• methanol,

• water,

• excessive sulfur (beyond what is intentionally added),

• fluorides,

• metal particles from fuel system components (Cu, Zn, Pb, Fe, etc),

• common dirt,



• heavy hydrocarbons (lubricating oils),

• plasticizers (phthalates, adipates, sebacates) leached from hoses, and

• excessive ethane, butane ethylene, propylene, etc.

One critical characteristic of the contaminants that will weigh heavily on the selection of sensor

technologies is the phase in which the contaminant primarily exists. Since the components of

LPG exist in both liquid and vapor form inside a tank, the contaminants may also exist in one or

both phases (Figure 4). If LPG applications only drew vapor off the tank or if contaminants

remained in the liquid phase then the problem would be simplified. Unfortunately, neither are

true. Vapor pressure is the driving force that will determine whether a contaminant will enter the

vapor phase. Fortunately, most of the contaminants listed above have such small vapor pressures

that they do not exist to an appreciable extent in the vapor phase. Again, this brings up the

question of how much contaminant is a problem. How this all plays into sensor selection will be

discussed further in the section on Sensor Technologies (Section 10.0).

Gas Phase

Liquid Phase

Tank Bottoms

Figure 4. Contaminant Distribution

6.0 INTERVIEWS

The following are summaries of discussions with local distributors regarding LPG fuel quality.

We've used generic names to protect their identities.

Company A

Conversations with Company A indicated that local distributors are only able to take

whatever the supplier or producer provides. In their case, they require HD-5 grade only

for all deliveries; however, they have received LPG from one particular supplier with

high water content. The only recourse for a local distributor is to choose a different

supplier, which is what they have done. The distributor does not check LPG quality

because no simple, inexpensive, and portable system exists. This implies that local



distributors could possibly impact LPG quality by only accepting bulk fuel that is

contaminant free.

Company B

Conversations with Company B confirm the bulk of the comments offered by Company

A in terms of quality of fuel purchased. Specifically, Company B purchases HD-5 grade

with no verification of fuel quality at the receiving terminal. In other words, their local

operations simply take what their suppliers deliver, although they smell the gas for any

obvious and easily detectable problems. Company B does routinely sniff containers that

they fill to insure that no leaks are present. This is accomplished with a small electronic

hand held device.

We inquired that if a simple inexpensive handheld device for fuel quality determination

were available, would they use such a device? They replied in the affirmative and would

be willing to pay several hundreds of dollars for such a unit. Company B said such a unit

would easily pay for itself if it prevented the loading of tanks with contaminated gas.

Many local companies load 25,000 gallons or more per day in domestic tanks. If a bad

load of fuel were delivered, the cleanup cost (pulling fuel out and inspection of tanks)

could be very significant.

7.0 LPG SENSOR SPECIFICATION

7.1 Objective

A sensor specification was generated for use as a guideline for developing a Design Specification

and/or a Performance Specification for selected LPG sensor technologies. Given that each

technology may have a unique goal (e.g. identifies a specific contaminant, measures a bulk

property, etc), the sensor characteristics defined herein should be considered for relevance when

designing a handheld or portable LPG instrument. Given current LPG quality requirements as

documented in such publications as GPA 2140-97 and ASTM D1835, the expected level of

performance should also be considered. The purpose of this specification is to document desired

sensor characteristics and provide a template for comparing selected LPG sensor technologies.



This specification is a living document and is subject to modification as requirements and

technology change. Current work involving the identification of LPG contaminants and their

effect on fuel-system components will undoubtedly affect the focus of future sensor

developments. The sensor specification can found in the Appendix, Section Error! Reference

source not found..

7.2 Subsystem Definition

A valid approach for designing any complex system is to break it down into a collection of

subsystems each performing a specific function. Detailed requirements are then generated for

each subsystem and interface protocols are defined between each subsystem. When designed and

integrated properly, the combined cooperation of these subsystems should yield an instrument

that performs according to the performance requirements established for that application.

Tentatively, we have chosen to divide the LPG sensor system into the following requirement

areas:

(a) Packaging

(b) Power

(c) Sample Handling and Preparation

(d) Sensor System

(e) Control System

(f) Operator Interface

(g) Data Interface

Detailed requirements for most of these subsystems will be beyond the scope of this

investigation. For instance, subsystems (a), (b), (e), (f), and (g) are strongly dependent on the

needs of a particular sensor system and its peripheral hardware. In our investigations, we will

primarily focus on subsystems (c) Sample Handling and Preparation and (d) Sensor System for

sensor technologies with potential merit. These subsystems are inherently tied together because

particular sensor systems typically have unique requirements for sample introduction and

preparation.



In this document, we will define the basic or overall characteristics that should be considered

while investigating selected LPG sensor technologies. Then where possible, detailed

characteristics for each subsystem will be identified. Requirements defined in this document

should be considered as minimum desired requirements.

7.2.1 Packaging

This requirement defines the external packaging of the instrument. Since the intended application

is a portable instrument, the instrument will be subjected to harsh environments. Therefore, the

packaging must accommodate the environment in which it is expected to operate, such as shock

and vibration, wet and potentially explosive environments, and atmospheric temperature and

pressure extremes. Depending on the intended area of usage, the area classification should be

considered and the relevant standards obeyed (e.g. NFPA, NEC).

7.2.2 Power

This requirement determines the means by which power can be supplied to drive the sensor. For

a given application, one should determine the expected area of usage and the types of power that

will be available (e.g. battery power, 12/24 VDC, 120 VAC). At least two options should be

provided and internal power regulation, if required, should also be defined. If battery operated,

then rechargeable batteries should be considered. Given the nature of the application,

consideration should be given to a design that is intrinsically safe and obeys relevant electrical

codes.

7.2.3 Sample Handling and Preparation

This requirement defines the hardware and procedures by which a collected LPG sample is

introduced to the sensor. LPG can exist in both a liquid or gaseous state and the means to present

a representative sample to the sensor is critical.

7.2.4 Sensor System

The sensor system provides the fundamental functionality of the instrument. This requirement

defines the characteristics of the sensor such as type, operational requirements, consumables,

power consumption, etc.



7.2.5 Control System

This requirement defines the control system for the entire instrument. Some computer processing

is inevitable given the state of most advanced sensor technologies. The control system should

define the requirements for instrument control, data acquisition, data processing, and display.

The Control System is the core of the instrument and binds all of the functionality together to

create a working system.

7.2.6 Operator Interface

This requirement defines the interface by which the operator interacts with the sensor system.

Depending on the needs of a particular sensor and application, the interface may be as simple as

buttons and LED indicators or as sophisticated as a touch-screen display.

7.2.7 Data Interface

It is rather likely that data collected with the instrument will become part of an audit trail for

tracking fuel quality. As a result, the data will need to be well documented for permanent records

and password protected to prevent tampering with data or calibration files. This requirement

defines the hardware and software protocols (e.g. serial) for externally interfacing to the

instrument and extracting stored records. The Data Interface may also be used to update internal

software. It is recommended that non-proprietary or open data protocols are incorporated into the

unit.

7.3 Summary Characteristics

Table 3 provides a summary of subsystem characteristics recommended for consideration when

performing trade-off studies of relevant LPG sensor technologies. Some modification of the

requirements may be needed for a given application.



Table 3. Summary Table

Requirement Type Characteristic Requirements

Ruggedness Designed to withstand moderateshock and vibration

Transportability

Can withstand airfreight cargopressuresX-ray hardened for securityinspection points

Operating Temperature-20°C (-4°F) to +40°C (104°F) min-40°C (-40°F) to +50°C (122°F)desired

Storage Temperature -20 to +70 °C desiredHumidity 0 to 95%, non-condensing

Area Classification reference current codes (NFPA,NEC, etc)

Chemical Compatibility Hydrocarbons/Water

Weather Conditions Sealed to withstand outdoorenvironments

Weight and Size

Portable by an individual or vehiclemountable

Should not interfere with normaloperations

Packaging

Hardware Commercial off-the-shelfcomponents where possible

Battery Power Rechargeable/replaceable batteriesBattery Life (NormalOperating Conditions) 3 hours

Vehicle Power 12/24 VDCFacility Power 100-240 VAC with universal adapter

Battery Backup Replaceable, internal battery to storeconfiguration, minimum 10-year life

Power

Battery Charging Internal or External

Working Pressure reference current ASME, DOT, etc.regulations

Burst Pressure 3X working pressure

Sample Temperature Maintain as needed for the specificapplication

Remote Sampling Allow sample to be collectedremotely and brought to instrument

Setup & Configuration Time Minimize

Automated Automate sample handling throughautomated controls where possible

Sample Handling andPreparation

Safety Reduce sample volumeEliminate ignition sources

Sensor System Reliability MTBF ≈ 1 year




Maintenance MTTR ≈ 2 weeksNot expected to be field repairable

Cyclic Maintenance Battery ExchangeDesiccant Exchange

Interface Must interface to Control SystemPortability Sufficiently small to be portable

Power Requirement Minimize power draw to increaseportability

Consumables Minimize consumables

Chemical Compatibility Hydrocarbons/Water/ExpectedContaminants

Warm-up Time <10 minutes

Measurement Time off-line ≈ 10 minuteson-line ≈ 1 minute

CalibrationCalibration only performed bymanufacturer. User can verifycalibration in the field.

Temperature Control Maintain required temperature

Detection Limit Meets minimum thresholdrequirement for target analyte

SensitivityPreferably a linear sensor responseto changes in analyte concentrationacross the concentration range

Specificity Responds to only the desiredanalyte (no interference)

Concentration RangeSensor responds to a wide range ofanalyte concentration (i.e. does notsaturate at low levels)

Embedded Algorithms Scale computing power to meet thedata processing needs

Processing PowerProcessor speed and memoryshould meet the requirements of theapplication and available power

Onboard Data Storage Sufficient memory to retain the last50 measurements

Real-Time Clock All records should be timestampedfor auditing purposes

Power Design to maximize battery lifeLevel of Automation Minimize operator interactionSystem Malfunction Provide visual error indication

Power Loss Graceful shutdown (preferred)Automatic reboot at power-up

Electromagnetic andElectrostatic Compatibility

Compatible with other electronicdevices

Temperature ControlMaintain internal temperature withinthe operating temperature range ofthe sensor

Control System

Diagnostics Built-in test at startup




Operator InterfaceProvide basic, intuitive interfaceLevel of information detaildetermined by operator login

Operational Impact Minimize operator interaction

IndicatorsPower-on indicatorError indicatorLow power indicator

Operator Display Pass/Fail indicator orGraphic Display w/ backlight

Operator Interface

Output Selectable engineering units

Protocol Serial, USB, or ethernet

Date Transfer Rate Minimum 9600 bps1.0+ MB/sec desired

Software Updates Accessible through Data InterfaceData Interface

Interface Software MS Windows compatibleconfiguration utility

8.0 SAMPLE CONDITIONING TECHNIQUES

So, why discuss LPG filtration in a report dedicated to LPG sensors? One of the concepts to be

discussed in this report is that of differential measurement. In this approach, the LPG sample to

be analyzed is split into two portions. One portion is preprocessed using a filter(s) that remove

specific compounds. The remaining portion of the original sample is left unchanged. The same

measurement technique is then applied to both samples and the results are compared. If there is a

noticeable difference between the original sample and the filtered sample then one might infer

that certain compounds were present in the original sample and were removed by the filtration

system. If one were to simply analyze the original sample without any baseline or reference point

it would be difficult to ascertain whether the signals are a result of the contaminant or interference

from other species. This approach mostly applies to spectroscopic measurements but may also

work with physical property measurements (e.g. density). In the latter case, it depends on the

contaminant level and the effect that it has on the physical property.



The information contained in this section of the report is provided for completeness and reference

in future work. The discussions that follow will introduce some of the more common filtration

techniques, which include:

• Molecular Sieves (zeolite or carbon-based)

• Activated Alumina

• Activated Carbon

• Silica Gel

• Coalescent Filters

• Strippers

• Particulate Filters

8.1 Molecular Sieves (Zeolite)

Molecular sieves are adsorbents consisting of crystalline aluminosilicates (zeolites) and clay. The

main characteristic of zeolitic molecular sieves is their well-defined pore-size and uniform porous

structure. Zeolites are "activated" by heating to remove the water trapped within their cage-like

network.

Molecular separation by zeolite-type absorbents occur in two ways:

• Separation by size - only molecules with a diameter smaller than the zeolite pore size can

enter and be absorbed. Molecular diameter is measured in angstroms (Å) or 10-10 meters.

• Separation by polarity - zeolites have an affinity for highly polar compounds

Critical Diameter (Å) ExamplesH2O, CO2, CO, O2 2.8N2 3.0NH3, H2S 3.6methane 4.0ethane, methanol, ethanol 4.4propane, nC4-nC22 4.9propylene 5.0ethyl mercaptan, butenes 5.1

Increasing Polarity / AdsorptionH2ONH3alcohols (methanol), aldehydes,ketonesSO2H2S, mercaptansalkynesalkenesCO2alkanesmethaneCO



The selectivity of a zeolite toward specific chemical adsorbates is affected by several parameters:

• zeolite type• pore diameter• adsorbate molecule's shape, chemical structure, critical diameter, polarity, and

polarizability.• adsorbate concentration or partial pressure• temperature

Critical diameter is an important factor that determines whether or not a molecule will fit through

the zeolites pore opening and into the crystalline lattice where it can interact with the active

surface. The only exception to this criterion may be at elevated temperatures where the elasticity

of the adsorbate (polarizability) or the slight mobility of the molecular sieve network may allow

larger molecules to "squeeze" through. Polarity is also a driving factor in adsorption - zeolites

have an affinity for strongly polar compounds.

Unlike alumina and silica, zeolites have a high adsorption capacity for relatively low

concentrations of adsorbate. This is important for LPG whose contaminants are generally in very

low concentrations. Furthermore, zeolites adsorption capacity is less affected by temperature

then other adsorbents.

Regeneration of the molecular sieves can be accomplished in several ways including thermal

regeneration, pressure swing desorption, or elution. Thermal regeneration is the most common

and involves heating the adsorbent bed to desorb the contaminant molecules. Pressure swing

desorption works by pulling a vacuum across the bed to accelerate the degassing process. Elution

displaces the adsorbed compounds with another liquid/gas that preferentially adsorbs in the sieve.

Common uses of zeolites in LPG treatment

include removal of H2S, H2O, and

mercaptans.

8.2 Activated Alumina

Alumina is commonly used in LPG drying

and is reportedly the most economical

desiccant given its range of application.

Chemisorption

Involves the formation of strongchemical bonds between the adsorbatemolecules and the surface locationsthat are chemically active.

Physisorption

Caused mainly by van der Waals forcesand electrostatic interactions betweenadsorbate molecules on the adsorbentsurface.



Activated alumina is less expensive than silica gel or molecular sieves but has a lower capacity

for water. Alumina generally has a high affinity for polar compounds like water.

Adsorption, the reversible fixation of molecules to a surface, occurs in three ways with alumina:

• Chemisorption - forms the first layer at the alumina surface

• Physisorption - formation of subsequent layers through hydrogen bonding

• Capillary condensation - local condensation of "contaminant" molecules at temperatures

above the bulk fluid's dew point

Benefits of alumina include a high surface area, a high pore volume, and high mechanical

strength. Pore size distribution can sometimes be tailored for a specific application allowing

elimination of selective species. Factors that favor adsorption into the alumina include low

temperature, high partial pressure, and high condensation temperature.

When used on a large scale, alumina beds can be regenerated by desorption. Regeneration can be

accomplished by exposure to a hot inert gas at reduced pressure. Like other adsorbents, several

grades of alumina are offered. Each grade typically serves a specific purpose and can be

designed to selectively remove specific contaminants. Examples include drying of air or organic

liquids like LPG, CO2 removal from air, and selected removal of fluorinated hydrocarbons from

gases and liquids.

8.3 Activated Carbon

Combined with a high surface area and porous internal structure, activated carbon is commonly

used for the removal of organic chemicals from both liquids and gases. In industry they play a

significant role in the purification and recovery of chemicals. The amount of material removed

depends on the capacity of the activated carbon as well as the chemical affinity of the material for

the carbon. Their unique properties and low cost compared with that of competitive adsorbents

make them popular alternatives.

8.3.1 Molecular Sieves (Carbon)

Carbon molecular sieves (CMS) are a special class of activated carbons. The physical distinction

between activated carbons and carbon molecular sieves is not always clear. Generally, carbon



molecular sieves will have most of their pores in the same molecular size range. Some activated

carbons may also have very small pores. The primary difference is in their method of separation.

Activated carbons separate molecules according to their adsorption equilibrium constants (i.e.

capacity) while carbon molecular sieves provide molecular separations based on the rate of

adsorption. The rate of adsorption consists of two important factors: the capacity of the sieve at

equilibrium, and the diffusivity, or rate of diffusion into the porous material. The extent of

molecular sieving can be defined as the ratio of the rate of adsorption of one molecule to another.

Carbon molecular sieves are non-crystalline or amorphous and are described as platelets of

carbon separated by interstitial spaces. The size of the spaces are influenced by several factors

including foreign atoms between the layers, side chains, and cross-linking. This is in contrast to

the inorganic oxide molecular sieves (zeolites) which have a well-ordered crystalline structure.

Although amorphous in nature, carbon molecular sieves have some advantages over zeolite

molecular sieves:

• High temperature stability (in some cases as high as 1500°C)

• Chemically compatible with acidic media.

• Low affinity for water.

• Easy to make relative to zeolites.

8.4 Silica Gel

Silica gel is a widely used desiccant. Common uses include natural gas dehydration and

hydrocarbon recovery units. Silica gel is easily regenerated by heating. Its capacity (and cost) is

higher than alumina but lower than molecular sieves.

8.5 Filter/Separators

Coalescer Elements

Coalescers are commonly used

throughout the petroleum industry to

filter solids and separate immiscible

liquids. The most common use for

coalescers is filtration and separation

of water from aviation and diesel.

Examples of Coalescent Filter Housings



Free water droplets, on the order of 30 microns, can suspend in hydrocarbon fuels giving them a

hazy appearance. Unless held in suspension (by a surfactant for example) this water will coalesce

and fall to the bottom under gravity given enough time. The function of a coalescer element is to

aid this process by merging the smaller particles of water into larger droplets so they will fall,

under gravity, to the sump where the accumulated water can be drained.

Coalescing elements generally consist of a hydrophilic medium such as glass. Unlike some of the

other filtration techniques discussed in this section, coalescent filters do not need to be constantly

regenerated because the particles of water do not permanently attach themselves to fibres. As the

wetted fuel flows through the filter, the water adsorbs onto the glass fibers. Repeated adsorption

leads to the formation of large water droplets on the fibers. The larger droplets are exposed to

increasing forces at their surface causing them to be pushed through the filter. When the large

droplets reach the outer layer of the filter, they are ejected from the filter. The ejection process is

still a matter of debate and may occur through a number of different processes (graping,

ballooning, droplet chains, pointing, etc.). Suffice it to say that most of the water droplets exiting

the filter remain large enough that they immediately sink to the bottom of the filter/separator unit

where they can be drained.

Separators

Separator (stripper) elements utilize a screen to prevent the passage of water entirely. The screen

is actually a hydrophobic (i.e. water-repellent) barrier, which allows the passage of hydrocarbon

fuel but prevents water from passing through. The screen has a small mesh size so that it can trap

suspended water droplets on the upstream side. As

additional water droplets collide with the screen

they begin to merge to form larger droplets. These

droplets fall, under gravity, to the sump. Stripper

materials may include silicon-coated paper, Teflon

mesh, or other synthetic materials.

Gas coalescers

Liquids can also be separated from gases using the

coalescer technique with some minor differences.

Gas coalescers reportedly use a baffle system for

the pre-separation of liquid slugs before the gasExample of Gas Coalescer Housing



enters the coalescer. This improves the separation efficiency of the coalescer element and

increases its life by preventing a large differential pressure across the filter. The process whereby

water droplets fall out of a hydrocarbon fuel under gravity is strongly influenced by the viscosity

of the fuel. The lower the hydrocarbon viscosity, the more easily the water droplets can move

through the fuel. Since the viscosity of a gas is even lower than that of a liquid, gas coalescers

may operate without separators. The droplets formed by the coalescers settle under gravity

without the need for separators to remove carry-over droplets.

8.6 Particulate Filters

Particulate filters are used to remove solid debris from liquid hydrocarbon streams. Other than

basket strainers to remove large objects like nuts and bolts, the particulate filter is usually the first

line of defense in the filtration sequence. Although coalescer elements may also remove

particulate matter, they're not designed to handle large amount of particulate matter. Particulate

filters are generally constructed of cellulose or synthetic fibers and are rated according to the

micrometer (a.k.a. micron) size of the debris that they can efficiently remove. A common

practice is to use a sequence of particulate filters with decreasing micron ratings as you move

downstream. Each successive filter removes smaller and smaller particulates. This prevents tight

(i.e. small micron) filters from clogging too quickly when large particles are encountered.

8.7 Sample Conditioning - Application to Portable LPG Sensors

Selective filtration may play a significant role in the ability to analyze for certain compounds.

For instance, the ability to remove compounds that interfere with certain measurement techniques

or the ability to separate and gather other compounds may prove useful (e.g. detecting coalesced

water). As mentioned earlier, the use of differential measurement techniques will also rely on

filtration to create a reference sample void of contaminants. While some filtration media (e.g.

silica gel and zeolites) can be regenerated, there is no reason to require that to be done at the

instrument. Small filter canisters that use swappable cartridges can be used and if possible the

canisters can be regenerated elsewhere.



9.0 LPG SAMPLING

9.1 The Sampling Environment

The local LPG retailer usually encounters the expense of servicing and support for their

customers who range from businesses that run fleets of LPG powered forklifts to the servicing of

the many storage tanks in rural America. Many of these expenses for service calls and

component replacement are related to off-specification and contaminated fuels. Local retailers

would have better bottom line fiscal control of their businesses and provide improved services if

they would verify the quality of LPG as a condition of acceptance. Therefore, an important

question is where (and by whom) should fuel be sampled and what measurements make sense?

When accepting shipment of LPG, the common practice for the local LPG retailer is to assume

that the shipment from the supplier is of specified quality and generally no confirmation of the

LPG quality is obtained. Unfortunately, the retailer has no knowledge of the processing history

of the LPG nor does the retailer have any history of the shipping route taken by the fuel. Since

there are numerous opportunities for the fuel to pick up contaminants during shipment, it is not

uncommon for fuel loads to contain water, ammonia, hydrogen sulfide, and plasticizers, to name

a few. In the context of obtaining sufficient information at the custody transfer point (shipper to

local retailer) needed for shipment acceptance, LPG quality determination by a relatively simple

and quick method is essential which also implies that a practical sampling method is required.

A second area where sampling is important is within a storage tank, especially those that have

been in use for several years. These tanks collect contaminants such as water and particulate

matter. Eventually, these lead to fouling of downstream regulators and burners. Early detection

of accumulated contaminates will allow for scheduling of tank reclamation or replacement in a

manner least disruptive to service.

If LPG quality and contaminant determination could be obtained by well-established non-

invasive measurement techniques, then the physical gathering of fuel samples for analysis would

not be necessary. Non-invasive techniques may become available, but in the immediate future

(five years), sampling will be necessary.

Accurate quality determination begins with obtaining an LPG sample that truly represents the

characteristics of the fuel. As suggested above, samples are required at the custody transfer point



and within tanks. Also, tank samples may be extracted from the gas or liquid phase. Sampling of

tank bottoms can present challenges due to limited physical accessibility to the interior of the

tank. Once these samples are obtained, specific sensors or sensor arrays will be immersed into a

representative LPG sample. When considering the development of hand held or portable

instrumentation, the sampling system needs to be integrated with the sensor systems in such a

way to maintain overall simplicity and ease of operation.

In this section, sampling in various regions is discussed especially in the context of exploring

methodologies that are applicable to portable systems.

9.2 Common Methods in Use For Sampling

The issue of sampling LPG in containers other than laboratory testing apparatus is addressed in

ASTM D1265 – 92 entitled ”Standard Practice for Sampling Liquefied Petroleum (LP) Gases

(Manual Method).” For custody transfer operations, the samples must be in the liquid phase only.

Furthermore, if corrosive compounds or sulfur compounds are to be analyzed, the sample

containers and valves should be made of stainless steel. A typical sampling container is shown in

Figure 5. The sample bottle contains an outage tube (ullage) at the top. Ullage volume is needed

for allowance of thermal expansion. By following the procedures outlined in D1265, this

particular sample bottle will always provide 20% outage (ullage) and 80% liquid. The D1265

sampling method is manual and requires some minor training of personnel that acquire LPG

samples.

Figure 5. Configuration for a Sampling Container (from ASTM D1265 – 92)



A liquid sampling method that can be automated is described in GPA Standard 2174-93 entitled

“Obtaining Liquid Hydrocarbon Samples for Analysis By Gas Chromatography.” This procedure

incorporates a floating piston cylinder arrangement with one typical configuration shown in

Figure 6 (other arrangements are discussed in the standard). One advantage of this system is that

the pressure of the samples can be regulated.

Figure 6. Typical Visual Indicator Sampling System (from GPA Standard 2174-93)

If it is necessary to obtain a sample and then physically move the sample to another location for

injection into portable analytical equipment, then a Closed Loop Cylinder Sample Panel such as

made by Sentry Equipment Corporation may be useful. A diagram of its operation is shown in

Figure 7. Note that this particular setup shows a filter at the supply valve. This filter may or may

not be necessary and is dependent on the particular sensor application. For instance, the filter

may be a simple particulate filter that prevents fouling of the instrument. This particular unit

mounts on an aluminum plate that is approximately 28 x 18 inches. A system built along the

same layout could be miniaturized (depending on the volume required) and incorporated into a

portable system or used as-is in conjunction with a sensor system.



Figure 7. Cylinder Sample Panel shown in three modes of operation(from Sentry Equipment Corp., Oconomowoc, WI.)

9.3 Sampling Considerations for Portable Systems

Portable LPG quality measurement systems will obviously need to operate in a custody transfer

environment (tanker to retailer terminal for example) and in the field where measurements are

needed to troubleshoot problems suspected to originate in a storage tank. Not discussed in this

study but an important issue is the safety requirement for operating equipment in an atmosphere

that contains LPG vapors and dealing with emissions from that testing. The size, weight, and

volume of the systems must be manageable by service personnel without undue encumbrances of

operational complexity. For most concepts, the sampling system will include a small, high

strength bottle, a method for introducing the sample, and a method for expelling the sample.

Processing of the sample could be effectively incorporated into the portable sampling unit. For

example, LPG is the only fuel that must be sub-saturated to meet specification, hence the

detection of any free water will verify that the fuel is grossly off-specification. Therefore a

simple filtration/separation unit that can drop out water could be used as a simple detection

method.

In summary, a sampling system will be needed for portable units at least in the near term. The

main features include small volume, ease of connecting to the desired sample point, use of non-

leaking quick-connect fittings, and on board purging and venting. Provisions for preconditioning

of samples, like selective removal of interferents, may aid in improving measurement accuracy.



10.0 SENSOR TECHNOLOGIES

In this section we describe a variety of different sensing techniques that are relevant to both gas

and/or liquid phase substances. Advantages and disadvantages will be highlighted. Some sensors

are only useful in detecting bulk contamination while others can detect and quantify a single

compound in trace quantities. While all of these technologies could be used to analyze LPG, we

will show that they may not be well suited for the intended application or point of use.

Obviously, sensor complexity, selectivity, and sensitivity are normally directly proportional to

cost, and it appears that most of the sensor technologies will likely be outside the budget of small,

local-area retailers. Although, the decision to buy a particular instrument should be traded off

against the financial risk of a catastrophe (fouling of storage tanks) that may have been avoided

by use of the instrument. Therefore, a broad investigation of relevant sensor technologies will be

considered for comparison and completeness.

Once the sensor technologies have been discussed, a trade-off analysis will be performed. The

trade-off analysis will grade sensors on their ability to provide accurate LPG analysis based on

the sensor’s intended purpose. For example, whereas some sensors are designed to detect trace

level contaminants, others are designed to detect large changes in overall composition. In this

case, the latter would not be scored lower provided it performs well with respect to its intended

usage.

Because LPG is normally contained within a closed system, we can distinguish between two

different types of sensor/instrument applications: on-line and batch measurements. There are

other sampling/measurement variations but these carry the widest distinction. On-line

measurements would require the sensor to be permanently installed in a transfer line or in a

storage tank. In an on-line mode, the sensors are generally intended to run unattended and

provide a continuous output. This type of installation is counter to the purpose of this report

because the instrument would become a permanent fixture of the LPG system. For batch

measurements, an operator usually withdraws a representative sample from an LPG source and

transports it to the instrument. Though not as common, the instrument may also be temporarily

interfaced to the main LPG source and an analysis performed. Each mode of operation has

challenges that must be overcome. In the on-line mode, the sensor must be incorporated into the

LPG system such that it does not affect normal operations. Automated sampling and



conditioning equipment may be needed to extract and prepare the sample before introducing it to

the instrument. Batch-mode sampling will also require peripheral equipment to allow samples to

be withdrawn from a large LPG source. A separate system may be needed to interface the

collection cylinder to the instrument.

Considering the manner in which LPG is handled, a truly “handheld” instrument presents

challenges for this application given the expected need for peripheral equipment to accommodate

sampling. A handheld instrument would need to be coupled directly to the LPG system; this is

feasible but may create some safety concerns (high pressure leaks, need for grounding and non-

sparking tools, etc.). Also, special ports or adapters may be required which makes this option

appear less viable. More likely, a representative LPG sample will need to be withdrawn from its

source for subsequent batch processing. The sample would be taken to a portable instrument,

perhaps mounted on a vehicle.

The types of sensors investigated in this study are shown below grouped into common sensor

families. However, it should be noted that some sensors might serve multiple purposes. For

example chemical array sensors and spectroscopic devices are sometimes used as gas monitoring

sensors but they may be used as analytical instruments too. This isn't normally true for the "gas

monitoring systems."

• Gas Monitoring Systems

• Catalytic Bead (Section 10.1)

• Electrochemical (Section 10.2)

• Metal Oxide Semiconductor (MOS) (Section 10.3)

• Chemical Array Sensors

• Bulk Acoustic Wave (BAW) (Section 10.4)

• Surface Acoustic Wave (SAW) (Section 10.5)

• Metal Oxide Field-Effect Transistor (MOSFET) (Section 10.6)

• Conducting Organic Polymers (COP) (Section 10.7)

• Chemoresistors (Section 10.8)

• Spectroscopic (Section 10.9)

• Mid-Infrared Spectroscopy (Section 10.9.2)

• Near-Infrared Spectroscopy (Section 10.9.3)



• Laser Spectroscopy (Section 10.9.4)

• Dielectric Measurement (Section 10.10)

• Gas Chromatography (Section 10.11)

• Ionization Detectors (Section 10.12)

• Gas Detection Tubes (Section 10.13)

• Coriolis Measurements (Section 10.14)

• Interface Detection (Section 10.15)

• Particle Counters (Section 10.16)

10.1 Catalytic Bead

Catalytic bead, or hot-wire, sensors are commonly used to detect flammable gases in the lower

explosive limit (LEL) range. They have numerous strengths, such as low cost, long life, simple

design, zero stability, and linear response over a wide temperature range, but they can be affected

by "catalytic poisons" such as silicones, plasticizers, and sulfur compounds that coat or corrode

the sensor's catalyst. These sensors are normally used in area monitoring.

Principle of Operations

Catalytic bead sensors consists of two ceramic beads with embedded platinum coils heated to

~450°C (Figure 8). One bead is passivated so that it will not react with combustible gas. This

bead compensates for changes in ambient temperature, humidity and pressure variations. The

other bead is activated by a catalytic material, which oxidizes the gas forming additional heat.

This heat creates a measurable change in resistance of the platinum coil. The sensor electronics

detect this increase in resistance and reduce the electrical power to the bead until the original

platinum coil resistance is restored. The beads are placed on two separate legs of a Wheatstone

bridge which creates a bridge current that is approximately proportional to the gas concentration.

The concentration range is generally 0%–100% of the LEL.

Figure 8. Catalytic Bead Sensor



Catalytic bead sensors operate above a threshold or voltage, which corresponds to the bead

temperature at which the presence of the catalyst and oxygen can first ignite the gas. As the

sensor ages, the catalyst is slowly used up on the bead causing the threshold voltage to gradually

increase and the sensor to become less sensitive. The platinum coil ages simultaneously causing

increased zero drift and noise. Ultimately, the sensor must be replaced.

These sensors are small, rugged, analyte specific, inexpensive, are relatively simple to maintain

and have a long life of 2 to 4 years. However, the potential for catalyst poisoning exists and the

sensor can be damaged (span loss) by repeated exposure to high concentrations of gases.

10.2 Electrochemical Sensors

Electrochemical sensors are excellent for detecting low parts-per-million concentrations of a

select gas and are generally the most popular for sensing toxic gases. These sensors are not used

for measuring combustible gases. The sensors contain an electrolyte that reacts with a specific

gas, producing an output signal that is proportional to the amount of gas present. Electrochemical

sensors exist for gases such as chlorine, carbon monoxide, hydrogen sulfide, and hydrogen, but

cannot be used to measure hydrocarbons.

Principle of Operation

Generally, an electrochemical sensor consists of a diffusion barrier, a sensing-electrode (also

called the working-electrode, measuring-electrode, or anode), a counter-electrode (also called the

cathode), and an electrolyte (Figure 9). The electrochemical cell typically consists of a casing

containing an electrolyte gel and electrodes. The top of the casing has a gas permeable

membrane and a gas capillary. The electrodes are constructed with a large surface area to

promote maximum sensitivity and longer life. In turn, this provides a larger signal and a quicker

response time.



Figure 9. Electrochemical Sensor

Oxidization takes place at the anode and reduction at the cathode. In an environment free of

chemically reactive gases, oxygen diffuses into the cell and adsorbs on both electrodes creating a

stable potential between the two, i.e. no measurable current flow. However, when a chemically

reactive gas passes through the diffusion barrier it is either oxidized or reduced depending on the

gas. The result is a potential difference between the two electrodes, which causes a measurable

current flow. Since electrochemical reactions are temperature dependent, an electrochemical

sensor should include a temperature sensor to provide temperature compensation.

The cells are diffusion limited so the rate the gas enters the cell is only dependent on the gas

concentration. The current generated is proportional to the rate of consumption of the subject gas

in the cell. Modern electrochemical sensors also utilize a third electrode called a reference

electrode. This electrode has a stable potential from which no current is drawn. It is used to

eliminate interference from side reactions with the counter-electrode and it allows the sensing-

electrode potential to be biased with respect to its rest potential. Biasing is one method of

controlling sensitivity to a particular gas. Extended storage of the sensor is achieved by placing a

shorting clip across the sensing and reference terminals, which maintains the electrodes at the

same potential and keeps current from flowing through the cell.

10.2.1 Interfering Gases

Like other sensors, electrochemical sensors are not completely specific to one analyte. Despite

the effort to engineer a sensor to be target specific, it is difficult to create a catalyst that responds

to only one chemical compound. Many of the target gases are chemically similar. Therefore,



sensors are said to be "cross sensitive" meaning they can react with other chemical species

besides the intended target. Although filters may help to prevent certain species from entering the

cell, it may also slow the response of the cell to the target compound.

10.2.2 Blocking Mechanisms

When blocking occurs, the cell may function erratically or not respond at all. Unlike poisoning,

which usually renders the cell useless, once the blocking condition is removed the cell will likely

return to normal. Common blocking mechanisms are described below.

Electrolyte Freezing

As the temperature of the cell decreases, the chemical reactions that create the

measurable current begin to slow and consequently the output signal begins to decrease.

Generally, at temperatures near freezing (0°C) no current flow can be measured in the

cell. However, once the temperature returns to normal, the cell will "thaw" and being to

function properly again. The cell can be heated if freezing temperatures are expected.

Oxygen Depravation

Oxygen must be present in the cell to react with the target gas. If, for any reason, the

oxygen supplied to the counter-electrode is cut-off then the current cannot be sustained.

Sources of oxygen depravation are described below.

1. The sensor is located in an environment saturated with reactive gases. This limits the

amount of oxygen that can diffuse into the cell or the oxygen may simply get used up.

2. The sensor is located in a low-oxygen environment. If necessary, air may need to be

supplied to the sensor.

3. The cell's diffusion barrier or flame arrestor becomes clogged. This may prevent both

oxygen and target gas from entering the cell. The sensor may require routine cleaning.

4. Water or gas vapor condenses in the cell's diffusion barrier or flame arrestor. If the

sensor temperature is lower than the ambient temperature condensation can occur.

Heating or the use of dry air may be needed to prevent this problem.



Oxidation/Reduction Reactions

Electrochemical sensors are normally designed to detect a specific target gas. Based on

the chemical makeup of the gas, it will either be oxidized or reduced in the cell.

However, if a reducible gas enters a cell designed to detect an oxidizable gas or vice

versa, the reverse reactions may mask the presence of the target species.

10.2.3 Poisoning

Poisoning is a common problem in chemical-based sensors. A poison is a chemical species that

blocks and/or degrades the sensor’s operation. After prolonged exposure to a poison, the sensor

usually becomes completely inoperable. Common sources of poisoning are described below.

Solvent Vapors

Depending on the design of the sensor, a high concentration of solvent vapor may attack

the sensor housing or filters. Solvents that are commonly problematic include alcohols,

ketones, phenols, pyridine, amines, and chlorinated solvents. Sensors that are routinely

exposed to large doses of these compounds may experience reduced lifetimes.

High Temperatures

Operation of the sensor at high temperatures (e.g. 40°C (104°F)) for long lengths of time

may degrade the electrolyte in the cell.

10.2.4 Pressure Effects

Like other diffusion-based chemical sensors, electrochemical sensors are designed to work at

atmospheric pressure ±10%. Sudden changes in pressure may produce high transient signals,

which may trigger false alarms.

10.2.5 Humidity

Electrochemical sensors are not affected directly by humidity but continuous operation at extreme

high or low humidity can change the water content of the electrolyte and affect the output of the

cell. Extreme humidity may cause the electrolyte to swell and the cell to leak. Excess water may

also cause the cell to freeze more easily. At low humidity, the cell may begin to dry out causing



the acid concentration to increase and either crystallize out or attack the cell. A hot and dry

climate is the worst-case scenario for an electrochemical sensor.

10.2.6 Sensor Life

Even when not in use, electrochemical cells have a limited shelf life because they are constantly

"active." Typical life expectancy may be three years whether used or not. Storage in a

refrigerator and avoidance of low humidity, high temperature, and poisons is recommended.

Continuous exposure to a signal producing gas (target or otherwise) will shorten the life of the

cell because it destroys a small portion of the electrolyte. The onboard filters also have a finite

life and will only remove a fixed amount of gas before becoming inoperable.

10.2.7 Advantages/Disadvantages

Advantages of electrochemical gas sensors:

• Good selectivity - can be made sensitive to a specific gas

• Normally very accurate to ppm levels

• Resistant to poisons

• Very linear response

• Excellent repeatability

Disadvantages of electrochemical gas sensors:

• Narrow temperature range

• Limited shelf life

• Subject to potential interfering gases

• Sensitive to temperature and humidity conditions

• Sensitive to EMF/RFI

• Slow start-up if depolarized

• Electrolyte contains a strong acid - may be dangerous if leakage occurs.

10.3 Metal Oxide Semiconductor (MOS)

Solid-state sensors, typically based on a tin oxide semiconductor, respond to gases by changing

resistance. Semiconductor sensors are commonly used for measuring H2S and combustible



hydrocarbons. Semiconductor sensors are one of the best sensors for H2S gas monitoring where

sensitivity to low concentrations (ppm range) is required. However, interferences caused by non-

specificity create problems for combustible hydrocarbon monitoring. The quality of sensors

reportedly varies widely from manufacturer to manufacturer with substantial variations in

performance often found from a single manufacturer.


MOS sensors are constructed by applying a semiconducting material to a non-conducting

substrate between two electrodes. The substrate is heated to a temperature such that the target gas

being monitored can cause a reversible change in the conductivity of the semiconducting

material. When the target gas is not present, oxygen atoms or other electron acceptors adsorb on

the surface of the semiconductor material and trap free electrons from the conduction band of the

semiconductor. This process inhibits electrical flow (i.e. increased resistance). As the target gas

is introduced, they react with the adsorbed oxygen atoms, which releases the trapped electrons to

the material thereby decreasing resistance. This change in resistance is measured electrically and

is proportional to the concentration of the gas being measured. To achieve some specificity, the

sensors can be impregnated with dopants or the working temperature can be changed so that the

sensor’s resistance changes when specific gases react with the adsorbed oxygen.

Figure 10. Example of a Metal Oxide Semiconductor (MOS) Sensor


MOS sensors have a relatively long operating life (3-5 years). They have a high sensitivity to

most combustible gases (i.e. saturated hydrocarbons), NO and CO. MOS sensors have a fast



response time, good reliability, good resistance to corrosive gases and humidity, high mechanical

strength, and low production cost.

However, MOS sensors have low selectivity and background gases can create inaccurate

readings. This problem may be improved somewhat by dopants and temperature adjustment

during the measurement. They also show poor distinction between different polar compounds.

Operating at high temperature, they consume more power than other similar sensors and the

sensor's nonlinear output signal makes calibration more complicated. Table 4 provides summary

characteristics for two common types of MOS.

Table 4. Comparison of MOS Sensors

Semiconductor Type Advantages Disadvantages

Sintered bulk semiconductorcomposed of tin dioxidedeposited on a ceramictubular former (CHC)

• Small• Mechanically

Rugged• ppm sensitive• Inexpensive

• Sensitive to Humidity• Sensitive to Temperature• Non specific to gases & vapors

Solid state thin film metaloxide semiconductordeposited on a ceramicsubstrate (H2S)

• Small• Somewhat rugged• ppm sensitive• Can be specific• Wide

TemperatureRange

• Needs linearizing output• Can saturate• Response may slow on aged sensor• Needs temperature controlled heater• Expensive• Sensitive to high vibration

10.4 Bulk Acoustic Wave (BAW) Sensors

BAW sensors are more commonly known as Quartz Microbalances (QMBs) or Quartz Crystal

Microbalances (QCMs) because they are mass sensitive. These types of sensors are commonly

used in sensor array systems especially those considered to be "chemical noses." The sensors are

typically made from thin discs of quartz, lithium niobate, lithium tantalite, or related compounds

and then coated with different polymers similar to those used in gas chromatographic columns.

These polymer coatings give the sensor its selectivity and sensitivity towards certain analytes so

the polymer is chosen according to its strength of interaction with the analyte. A large variety of

available polymers allows the tailoring of the sensor to meet the needs of the application.


When an alternating voltage is applied at a constant temperature, the crystal substrate vibrates at a

very stable and measurable frequency. Upon exposure to volatile compounds, the volatiles



absorb into the polymer coating of the sensor, which causes a change in the mass of the sensor.

The change in mass creates a measurable change in the oscillation frequency of the sensor; the

frequency change is linear with the additional mass loading in the polymer matrix. QMB sensors

have found a niche in the electronic nose market because they produce stable responses and are

easily manufactured.

QMB sensors must be operated in a temperature controlled environment because the signal is

temperature dependent. If the temperature is too low, absorption/desorption is slowed. On the

other hand, high temperatures increase desorption causing a decrease in the amount of absorbed

molecules.


The operation principle restricts the use of QMB sensors to the detection of gaseous analytes

although they may have sensitivities as low as 10 ppm.

Advantages include:

• High stability of the signal

• Non destructive analysis

• Operation at room temperature

• Sensor reproducibility in production

• Resistant to moisture

Disadvantages include:

• Large size and high cost of a sensor

• QMBs require higher concentrations of sample gas relative to MOSFET or MOS sensors.

10.5 Surface Acoustic Wave (SAW) Sensors

SAW sensors are based on a similar principle as the QMB sensors described above - a change in

mass is registered as a frequency change. This device uses, as referred by the name, surface

acoustic waves, which have a frequency as high as 600 MHz.




A SAW device consists of two transducers of thin metal electrodes on a polished piezoelectric

substrate. The spacing of the transducers determines the wavelength. An alternating current is

applied to one of the transducers causing the surface to expand and contract. This motion

generates surface waves, which propagate across the substrate and are received at the other

transducer. An uncoated SAW sensor may also be used as a reference signal to minimize noise

and temperature effects.

The wavelength/frequency of the surface wave is influenced by the physical properties of the

surface. The bare substrate between the transducers may be coated with a thin layer of polymer

or the transducer may be covered by a polymer layer. In some cases, the substrate itself can be

used as sensitive layer. Gas absorption changes the mass and/or the physical properties of the

sensitive layer, which disturbs the propagation of the surface waves. SAW sensors are not only

mass sensitive, they are also affected by changes in the dielectric constant and viscosity of the

polymer phase, temperature, and pressure.


SAW devices are coated with much thinner sensitive layers than QMB sensors. This reduces

analyte absorption because mass uptake takes place into the bulk of the polymer layer. However,

unlike QMB sensors, the SAW signal is dependent on a number of other factors, which can be

used to get additional information about the polymer/analyte absorption process.

10.6 Metal Oxide Field-Effect Transistor (MOSFET)

Gas sensitive devices based on field effect transistors are commonly GASFETs (Gallium

Arsenide Field Effect Transistors). In general, the MOSFET sensor consists of a thin, catalytic

metal layer on top of a metal-oxide-semiconductor field-effect transistor. The type of catalytic

metal (palladium for hydrogen or iridium/platinum for NH3, H2S, and ethanol) and the layer

thickness can be varied in order to create different types of MOSFET sensors. Typical devices all

share a common structure: a gate on top of an insulating layer (SiO2), a p-doped silicon channel

and an n-doped source and drain. The catalytic metal forms the gate electrode that is electrically

connected to the drain electrode and the source electrode is grounded. The whole structure is

heated with a resistive heater. The specific name of the sensor in the literature denotes the gate



material and/or the set-up. Systems based on MOSFETs usually include multiple sensors where

each is made sensitive to different groups of chemical compounds.


In MOSFET sensors, molecules in the gas phase react at the catalytic surface. Intermediate

products and reaction products may polarize and adsorb onto the metal surface or onto the

insulator surface. Hydrogen, formed in the reaction at the surface, diffuses through the catalytic

metal and forms dipoles at the metal-insulator interface. The polarized species at the insulator

surface and polarized hydrogen at the metal-insulator interface remain in equilibrium with the

concentration in the gas phase. A dipole layer is formed which adds to the electric field between

the metal and semiconductor. This changes the voltage between the gate and the source electrode

and modulates the drain-source current through capacitive coupling to the channel.

Selectivity to different gases is achieved by changing the operating temperature (catalytic

dehydrogenation is temperature dependent), changing the metal on the catalytic surface, and

changing the thickness and morphology of the gate metal. Thus, MOSFET sensors can be made

sensitive to a broad range of hydrogen-containing or polar compounds. They are generally stable,

exhibit a relatively low sensitivity to moisture, and can be produced with high reproducibility.

10.7 Conducting Organic Polymers (COP)

Sensors based on changes in resistivity of polymer films to gas exposure have been studied

extensively. The most studied polymers are that of polypyrrole and polyaniline. In general, all

suitable polymers have a conjugated π–electron system along the polymer backbone. The choice

of the polymer is limited to conducting polymers or those which can be made conduct. Polymers

are made conductive by doping them with counter ions from an electrolyte solution by reducing

or oxidizing the polymer. The polymers are formally described as semiconductors because they

have a band gap in their electronic structure at room temperature. Conducting polymers are

fabricated by depositing a thin polymer film on a sensor structure consisting of a narrow electrode

gap. They are sensitive to polar compounds and the bulk absorption is normally reversible.


The conductivity of the polymer is measured at a constant current or voltage over a resistor. As

the target gases are absorbed into the polymer matrix, their interaction with the matrix causes a



change in the conductivity of the polymer. Unfortunately, the response of the sensor is not

necessarily a linear function of the target gas concentration. Therefore, quantification is difficult.

As with other sensors of this nature, the responses are typically measured as the differential

resistance (R2-R1)/R1 where R1 is the baseline resistance in clean, dry air and R2 is the resistance

after exposure to the target gas.


The sensitivity and selectivity of a conducting polymer array can be altered extensively by

manipulating the chemistry of the polymer backbone, changing the selection of doping ions,

varying the polymer chain length, and changing the method of polymerization. COP sensors can

be used at ambient temperature unlike many of the gas monitoring sensors. However, they are

also sensitive to temperature changes. Their primary advantages are high sensitivity and small

size.

Their primary disadvantage is poor reproducibility of fabrication, which is dependent on polymer

coating of the substrate. Water vapor is a strong interferent, and the baseline may drift over time

as a result of conformation changes within the polymer after exposure to inappropriate

compounds.

10.8 Chemoresistors

Chemoresistors (or chemiresistors), consist of thin, typically polymeric, films deposited across

two electrical leads on an alumina or silicon substrate (Figure 11). When the film is exposed to a

gas phase analyte, the polymer matrix swells as it absorbs the analyte. The increase in volume

causes an increase in resistance because the conductive pathways through the material are

disrupted. When the gas is removed the polymer releases the analyte and returns to its original

size, restoring the conductive pathways. A sensor system usually consists of many individual

sensors each with a chemically unique polymer that responds to different types of chemicals

based on size, polarity, functionality, etc. As a result, the sensor array will respond differently to

different analyte gases, even those with similar chemical structures. The output of the sensor

system is a "fingerprint" or pattern consisting of each individual sensor output. In this

configuration, these sensors are not typically used for quantitative purposes. Instead, they are



used for discriminatory purposes (i.e. identifying a sample from a small set of possibilities.) and

the instrument is usually calibrated to recognize and distinguish certain classes of compounds.

Example: metaloxides, organiccrystals, conductingpolymers

Figure 11. General Schematic for a Chemoresistor

10.9 Spectroscopic

The regions of the electromagnetic spectrum that are most often utilized are ultraviolet/visible

(UV/VIS), near-infrared (NIR), and mid-infrared (MIR). By far, the largest number of analytical

techniques for analysis of organic compounds is based on infrared analysis. In the NIR and MIR

regions one will find a large number of instrumental techniques used to extract useful

information. For all practical purposes, most spectroscopic techniques that use UV/VIS or

infrared light are based on absorption spectroscopy, i.e. the absorption of a particular wavelength

of light. The specific mode of absorption is dependent on the region of the electromagnetic

spectrum being utilized. Ultimately, the choice of spectroscopic technique is based on the

composition of the sample and the chemical nature of the species to be measured. Each of these

regions will lend themselves to a particular chemical species based on their chemical

functionality giving them an advantage over the other techniques.

UV spectroscopy may find little application in LPG analysis. Ultraviolet detectors are sometimes

employed as detectors in gas and liquid chromatography systems. For practical purposes, the

ultraviolet region is that between 200 and 380 nm. Energy absorbed in the UV region produces

changes in the electronic structure of the molecule as a result of transitions of valence electrons to



higher energy orbitals. This generally limits its usefulness to compounds containing conjugated

double bonds or those containing aromatic rings (such as benzene).

Outside of the laboratory, most spectroscopic techniques for gas monitoring are based on infrared

(both near and mid-band). Furthermore, the measurement is typically made from a remote or

standoff position. For instance, a common technique is to measure the vapors being emitted from

a smokestack for the purpose of environmental monitoring. This is done from a distant location

by using an infrared telescope or similar device, which never comes in direct contact with the gas.

The wavelength of light absorbed by the gas is measured and used to determine the composition

of the gas. Similar to catalytic sensors, small infrared devices can also be used for area

monitoring. Their advantage is that they are not subject to poisoning and interferences can be

eliminated through wavelength selection.

Infrared technology is well suited for process monitoring, quality assurance, or sample

identification. Mathematical correlations between infrared spectral data and bulk fuel properties

have already been shown to provide reasonable estimates. Indeed, many refineries utilize this

technology to track their processes. The correlations use calibration data based on classical

laboratory procedures (e.g. ASTM), and although the degrees of accuracy will vary, many of the

properties fall well within the reproducibility limits stated in the laboratory procedures. This

means that these technologies could generate results that correlate to the classical methods as well

as two independent laboratories running the same procedures on the same sample would correlate

with each other.

Advances in manufacturing and computing have created powerful analytical instruments capable

of both qualitative and quantitative analyses of multi-component samples in seconds. Currently,

versions of these instruments are available for benchtop, on-line, and semi-portable applications.

The future for these technologies will see additional miniaturization and increasing sensitivity.

As a result, powerful hand-held instruments will ultimately emerge for various applications.

Such an instrument for LPG analysis in the field could conceivably be developed now and

refinements made as the technology advances.

The following sections briefly describe the two most common forms of spectroscopy, near- and

mid-infrared and the advantages inherent to each one. A discussion of laser spectroscopy also

follows.



10.9.1 Spectroscopic Instrument Technologies

In general, infrared instruments can be grouped into one of two broad categories: dispersive and

Fourier transform. A brief description of each is provided below.

Dispersive

Spectroscopic results are typically presented in the form of a two-dimensional plot of intensity at

each individual frequency. In order to do this, the instrument must separate the different

wavelengths of radiation either before or after passing through the sample. A dispersive

instrument accomplishes this through the use of a prism, grating, or filter. There are many forms

of each, and the design is usually tailored for a specific purpose, e.g., the wavelength range of

interest. Dispersive-type instruments are considered to be an older technology compared to

Fourier transform instruments. However, they still serve an important role in some applications

where ruggedness and size are a factor. For the near-term, hand-held and field-portable fuel

quality instruments will likely incorporate some form of a dispersive instrument.

Fourier Transform (FT)

Fourier transform instrumentation was originally developed to overcome some of the limitations

with dispersive instruments, e.g., scanning speed. In order to measure all of the infrared

frequencies simultaneously a device called an interferometer was developed. The interferometer

produces a signal called an interferogram in which every data point contains information about

every frequency emitted by the radiation source. This signal can be measured very quickly,

usually on the order of one second. The measured interferogram signal can not be interpreted

directly and must be decoded using a well-known mathematical technique called the Fourier

transformation. The result is a plot of the intensity at each individual frequency.

Fourier transform instruments are the preferred choice of instrumentation for research and testing

because of their speed, sensitivity, low noise levels, and internal wavelength calibration. For field

use, special precautions would need to be taken with FT instruments because they incorporate a

moving mirror that is vibration sensitive.



10.9.2 Mid-Infrared Spectroscopy


The principle behind infrared spectroscopy is absorption of infrared radiation. Infrared radiation

is passed through the sample where some is absorbed and some passes through unchanged. The

resulting decrease in intensity at each wavelength is recorded and used to create an infrared

spectrum.

The most utilized portion of the mid-infrared region is between 4000-400 cm-1 (2.5 – 25 µm).

The infrared radiation in this region is absorbed causing vibrational and rotational changes to

organic molecules. The frequency of absorption is related to the functional groups that are

present in the molecule and the environment in which they exist. Functional group absorption

frequencies are well documented. An infrared spectrum represents a fingerprint of a sample with

absorption peaks corresponding to the fundamental vibrational frequencies between the atoms

that make up a chemical compound. The absorption peaks are usually sharp and well defined.

Therefore, infrared spectroscopy can be used qualitatively to identify different types of materials.

Practically every compound is a unique combination of atoms, so no two compounds produce the

exact same infrared spectrum. The same is true for complex mixtures of many compounds such

as fuels. In addition, the size of the absorption peaks in the spectrum is directly correlated to the

amount of material present. Therefore, with proper calibration, software algorithms can provide a

quantitative analysis for individual components or bulk properties.

Mid-Infrared Instrumentation

Most mid-infrared instruments are bulky, benchtop units designed for research and testing

purposes. Essentially, all new instruments are interferometer-based and commonly referred to as

FT-IR (Fourier Transform Infrared) spectrometers. In recent years, some companies have

developed smaller units that are considered semi-portable, but none would classify as hand-held.

While these instruments probably provide the most information about a material, the hardware

they employ will likely limit their usefulness to on-line fuel monitors for the time being. The

methods for introducing the sample to the instrument are limited and this presents a minor

disadvantage for mid-infrared spectroscopy. Techniques for sample introduction include the use

of fiber-optic probes, flow-through cells, and horizontal attenuated total reflectance (ATR) cells.



Advantages/Disadvantages

Several of the disadvantages inherent to mid-infrared spectroscopy have already been noted. Its

bulkier equipment and limited modes of sample introduction restrict its use to applications that

are not hand-held. Because water absorption (humidity in the air) is a major problem for mid-

infrared spectroscopy, the instrument and optical path leading to the sample must be purged with

dry air or nitrogen. Furthermore, its strong dependence on the use of FT instrumentation requires

special precautions to be taken when handling the instrument; however, careful packaging of the

instrument can overcome these problems and make it just as usable as any other piece of

electronic equipment. Despite these disadvantages, mid-infrared spectra may provide the most

information about a sample. Sample preparation is normally not required. Its sharp peaks and

high signal to noise ratio provide a fingerprint of the chemical makeup of the sample. This

fingerprint contains responses from nearly every type of functional group present in the sample

and can be used for quantifying chemical and physical fuel properties. Complex mixtures are

more difficult to analyze because all of the responses from all of the sample constituents overlap.

10.9.3 Near-Infrared Spectroscopy


The principle behind near-infrared spectroscopy is the same as that for mid-infrared spectroscopy

– absorption of infrared radiation by a sample. The fundamental differences are the wavelength

region and the mechanics of absorption. The most utilized portion of the near-infrared region is

between 0.9 and 2.5 µm (10,000 - 4,000 cm-1). For organic molecules, near-infrared spectra are

the result of combination and overtone bands of the fundamental vibrational frequencies seen in

the mid-infrared region. Overtone bands appear at integer multiples (approximately) of the

fundamental vibrational frequencies, and each subsequent overtone is dramatically weaker in

intensity. The first overtone for a fundamental vibrational frequency in the mid-infrared region

appears at twice the wavenumber (or one half of the wavelength), the second overtone appears at

three times the wavenumber (or one third of the wavelength), etc. For this reason, near-infrared

spectra are typically comprised of first and second overtones of C-H, N-H, and O-H vibrational

modes. This makes near-infrared spectroscopy highly suitable for use with hydrocarbon fuels.

The relatively smooth features resulting from overlapping and collinear spectral responses make

the spectra nearly useless for qualitative spectral interpretation. However, with the increasing

power of desktop computers and emerging mathematical treatments, the correlation of chemical



and physical properties to small spectral differences can now be made. As a result, near-infrared

spectrometers can be used to rapidly identify a sample and estimate a variety of its properties.

Near-Infrared Instrumentation

Near-infrared instruments can be purchased in several forms including filter-based and Fourier

transform. Many research-grade instruments allow a user to easily interchange between mid-

infrared and near-infrared modes. For field use, a grating or filter-based instrument would be

appropriate for either a handheld or an on-line monitor. Similar to mid-infrared spectroscopy, the

methods of introducing the sample to the instrument are limited. Fiber-optic probes are

commonly used for both on-line and benchtop applications.


Compared to mid-infrared, near-infrared spectroscopy represents the other extreme for this form

of analysis. The hardware commonly used for near-infrared instrumentation is more conducive to

being miniaturized and used in hand-held applications. The fibers-optic cables used in near-

infrared spectroscopy are common low-hydroxyl silica core fibers. The use of mid-infrared via

fiber optics is much less common owing to the highly specialized fibers that are required. Near-

infrared spectra are not as susceptible to water absorption so purging is not normally required.

However, despite these advantages, near-infrared spectroscopy has several drawbacks.

Wavelength stabilization can be a problem and may require frequent calibration. Furthermore,

the information contained in its spectra is far less detailed than mid-infrared spectra and its ability

to quantify chemical and physical properties will therefore be reduced.

10.9.4 Laser Spectroscopy

Laser-based spectroscopic techniques in the infrared region are gaining ground in applications

that require highly sensitive, trace gas analysis. While FTIR is ideally suited for multicomponent

analysis and spectral analysis of unknown gases, laser spectroscopy is the preferred method for

trace gas analysis because of its high spectral resolution. Whereas NIR and MIR systems

normally operate over an entire spectrum (100s of wavelengths), laser-based systems operate over

an isolated absorption line (e.g. 1531.7 nm). The selection of a suitable absorption line is not

trivial and is dependent on the availability of a laser with the appropriate output and interference

by other gas species. Water and carbon dioxide are usually the biggest problems because they

show absorption through the infrared. Databases of spectral lines have been compiled - the most



popular of which is HITRAN. HITRAN includes information on over one million spectral

absorption lines; however it's use is primarily limited to atmospheric gases such as CO, CO2,

H2O, NH3, etc. Of those listed in the HITRAN database, the only ones that appear applicable to

LPG analysis are ammonia and water. Indeed, several examples in the literature used laser-based

systems specifically for ammonia monitoring.

Laser Spectroscopy Instrumentation

A typical laser-based gas measurement system might consist of the following components:

• a diode laser (fixed wavelength or tunable)

• a multipass cell (fiber coupled to the laser)

• a laser driver

• a temperature controller

• a dual-beam detector (one detector for reference)

• a computer for data acquisition

A multipass cell increases the optical pathlength of the light allowing it to stay in contact with the

sample for a longer time thereby increasing it sensitivity.


Diode laser spectroscopy has many advantages over other multiple wavelength techniques. It

allows highly sensitive measurements often in the sub-ppm range. Because absorption at a single

wavelength is measured, it is highly specific. The latest advancements in tunable diode lasers

permit such a system to be highly portable. Like other gas sensors, a single sensor is typically

dedicated to a single analyte. However, if several analytes have absorptions that fall near one

another, a tunable laser may have enough range to measure them both.



10.10 Dielectric Measurement

Gas and liquid densities can be inferred by measuring their dielectric constant. For example, gas

mixture density, ρmix, is related to the dielectric constant of the mixture through the Clausius-

Mossotti equation, or

ρmix = [(∑ xi Mwi)/ (∑ xI C-Mi)] • [(ε- 1)/ (ε+2)]

where

ε = dielectric constant

xi = mole fraction for each i component

Mwi = molecular weight of each i component

C-Mi = Clausius-Mossotti function of each i component

Knowledge of the component mole fractions and their associated Clausius-Mossotti functions are

required to calculate the density based on a dielectric measurement. For hydrocarbon gas

mixtures, MwI / C-MI is nearly constant which implies that gas density is directly proportional to

dielectric constant.

The density-dielectric relationship may be employed in several ways. In one embodiment of the

dielectric application, differential or comparative measurements of a base gas and the same base

gas with a contaminant, for example H2S, can be detected by dielectric changes. A sensitivity of

one part in 1000 (0.1%) is easily achieved and sensitivities of one part in 10,000 or more can be

achieved with more complex electronics. While this discussion is in the context of gas phase

samples, the same philosophy works well with liquids also.

The sensor system is generally composed of a coaxial capacitor that is filled with a gas or liquid

that is to be sampled (any type of liquid, like water, that provides an electrical short across the

plates of the capacitor, can be a problem and must be considered). Very small changes in

dielectric constant may be determined by configuring the sample cell into a single active arm AC

bridge network (Figure 12). Excitation (3 to 5 kHz depending upon cell design) and

demodulation of the bridge circuit is normally accomplished by an AC bridge amplifier.



Flow of Liquidor Gas

Sample Cell

Oscillator(5 kHz)

Demodulator andAmplifier Circuit

Figure 12. AC Bridge Network for Dielectric Sample Cell

10.11 Gas Chromatography

Gas chromatography (GC) is a common technique used throughout industry. Because of its

sensitivity, GCs are widely used in laboratories and refineries to analyze both gas and liquid

samples. GC is a method for separating, identifying, and quantifying the chemical components of

a sample. Analysis times vary from minutes to hours depending on the sample and very little

sample (micrograms) is usually required. In gas chromatography, a sample is injected into the

instrument where it is rapidly heated, vaporized (if necessary), and mixed with inert carrier gas

(e.g. helium). The carrier gas then transports the sample through a heated chromatographic

column. Sample components are separated based on their boiling points, size, and relative

affinity for the stationary phase in the column. The larger the molecule or the more affinity it has

for the stationary phase, the slower it travels through the column. The separated (a.k.a speciated)

components are then detected and represented as peaks in the form of a chromatogram (retention

time vs. quantity). There are a variety of GC detectors including photoionization and flame

ionization. GCs are often coupled with other instruments, such as mass spectrometers, in a

tandem configuration to yield even more definitive information about the chemical species

eluting from the column.


Combined with the proper column and detection method, gas chromatographs are essentially the

"gold standard" method when it comes to gas analysis (and liquids). Their sensitivity and their

ability to speciate samples into their individual components make them the preferred choice for

quality assurance. Their primary disadvantage is their size and cost. GCs are generally

considered to be laboratory-grade instruments. "Portable" GCs are available, for HAZMAT use



for example. However, they are only portable in the sense that they have been ruggedized to

withstand outdoor use and run on battery power. Their size is still roughly proportional to a small

benchtop laboratory instrument. And portable or not, GCs still require carrier gas. In portable

versions, a smaller cylinder simply replaces the large laboratory cylinder.

10.12 Ionization Detectors

Photoionization Detectors (PIDs) and Flame Ionization Detectors (FIDs) are instruments that are

typically used to detect low concentrations of volatile organic compounds (VOC). Although both

are designed to provide ppm level measurements, their operating principles and strengths are

different. FIDs and PIDs are commonly used as detectors in gas chromatographs. Because of

their size and overall complexity, FIDs are generally limited to non-portable applications. PIDs

are generally smaller and simpler to use and are preferred in portable applications (Figure 13).

Instruments that utilize PIDs are commonly used in handheld devices to detect the presence of

explosive or toxic gases.

Figure 13. Example of a Handheld PID Monitor Capable ofDetecting VOCs in the Parts-Per-Billion Range

The reason for their wide spread use is that they can achieve selective and/or highly sensitive

detection of specific analytes. However, FIDs and PIDs selectivity is somewhat different. In

general, PIDs respond to functional groups while FIDs respond to chain length. For instance, a

FID will give a similar response to propane, acetone, and isopropanol because all are three carbon

compounds. PID sensitivity to these three compounds will vary as follows: acetone >

isopropanol >> propane. PIDs are particularly useful in the selective determination of aromatic

hydrocarbons or heteroatom containing species.




PIDs use an ultraviolet light as a means of ionizing a gas phase analyte. The molecular ions that

are produced are collected by electrodes, which generate a measurable current. This current is

directly proportional to the analyte concentration. The general form for the photoionization

reaction is

R + hν → R+ + e-

The reason for the high selectivity is because the

only detectable molecules are those in which the

photon absorption is quantum mechanically

"allowed." If the energy of an incoming photon

is high enough then photoexcitation can occur to

the extent that an electron is ejected from its

molecular orbital and the molecule becomes

ionized. When the amount of ionization is

reproducible for a given compound, vapor

pressure, and light source then the current

collected at the PID's electrodes is also reproducible and is proportional to the analyte

concentration. Aromatic hydrocarbons or heteroatom containing compounds have ionization

potentials that are within the output range of commercially available UV lamps. Lamp energies

typically vary from 8.3 to 11.7 ev (electron volts) and wavelengths typically vary from 150 nm to

Increasing PID Sensitivity

aromatics

olefins, ketones, esters, amines,sulfur compounds

esters, aldehydes, alcohols, nC2+aliphatics, chlorinatedhydrocarbons

methane (no response)

Increasing FID Sensitivity

aromatics, long chain compounds

short-chain compounds (methane)

Cl, Br, I compounds

Examples of Ionization Potentials (ev)

CH3SSCH3 8.46CH3CH2SH 9.285CH3COCH3 9.69Elemental Sulfur ~9.5CH3CH=CH2 9.73CH3CH2CH2CH2OH 10.04NH3 10.2H2S 10.4CH3OH 10.84C3H8 11.1COS 11.17-NH2 11.4O2 12.063N2 15.576



106 nm. Although most PIDs have only one lamp, lamps in the PID can be exchanged depending

on the required analyte selectivity. Common lamp energies include 9.8, 10.6, and 11.7 ev.

An example of selective analyte detection using a PID is the co-elution of benzene and

isopropanol from a gas chromatograph. Benzene and isopropanol have similar boiling points,

80.1°C and 82.5°C respectively, and the two compounds might elute close together in a gas

chromatogram. However, benzene's IP is 9.24 ev while that of isopropanol is 10.15 ev. Using a

low energy lamp (e.g. 9.8 ev), the isopropanol would give a very poor response in the PID thus

allowing for selective detection of the benzene.

FIDs achieve the same end result by means of a hydrogen-air flame to ionize the sample.

Ionization occurs during the combustion process as the VOCs enter the flame. The need for a

hydrogen cylinder and the apparatus to handle the combustion process make FIDs much more

complex than PIDs not to mention the potential safety hazard of an incendiary source in a

propane environment. Table 5 provides a comparison of features for PIDs and FIDs.

Table 5. PID/FID ComparisonParameter PID FID

Size small, portable larger, requires hydrogensource

Linearity Best at low concentration good linearity throughoutrange

Range 5 ppb to 10,000 ppm 1-50,000 ppm

Compounds Mostly volatile organic compounds.A few inorganics

Selectivity selectivity based on lampselection broad sensitivity

Reliability Reliable, long lamp life flame-out issueshydrogen cylinder replacement

Safety generally safe requires flame arrestorCost PID << FID

In general, detection and analysis by ionization methods may be acceptable in an environment

that is normally free of VOCs, hence its use as an area monitoring system. However, without the

benefit of a speciation technique like gas chromatography, quantification of complex mixtures

like LPG would be difficult to analyze because many of the compounds have similar IPs. Since

propane's IP is relatively high, it would be essentially invisible, even to a moderately high-energy

lamp. However, many of the potential contaminants found in LPG have IPs that are so close

together that they may not be separable. Even worse, ethyl mercaptan, the common LPG odorant

has a low IP.



10.13 Gas Detection Tubes


Gas detector tubes are thin glass tubes that contain detection reagent(s) that are sensitive to

specific target compounds. The reagent(s) produce a distinct layer of color change when exposed

to the target compound. Calibration scales are printed on the tubes which indicate the

concentration of the substance being measured. To provide long-term stability (shelf life up to 3

years), the tubes are hermetically sealed to protect the reagents. Hundreds of variations of

detector tubes currently exist to measure a wide variety of chemical compounds. An example of

a detector tube specification sheet (sold by Sensidyne) for ammonia is shown in Figure 14. There

are actrually several tubes for ammonia depending on the concentration range. Some measure as

low as 1 ppm while others measure as high as 30%.

Figure 14. Example Detctor Tube Specification for Ammonia from Sensidyne



10.13.1 Gas Sampling Methods

Gas sensors and detection systems may utilize several different methods to draw gas through the

detector tubes. The most common are described below.

Vacuum method

The sample is drawn into the detector tube by manually operating a vacuum pump. This

is probably the most widely used technique (Figure 15).

Injection method

The sample is first drawn into a syringe before being injected into the detector tube.

Motor-driven pump method

The sample is drawn through the detector tubes by a motor-driven pump at a prescribed

rate for a prescribed time.

Diffusion method

The sample is not drawn but is allowed to diffuse slowly into the detector tubes.

Figure 15. Example of a Manual Detctor Tube Pump



10.13.2 Reaction Principles

Detector tubes generally react to chemical compounds in one of the following ways:

1. Sample reacts directly with a detecting reagent.

2. Sample reacts directly with several detecting reagents.

3. Sample reacts in a two-step reaction. The sample is first oxidized in a pretreatment

layer before reacting with the detecting reagent.

Substances that are chemically similar to the target compound may also react in the tube affecting

the results. These substances are called interferents and their effects may vary on the detector

tube type.

Interferences in direct reaction type detector tubes

1. The detector tube reagent(s) will also react with the interferents, giving a higher

indication. An example of such interferent is hydrogen sulfide in an ethyl mercaptan

detector tube. Common interferences and their effects are usually documented on the

specification sheet for the detector tube

2. If the detector tube contains a pH indicator then acids and bases will react as

interferents giving a higher indication.

Interferences in compound reaction type detector tubes

If a substance generated by the primary reaction(s) is the same as the target compound

then a higher indication will be given.

Interference in two-step reaction type detector tubes

If interferents consume the pretreatment oxidizer then its ability to oxidize the target

compound will be inhibited resulting in a lower indication.



10.13.3 Temperature Effects

Most detector tubes are either based on chemical reactions or physical adsorption, both of which

can be greatly affected by tube temperature. These effects are described below.

Influences on reaction rate

Chemical reaction rates are generally proportional to temperature. Below 20°C (68°F),

reactions will slow down and the sample will not completely react in the manner desired.

Some of the sample will diffuse further into the tube and react there creating a long pale

color change giving a higher indication. Above 20°C (68°F), reactions will accelerate

causing the sample to completely react in a shorter distance than the normal. This gives a

shorter layer of color change resulting in a lower indication.

Influences on physical adsorption

Chemical adsorption is inversely proportional to temperature. Below the prescribed

temperature, some of the sample will physically adsorb onto the detector tube reagent that

has already reacted with previous sample. Thus the subsequent sample will never reach

fresh reagent and react in the normal manner giving a lower than expected indication. At

higher temperature, adsorption/desorption will cause the sample to diffuse further into the

tube giving a higher than expected indication.

10.13.4 Correcting Tube Results

Detector tubes are generally resistant to minor fluctuations in temperature, pressure, and

humidity. When conditions are outside of predetermined limits the detector tube specification

sheet will normally provide instructions for correcting the results. Cases that may involve the

need to correct the indicated measurement are as follows.

Correction for temperature

Detector tubes are generally designed to be used at ambient temperatures 0-40°C (32-

104°F) and are calibrated based on a tube temperature (not sample temperature) of 20°C

(68°F). However, some tubes are more sensitive to temperature than others and may give

erroneous results at temperatures other than 20°C. For these tubes, a chart is generally

provided to correct the readings.



Correction for humidity

Most detector tubes are calibrated based on a specific relative humidity (e.g. 50%) and

their indications are not affected when the humidity is in the range of 0 to 99%.

Correction for atmospheric pressure

Since gas concentration is proportional to pressure, gas detector tubes are calibrated at

normal atmospheric pressure (760 mmHg). Their indications are not usually affected

within ±10% of this pressure. At pressures outside of this range, mathematical

corrections are applied to negate the effect.

10.13.5 Storage of Gas Detector Tubes

Detector tubes contain sensitive reagents and storage in a dark, cool (0-10°C (32-50°F)) place is

recommended.

10.14 Coriolis Measurements

The Coriolis principal states that if a particle inside a rotating body moves in a direction toward

or away from the center of rotation, the particle generates internal forces that act on the body.

Coriolis mass flow meters create a rotating motion by vibrating a tube or tubes carrying the fluid,

and the internal force that results is proportional to the mass flow rate.

In a Coriolis mass flow meter, an angular momentum is imparted to a tube conveying a fluid.

This angular momentum is delivered by harmonic vibration of the tube. As a result, forces

proportional to the product of fluid density and velocity act through the fluid and generate forces

on the tubing wall, producing a measurable effect. Also, mass measurement is inherently very

practical because one does not need to deal with temperature and pressure compensations. There

are several physical configurations of the tubes, for example U-shaped and omega-shaped. While

the details of their design are not particularly important at this point, the coriolis meter is unique

in several ways:

• It can directly measure the mass of a flowing stream.• It can measure the flowing density of the stream.• It can determine the volume of the stream.• It does not have rotating mechanical parts (although there is one vibrating component).



In the context of using Coriolis meters for the LPG custody transfer operations, this meter can

provide accurate flow rate information and it can provide some indicator of changes in fuel

composition caused by mixture changes or introduction of contaminants. Unfortunately, one does

not obtain any readout from the instrument to indicate what specific component of the mixture

caused the density change. Density accuracy is better than ± 0.001 g/mL for most applications

with repeatability of the order ± 0.0005 g/mL. This implies that one needs a change of mass

composition in the liquid phase of the order ± 0.1% to be detected. This may be a borderline

situation for employing the coriolis meter as a device to detect LPG contaminants.

10.15 Interface Detection

Another possibility for the detection of bulk contamination in large storage tanks may be to detect

the interface between the liquid hydrocarbon and the tank bottoms. Since the tank bottoms are

primarily going to consist of water, water-soluble material, and sediments that have collected

over the years, a distinct interface should be detectable. However, detection of a small amount of

tank bottoms is extremely challenging. The depth of a heavy, liquid or insoluble material

representing one percent by volume of a three foot six inch diameter tank would be

approximately 1.4 inches (Figure 16). As seen in the figure, installation of depth measuring

equipment on either the inside or outside (especially with submerged tanks) requires special

consideration.

d

Dia

Depth at Center-Bottom of Cylindrical Tank vs % Full

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6

Depth of liquid (inches)

% o

f Ful

l Cro

ss S

ectio

n

3.5' Dia Tank6' Dia Tank

Figure 16. Depth of Bottom Sludge as a Percent of Full Tank Cross Section



To investigate interface detection, we initially looked at commercially available tank gauging

systems. There are many different types of sensor technologies utilized in tank gauging systems.

The sensors may be initially categorized as either contact or non-contact with respect to the

substance being measured. The term invasive or non-invasive may also be used. Oftentimes, the

literature will use these terms interchangeably: non-contact = non-invasive and contact =

invasive. By our definition, sensors that are invasive are those that must penetrate the tank wall

in order to get access to the substance to be measured. By that definition, nearly all tank-gauging

systems herein are invasive. Invasive sensors are installed through an existing port in the tank or

one that is specifically created. However, an invasive sensor is not necessarily in contact with the

substance to be measured. Based on our understanding of the problem and the intended target for

these sensors (i.e. the retailer), we believe that the best "interface sensor" would be one that is

non-invasive and non-contact. Our reasoning for this is that existing tanks are unlikely to have a

suitable port in which a sensor could be installed.

Common tank level gauging systems that are relevant to this discussion are based on a few basic

measurement types:

• Ultrasonic

• Radar

• Capacitance

• Conductance

In their standard configuration, most of these sensors will be of little use in detecting the

LPG/water interface, primarily due to the need for tank penetration of the sensor. However, in

some cases the technology may be modified to make sensors that are non-intrusive. Another

technology, which may show promise, is a field effect sensor. This sensor detects interfaces,

which create disruptions in the electric field around the sensor.

10.15.1 Ultrasonic

Ultrasonic and sonic level instruments use sound waves to determine fluid level. The frequency

range for ultrasonic methods is ~20 to 200 kHz and sonic types use a frequency of 10 kHz. In

practice, these sensors are mounted in the top of a tank above the maximum fill point of the

liquid. A piezoelectric crystal inside the transducer converts electrical pulses into sound energy



that travels through the headspace of the tank in the form of a wave at a specific frequency and at

a known speed in the given medium (e.g. propane vapor or air). The sound waves are emitted in

bursts and received back at the transducer as echoes from the surface of the liquid. The

instrument measures the total time it takes for each sound pulse to traverse the gap between the

sensor and the liquid surface and return. This time is proportional to the distance from the

transducer to the surface and is used to determine the level of fluid in the tank. Alternatively, a

transmitter/receiver pair may be employed. Depending on a number of conditions, typical sensor

resolution accuracy will be on the order of ±10 mm.

Ultrasonic technology can also be used to detect the interface between immiscible liquids or the

presence of suspended solids. Two common techniques found in the literature are ultrasonic

attenuation (Figure 17) and reflection (Figure 18). Viscous liquids, emulsions, and liquids with

entrained solids usually have higher attenuation. If the attenuation difference between the liquids

is sufficient then the sensor gain can be adjusted to reflect off of the more attenuating liquid. The

primary disadvantage of this technique is that it is invasive and must be installed through the

sidewall of the tank.

Figure 17. Interface Detection by Attentuation



Figure 18. Interface Detection by Reflection

10.15.2 Radar

Comparable to ultrasonic measurements, radar (or microwave) is also a common technique for

tank level gauging. Radar uses electromagnetic waves, typically in the microwave X-band (5-26

GHz) range. For continuous level measurement, there are two main types of non-contact

systems: frequency-modulated continuous wave (FMCW) and pulsed radar or pulsed time-of-

flight (similar to the ultrasonic technique). Both of these techniques require an antenna to be

mounted inside the top of the tank (Figure 19). A variety of antennas are used depending on the

application. Under normal circumstances the antenna will not come into contact with the liquid.

A third common technique, guided wave or Time Domain Reflectometry (TDR), requires direct

contact with the liquid. However, of the three radar techniques this one claims to be able to

detect interfaces between liquids with different dielectric constants. Recall that a low dielectric

material, like propane (dielectric constant = 1.6), is non-conductive. Water on the other hand is

conductive and has a much higher dielectric constant (80). Dielectric measurements are

commonly used to find oil/water interfaces. In the guided wave technique, extremely short

microwave pulses travel along a cable or rod that extends to the bottom of the tank (Figure 20).



The radar waves reflect off the surface or interface of the liquid(s) and are detected back at the

sensor. The transit time of the signal is proportional to distance. In fact, tank-gauging systems

for low dielectric materials, such as liquid propane, often use guided wave sensors (or a horn and

stilling tube) to increase measurement accuracy. The radar measurement resolution should be

better than ultrasonic at ±5 mm.

Figure 19. Examples of Tank Radar Antennas

Figure 20. Guided Wave Radar

10.15.3 Capacitance

Sensors based on capacitance are commonly used for both point level and continuous

measurements. Electrical capacitance is the ability to store an electrical charge and exists



between two conductors separated by a short distance (Figure 21). Between the conductors is an

insulating medium, which in our application would be the non-conducting propane. The

conductors may be an electrode and the vessel wall, two plate electrodes, or two concentric

cylindrical capacitors.

Figure 21. Basic Capacitor

The capacitance is determined by the spacing, area of the conductors, and the electrical

characteristic (dielectric constant) of the insulating material. The lower the dielectric constant,

the less charge the system can build up. If the dielectric material (i.e. propane) is displaced by a

conducting medium, such as water, then a measurable change in capacitance will occur indicating

the presence of contamination.

10.15.4 Conductance

The conductance method of liquid level measurement is based on the electrical conductance of

the liquid. This technique is generally used for point level detection and the detected point can be

the interface between a conductive and nonconductive liquid (e.g. air/water or propane/water).

Conductance probes are usually employed as high/low switches to turn equipment on or off.

10.15.5 Field Effect

One example of a field effect sensor that we found is used to detect the presence of a material

with a dielectric constant above 2.0 (practically anything other than air). This sensor is offered by

Material Sciences Corporation (MSC) as a level switch (Figure 22). The sensor is composed of

two electrodes and an active device in close proximity. The active circuit provides an oscillating

electric field to the two electrodes creating an electric field around the cell. When the electric

field is interrupted, the active circuit senses the disruption and responds accordingly. If the

disruption affects both electrodes then there is no response. Sensitivity can be controlled by the



design of the electrodes, the gain, and the active circuit. Therefore, the cells can be designed to

meet an application's specific needs.

Figure 22. Field Effect Sensor

The major disadvantage of this particular sensor is that it must be attached to a substrate that is a

dielectric such as glass, plastic, or isolated (i.e. ungrounded) metal. This would obviously be a

problem for metal storage tanks. In the short time that we investigated this type of sensor, we

were unable to find a similar sensor that could get around this problem. However, a solution

most likely exists or could be engineered.

This sensor draws less than 16 µA of current in the inactive state. It also has a high signal-to-

noise ratio setting it apart from capacitive sensors and making it immune to low-level circuit

noise and environmental (radio frequency) noise. Field effect technology is inherently stable, and

should require minimal calibration, drift compensation, or digital filtering as other sensors

sometimes do.

10.16 Particle Counters

In the petroleum industry, a common application for particle counters is in the analysis of

hydraulic fluids - an estimated 80 percent of all hydraulic system failures are attributable to

particulate contamination in the fluid. In the pasts few years there has been a push (albeit a small

one) to augment/replace laboratory, filtration-based, particulate measurements for petroleum

products (diesel, aviation fuel, etc) with particle counting. This hasn't yet caught on because the

fuel specifications are still written in weight/volume requirements for particulate matter. Particle

counters return particle counts at various particle sizes and there isn't a clear conversion between

that and weight/volume. In addition, there isn't a good understanding (or agreement) on what

sizes of particles will be a problem. The same is true for LPG. Nevertheless, particle-counting



technology is well established and is routinely used in an on-line fashion and may be applicable

to particulate matter in LPG.

We will consider two basic types of liquid-borne-particle sensors: light scattering and light

blocking. Light-scattering sensors detect smaller particles than light-blocking sensors and are

more effective at counting light-colored particles. However, light-scattering sensors are more

expensive because they must contain collection optics for the scattered light. The basic

components of any particle counter include a light source (typically a laser), a photodetector, and

a sample cell. A typical, commercially available, particle counter will likely use a solid-state

infrared laser diode as a light source. These lasers have a life expectancy approaching 30,000

hours. In addition, the output of solid state laser diodes is stable and does not require frequent

calibration.

Particle sizes are measured in nanometers and micrometers (microns). Typical light-blocking

particle counters measure particle sizes in the micron range (e.g. 10 micron) while the more

sensitive, light-scattering technique is employed to measure sub-micron (100 nm) particle sizes.

A typical, on-line particle counter for liquid applications requires that the flow be restricted to

approximately 100mL/min. Sampling times can be as short as 1 minute.

10.16.1 Light-Scattering Particle Counters

Light-scattering sensors operate by detecting the amount of light reflected, refracted, or diffracted

by a particle. As a liquid sample enters the sensor, it passes through the optical cell where the

laser light is most intense. Particles in the sample scatter the light into off-axis collection optics

that focus the light onto a photodetector. The photodetector converts the light pulses into

electrical pulses where the magnitude of each pulse is proportional to the amount of scattered

light and thus particle size. The particle counter then sorts and counts the pulses according to

particle size.

10.16.2 Light-Blocking Particle Counters

Light-blocking sensors are more efficient at detecting opaque and dark-colored particles because

they operate by detecting how much light is blocked rather than scattered. As the liquid sample

flows through the cell, particles in the sample momentarily block the light. The amount of light



blocked is indicative of the particle size. As the particles pass through the light beam, a

photodetector detects the momentary decrease in light intensity and creates a corresponding

electrical pulse that is proportional to particle size. The pulses are counted and categorized by

size by the particle counter instrumentation.


In general, light-blocking sensors have many preferred features over light-scattering sensors:

• More effective at counting dark-colored particles

• Remain calibrated over a large range of flow rates

• Handle higher concentrations of particles (~20,000 counts/mL)

• Have a longer service life

• Simpler optical design

• Cost less

On the other hand, if light-colored or sub-micron particles are the target then light-scattering

sensors are preferential.

10.17 Summary Notes for Sensor Technologies

10.17.1 Gas Monitoring Systems

Notes on gas monitoring sensors:

• commonly used as area monitoring sensors but not in analytical applications

• sensors are usually very small and are used in handheld devices for portable personnelprotection

• some sensors are based on chemical reactions so they get used over time

• sensors have relatively short life span and finite shelf life

• detection limit is usually very high (ppm) and sensitivity is linear

• specificity is moderately good but interferents may co-respond with target analytes andthere is no way to tell them apart



10.17.2 Chemical Array Sensors

Notes on chemical arrays:

• some chemical arrays are small and handheld/portable

• sensors rely heavily on mathematical (multivariate analysis) techniques to extract and

model the response of the system

• potentially frequent re-calibration as the sensors age

• on average, detection limits are high ppm to %level concentrations - often dependant on

volatility and vapor pressure of compounds

• could probably increase specificity with some sensor and method development

• off-the-shelf sensors use large arrays (e.g. 32 sensors) of chemically different sensors and

they rely on calibration techniques for identification

• chemical array are not generally good in quantitative applications, especially when

measuring complex mixtures

• most common application is in discriminatory analysis

10.17.3 Spectroscopic Sensors

Notes on spectroscopic measurements:

• Calibration and maintenance may be difficult – light sources tend to drift over time

nullifying the current calibration

• Diode lasers are more stable than lamps and have long operating lives

• Could be made portable or even handheld.

• If analyte specific detection and quantification is desired, then a dedicated sensor

operating at a fixed wavelength would probably be needed for each contaminant.

• Multiple analyte detection requires multiple single-wavelength sensors, a tunable laser, or

a complex model based on multiple wavelength (broadband) spectra.

• Detection of bulk contamination through fingerprinting is plausible.

• Detection limits for broadband spectra are generally lower (% range) owing to

interferences and less sensitive instrumentation.

• Diode lasers have much higher sensitivity and can achieve the ppm level sensitivity that

is needed for this application.



10.17.4 Dielectric Measurement

Notes on dielectric measurements:

• Easy to use, simply fill chamber with gas or liquid

• Calibration with complex mixtures required

• Could be made portable but not likely handheld

• control circuit is simple but temperature control is required

• sensitive to changes in composition but must be calibrated for specific analytes

• Generally used for bulk contamination especially complex mixtures

10.17.5 Gas Chromatography

Notes on gas chromatography:

• generally considered to be the gold standard

• some method development may be required

• periodic calibration and frequent standardization required

• consumables - periodic column change and continuous use of carrier gas (hydrogen or

helium)

• handheld instrument not likely - portable systems exist but they're still large and bulky

• detection limit, sensitivity, and selectivity are very good

10.17.6 Photoionization Detector (PID)

Notes on PID:

• may be handheld - already exist as handheld gas monitoring sensors

• measurement is based on ionization potential (IP) which is sensitive and can be specific

to a variety of compounds depending on their IP

• detection limits are good - in the ppm range for most compounds

• propane should be invisible but most other contaminants will respond together, therefore

specificity in LPG is not good

• measurement of bulk contamination (i.e. compounds other than propane) without analyte

identification may be possible



10.17.7 Flame Ionization Detector (FID)

Notes on FID:

• comparable to PID

• instrumentation is bulky - requires a burner and a source of combustible gas like

hydrogen

• not considered portable

• detection limits are as good as PID and more linear at high concentrations

• specificity is generally poorer than PID because measurement is mostly sensitive to chain

length

10.17.8 Gas Detection Tubes

Notes on gas detection tubes:

• very simple to operate - uses a manual or automatic pump to draw air through tube,

therefore technology should be dependable

• tube selection process may require some standardization - a series of tubes in a particular

order to specifically identify a contaminant

• good for bulk contamination or analyte specific detection

• detection limits are tube dependent and there are a variety of tubes to choose from

• tube concentrations vary from ppm to %level

• minor temperature and pressure effects

• tube selection is critical because some tubes respond to multiple analytes; however, the

interferents usually respond with a different color so the interference is known.

10.17.9 Coriolis Measurement

Notes on Coriolis measurements:

• minor setup• periodic calibration• Available off-the-shelf but very expensive• may be vehicle mounted but vibration may be a problem• generally very reliable with few moving parts• not good for speciation because it is not selective - only makes bulk measurements• high repeatability/reproducibility• should detect slugs of water but not minor changes in low concentration contaminants



10.17.10 Particle Counters

Notes on particle counters:

• factory calibrated on an annual or semiannual basis

• the sensor is small (12 oz can size) but not generally used in a portable application

• may be truck mounted or at a fixed facility

• typically use laser diodes which are reliable and have a very long life

• can easily measure and quantify micron size particles

• can handle at least 16,000 counts/mL so it is sensitive yet difficult to saturate under

normal conditions

• linear output

• does not discriminate between solid materials

• intended for bulk particulate measurement

11.0 SENSOR TRADE-OFF STUDY

A sensor trade-off analysis was undertaken to identify those technologies that show promise for

incorporation into portable or handheld devices for detecting contaminants in LPG. The

technologies were assessed in terms of six major attributes: 1) low operation complexity, 2)

portability, 3) high reliability, 4) detection limit, 5) good sensitivity, and 6) high specificity.

When assigning ratings to each attribute, consideration was given to the ultimate end user - the

LPG retailer. While many of these technologies are certainly applicable to LPG analysis, a

suitable instrument package may not be currently available or ready for direct employment at the

retail level. In those cases, we considered only the technology and its suitability for the intended

purpose assuming that it could be designed into a suitable instrument package. The assigned

scale (or grade) for the attributes used in the trade-off study are described below. The rating

scales are set up to provide the highest number for the most desired characteristic.



Low Operational Complexity

Operational complexity embodies the overall difficulty of using the sensor/instrument.

This parameter encompasses several aspects of the use of the system including setup,

calibration, and general handling.

The rating scale for this parameter is

1 difficult

10 easy

Portability

The focus of this entire effort was to investigate portable or handheld sensors for LPG

analysis. While many applicable sensor technologies exist, some are simply not portable.

Therefore, the portability parameter distinguishes a sensor's potential to be portable.

The rating scale assigned to this parameter is

1 Laboratory or other climate controlled facility

5 Vehicle-mounted or fixed site

10 Handheld

High Reliability

Our definition of reliability is the operational or shelf life of a sensor or instrument.

Some instruments have a finite life span. For instance, spectroscopic instruments that use

lamps deteriorate over time and the lamp must be replaced. Some chemical sensors

utilize catalysts that are also used up under normal operation. And finally, some sensors

have a finite shelf life. This parameter attempts to quantify that reliability characteristic

which is indicative of maintenance or upkeep that the retailer will face.


1 <1 year operational or shelf life

5 2 year operational or shelf life

10 4+ year operational or shelf life



Detection Limit

The detection limit is the threshold value above which a sensor can detect an analyte.

Obviously, most sensors are tuned to make them particularly sensitive to just a few

chemical compounds. For rating detection limit, we only considered a sensor's ability to

detect the chemical species for which it was designed.


1 does not detect

5 detects, but at higher concentration (e.g. % range)

10 detects low concentration (e.g. ppm range)

Good Sensitivity

In many applications, just detecting a gas in not sufficient. A reproducible response

according to the concentration is usually desirable as well. The sensitivity parameter is a

measure of change in sensor response to changes in analyte concentration. With modern

electronics and signal processing, many shortcomings of a particular sensor can be

improved. Therefore, the ratings used here are directed as much as possible towards the

inherent characteristics of the sensing element.


1 saturates at all concentrations (serious shortcoming)

5 nonlinear response or moderate output

10 linear response and inherent high output

High Specificity

In addition to being able to respond to low concentration analytes in a predictable

manner, specificity in detection is also a desirable characteristic. As we have seen, some

sensors are affected by interferents, which are co-detected along with the desired target

analyte.


1 non-specific, multiple interferents

5 at least 1 major interferent

10 only detects desired analyte



11.1 Weighting Factors

Each of the parameters is assigned a weight based on their perceived importance in the selection

process. The weights range from 1-10, with 10 being the most important. The following weights

were selected:

Low Operational Complexity 6

Portability 8

High Reliability 6

Detection Limit 10

Good Sensitivity 5

High Specificity 8

Relative to the other tests, we ranked "Low Operation Complexity" and "Good Sensitivity" lower.

We believe that once a sensor or instrument is identified, standard methods for sampling and

testing will be documented which will remove most of the complexity for the operator. With

respect to sensitivity, we believe that detecting the contaminants is more critical than providing a

predictable or linear response to changes in their concentration. In fact, detectability is the most

important attribute of a sensor.

11.2 Trade-off Summary

Based on the trade-off parameters and weighting criteria described above, a trade-off analysis was

performed for the LPG sensor candidates. Sensors that share similar characteristics, serve a

common purpose (e.g. gas monitoring sensors), or are based on variations of a particular

technology (e.g. spectroscopic) were grouped together. The results of the trade-off analysis are

summarized in Table 6. The sections that follow describe the factors that were considered in

rating the various sensors. The trade-off analysis was performed in MS Excel to facilitate sharing

of information. The composite score for a particular technology is simply the product of an

attribute's grade with its corresponding weighting factor summed across all attributes. Note that

the interface detection methods were not included in the trade-off analysis. Since these

measurements are more physical than chemical we'll discuss them separately in the

recommendations section.



Table 6. Sensor Trade-Off Table

Attribute Color Legend: 1-3 (red), 4-7 (yellow), 8-10 (green)

Composite Score Legend: 43-236 (red), 237-353 (yellow), 354-430 (green)Desired Attributes Composite

Low Operational High Detection Good High ScoreComplexity Portability Reliability Limit Sensitivity Specificity sum XiGi

Weighting Factors [Xi] 6 8 6 10 5 8Sensing Technology [grade, Gi]Gas Detection Tubes 9 10 8 9 7 8 371Gas Chromatography 5 7 9 10 10 10 370Particle Counter (bulk solids) 8 5 9 9 9 10 357Ionization - PID 8 10 7 9 9 5 345Spectroscopic Sensors 8 7 8 6 7 8 311Gas Monitoring Sensors 7 9 3 6 9 7 293Dielectric Sensor 7 6 7 8 7 2 263Chemical Array Sensors 6 8 5 5 6 5 250Ionization - FID 2 3 6 9 9 3 231Coriolis Sensor 9 5 8 3 7 2 223

The composite score is based on the following percentage of the total range:

50%20% 30%

(green) (yellow) (red)

In the following sections, a compilation of notes and explanations to support the attribute ratings

(grades) for each sensor technology is provided.



BLANK



APPENDIX BInstrument Evaluation



12.0 Background

An electronic nose is an instrument that is capable of analyzing and recognizing gaseous samples

using chemical sensors and mathematical pattern recognition algorithms. This distinguishes it

from classical analytical instruments, like gas chromatography/mass spectrometry (GC/MS),

which can determine the amount and the chemical structure of the analytes. Electronic noses are

generally used to classify samples into a predetermined set of sample classes.

An electronic nose is normally supplied with several chemically different and non-specific

chemical gas sensors. A chemical gas sensor is a device that responds to the chemical

composition of the ambient gaseous phase. There are a variety of different "nose" sensors that

operate on different gas sensing principles (surface acoustic wave, conducting organic polymers,

etc.). The different functional principles lead to different types of sensors and some types of

sensors are more suitable for certain types of analytes in the gas phase. It is entirely feasible to

affect the response of the sensor to a specific analyte by changing its design during production or

changing the operation of the sensor. It is possible to construct highly specific sensors, which

respond only to a few kinds of molecules or even only to one target compound. These are not

normally used in electronic noses, because the samples often contain a complex mixture of many

different analytes in varying quantities.

The sensors in an electronic nose are said to be partially specific, i.e. each individual sensor

responds best to a certain group of analytes. The response of a particular sensor is always the

total response for all compounds that it is exposed to at that moment. Some compounds may

generate a strong response while others may not interact with the sensor at all adding little to the

signal. It is not possible to associate a part of the response to a specific compound, especially

because sometimes the response to one compound is influenced by the presence of another

compound. This is known as cross sensitivity, matrix effects, or interaction effects. Therefore,

the electronic nose is generally a limited number of non-specific sensors with overlapping, yet

varying, sensitivities, which responds to a large range of volatile compounds. The result is a

unique response pattern, or “fingerprint” for each sample that it encounters. The same sample

should ideally elicit the same fingerprint pattern on each exposure allowing for classification.

For example, if an electronic nose is supplied with 32 different sensors, a vapor sample provides

32 different responses, each containing different information. These 32 signals, represented as



numerical values, can be seen as a pattern which is characteristic of the measured sample. If a

different sample is measured, the 32 signals should differ in some way so that the pattern is

different. Pattern recognition algorithms are used to compare known samples (the "training set")

to new, unknown samples.

Although a sensor will not respond to every compound, the response from the desired analyte

compound can be masked by changes in other components in the mixture. This can usually be

avoided by choosing selective sensors, i.e. those that only respond to the target compounds.

13.0 MATERIALS

Our experimentation was aimed at determining the sensitivity of the instrument to a variety of

chemical compounds found as contaminants in LPG. The contaminants were identified

previously through other PERC work at SwRI. The contaminants utilized in these tests are

tabulated in Table 7. The index and abbreviated name is used to identify them in subsequent

plots and discussions. The plasticizer compounds were ordered from Alfa Aesar. The water was

deionized water from an in-house filtration system. The remaining materials in Table 7 were

reagent grade or better and were already on-hand.

Table 7. Test Compounds

Index Compound Structure

A

Bis(2-ethylhexyl) sebacate

Abbrev: BEHSeb

BDi-n-butyl sebacate

Abbrev: ButylSeb

CDi-n-octyl phthalate

Abbrev: OctPhth

DBenzyl butyl phthalate

Abbrev: BenButPhth



Table 7. Test Compounds

Index Compound Structure

EBis(2-ethylhexyl) phthalate

Abbrev: BEHPhth

IHexadecane

Abbrev: I_Hexadec

KIsooctane

Abbrev: K_Iso-oct

LAcetone

Abbrev: L_Acetone

MWater

Abbrev: M_Water

NMethanol

Abbrev: N_Methanol

A cylinder of pure propane (Grade 5.0, 99.999%) was acquired from Advance Gas Technologies,

Inc (Palm, PA). The certificate of analysis provided with the cylinder is shown in Table 8. For

comparison testing, a small cylinder of Coleman propane was acquired from a local retail outlet

and a cylinder of normal butane (already in hand) was selected for testing. Septa vials (40 mL)

for sampling were acquired from Fisher Scientific.



Table 8. Propane Certificate of Analysis

Impurity Specification (ppm) AnalysisNitrogen <2 0.3Oxygen <1 0.2Carbon Dioxide <1 <1Propylene <2 <1Other Hydrocarbons <8 <4.4Moisture <2 <1Total Impurities <10 <7.9

14.0 INSTRUMENTATION

The handheld instrument, know as the Cyranose 320 (Cyrano Sciences Inc), consists of 32

individual thin-film chemiresistors configured into an array (Figure 23). Each individual detector

of the sensor array is a composite material consisting of conductive carbon black blended with a

non-conducting polymer. When a composite is exposed to a vapor-phase analyte, the polymer

matrix absorbs the analyte, which causes a concomitant increase in resistance. When the analyte

is removed, the polymer de-gasses causing the film to shrink and return to normal. The collective

output of the array is used to identify an unknown analyte using standard data analysis

techniques.

Figure 23. Cyranose 320



The unit is trained by measuring vapor "fingerprints" representative of the samples that will be

analyzed later. Future measurements are then compared to these patterns to identify (i.e. classify)

the vapor. The unique polymer composite sensors have been shown to respond to a wide range of

organic compounds. Such a device, which is sensitive to a variety of volatile organic compounds,

might be used to "fingerprint" LPG (that meets required specifications) and form a basis for

future comparison samples of unknown quality. The Cyranose 320 sensor specification is shown

in Table 9.

Table 9. Cyranose 320 Sensor Specification

Parameter DescriptionWeight <32 ounces (0.91 kg)Dimensions 10 x 22 x 5 cmSensor Array Module 32 polymer carbon black compositesBattery Type NiMH battery packUniversal Power Adapter 110-240 V AC external power adapterBattery Life (Normal Operating Conditions) 3 hoursBattery Charging <3 hours with external adapterDisplay 320x200 graphic with LED backlight

Inlet ProbeOne 2” and one 4” needle interchangeablewith any standard female Luer Lock or slipadapter.

Keypad Scroll up/down, select, run and cancelWarm up time (at room temperature) < 5 minutesCommunication RS–232 @ 9600 to 57,600 bpsSampling Pump 50–180 cc/minute

Algorithms PCA, KNN, K-means, CDA(CanonicalDiscriminant Analysis)

Operating Temperature 0 to 40°C (32 to 104°F)Humidity 10 to 95%, non-condensingStorage Temperature -20 to 50°C (-4 to 122°F)

PC compatibility Minimum requirements Pentium II, 266 MHz,Windows 95, with CD ROM drive.

The general operation of the Cyranose 320 is described in the Users Manual as follows.

The Cyranose 320 consists of three functional components:

1. Sampling system

2. Chemical sensors

3. Signal processing and data manipulation



These components are designed to work together to provide sophisticated measurement and data

processing capability while maintaining simplicity of operation.

The measurement is based on a change in resistance of each chemical sensor in the 32-sensor

array when exposed to a chemical vapor. This is a differential measurement with the sensor

response measured as (Rmax-Ro)/Ro, where Ro is the resistance during a baseline gas flow and Rmax

is the maximum resistance during exposure to the sample vapor.

14.1 Sampling System

The sampling system requires two independent sample paths. The sampling system provides

independent sample paths to allow for the differential sensor resistance measurements. Inside the

Cyranose 320 a valve and pump are used to pull the purge gas and sample vapors over the

sensors. The purge path supplies background air to the sensors for the Ro measurement, as well

as a means to purge the sensors of sample vapor after sampling. The sample path provides a

means for the sample vapors to reach the sensors that are kept at constant temperature inside the

Cyranose 320.

14.2 Purge Cycle

In the purge cycle (Figure 24), the valve is in the default position (open) and allows vapors from

the purge inlet on the side of the Cyranose 320 to be pulled across the sensor array. The vapor is

then emitted at the exhaust port on the opposite side of the Cyranose 320.

Figure 24. Schematic of the Purge Cycle

This cycle provides the baseline or Ro sensor measurement. An optional filter may be added to

the baseline flow to remove either background chemicals or moisture, which might interfere in

the analysis. This cycle is also used to refresh the sensors after the sampling cycle.



14.3 Chemical Sensors

The chemical sensors respond to the vapor headspace to which they are exposed. Across the

array of unique sensors the responses are different and a response pattern is obtained that

represents each particular headspace. The proprietary sensor technology consists of 32 individual

thin-film carbon-black polymer composite chemiresistors configured into an array. The sensor

materials are thin films deposited across two electrical leads on an alumina substrate, creating the

conducting chemiresistors. When the composite film is exposed to a vapor-phase analyte, the

polymer matrix acts like a sponge and swells while absorbing the analyte. The increase in

volume causes an increase in resistance because the conductive carbon-black pathways through

the material are disrupted. When the analyte is removed the polymer releases the analyte and

shrinks to its original size, restoring the conductive pathways. Each polymer used in the array is

chemically unique and absorbs the analyte gases to a different degree, thus creating a pattern of

differential response across the array.

14.4 Signal Processing and Data Analysis

Signal processing converts the raw sensor response into the actual value that is used in the data

analysis to enable the pattern matching. The instrument measures the voltage drop across each

sensor in the array and converts it into a resistance reading. It stores the differential changes in

resistance across the array which it later uses for statistical analysis. The raw data is digitally

filtered using a Savitsky-Golay filter is used to smooth the response curve using a polynomial fit,

which reduces high frequency noise. The data is then converted to the maximum resistance

change caused by the analyte exposure to the sensors. The relative change in resistance for a

particular sensor is defined as the sensor response.

R/Ro = (Rmax-Ro)/Ro

A smellprint (Figure 25) is generated after the data is reduced for all 32 sensors. In the next step,

post-processing, normalization, and scaling techniques are applied to the pattern. Typically the

responses of the 32 sensors are normalized using a simple weighting method.

Normalized (∆R/Ro)i = (∆R/Ro)i / Σ|∆R/Ro|j



The normalized data is usually auto-scaled to remove the effects of response size on the

smellprint pattern. Once preprocessing and post-processing is complete, one of several on-board

pattern recognition algorithms is applied: K-Nearest Neighbor (KNN), K-means, or Canonical

Discriminant Analysis.

Figure 25. Smellprint Containing 32 sensor Responses

14.5 Outlier Diagnostics

Currently, principal component analysis (PCA) is used for visual outlier diagnostic. PCA plots

are used to find the data points that lie far away from the centroid of a particular class.

14.5.1 Principal Component Analysis (PCA)

The Principal Component Analysis (PCA) is a model-based and unsupervised method and is

widely used for a variety of data types to extract the main relations in the data matrix containing

sensor responses. The aim of the Principal Component Analysis is the optimum (in terms of data

set variance) description of a given data set in a dimension smaller than the number of sensors, N,

which span a vector space of N dimensions.

PCA data is depicted graphically and attempts to describe the majority of the variation in the data

set. Therefore, the data set is plotted in a minimal new set of axes, which represents a maximum

amount of variation. The coordinate axes of the newly defined space are formed by linear

combination of the original axes and need to be uncorrelated or orthogonal (eigenvector analysis).



One axis, or base vector, represents one virtual quality independent of all others and is called the

principal component (PC). The first axis is chosen in a way that it represents the direction along

the largest variance in the data set, the second axis shows the second largest variance, and so on.

Two plots are often used to represent PCA data: a plot of PCA scores and a plot of PCA loadings.

The plots may be two or three-dimensional depending on the number PCA factors chosen to

model the data set. For the scores plot, the distance between points (samples) corresponds

directly to their overall likeliness. The Mahalanobis Distance gives a numeric value for the

likeliness of the samples and can be specified for samples or sample groups.

The loadings plot shows the contribution of individual features (i.e. sensor response) to the

calculation of the PCs. By evaluating the loadings plot redundant features not contributing to the

evaluation can be chosen for removal. Features placed close to the origin (0,0) of the loadings

plot have little influence; features close to each other, under the same angle, or mirrored across

the center carry may carry redundant information.

14.6 Algorithms

Pattern recognition algorithms are powerful tools used to deal with large sets of data such as that

collected from the Cyranose 320. The Cyranose 320 uses PCA for outlier diagnosis and three

supervised algorithms for building a model and predicting the unknowns. These three supervised

algorithms are K-nearest neighbor (KNN), K-means and Canonical Discriminant Analysis

(CDA). Normally, CDA requires a matrix such that the number of rows is equal or greater than

the number of the columns. Therefore, the number of exposures of each sample should be equal

or greater than the number of sensors used in the electronic nose. However, the number of

exposures in the training set or class may be smaller than the number of sensors (e.g. 32). For

CDA, the first step is to employ a data compression technique, such as PCA, to the 32

dimensional sensor response data and use a subset of significant principal component scores as

inputs in the CDA. This guarantees a square matrix and avoids overfitting the model.

14.6.1 KNN

The k-nearest neighbor technique classifies an unknown sample according to the majority of its k

nearest neighbors. The nearest neighbor is that which is the shortest distance away in the model



space. For the calculation of the distance, the most common metrics are Euclidean distance and

Mahalanobis distance. The sample with k = 1 is the closest neighbor in the training set of data.

14.6.2 K-Means

This is a prediction algorithm in which an unknown is assigned to a class on the basis of the

Euclidean distance between the class centroid and the sample in sensor space.

14.6.3 Canonical Discriminant Analysis (CDA)

In Canonical Discriminant Analysis, an unknown sample is assigned to a class. A canonical

model maximizes the distance between the different classes and minimizes the distance between

the training samples in one class. The selection criterion is the class with the shortest

Mahalanobis distance between its centroid and the sample in canonical space.

14.7 Sensor Selection

Sensor selection is often a difficult and time-consuming process and requires intimate knowledge

of the sensor array material and its compatibility with the sample vapor. In this case, the sensor

array is proprietary, so all 32 of the sensors will be used. Some of the sensors (namely 5, 6, 23,

and 31) are known to be sensitive to water vapor and those may be removed to prevent

adulteration of the analysis. However, in tests that require the detection of water, those sensors

will need to be used.

14.8 Sensor Conditioning

The Cyranose 320 (and other chemical noses) are sometimes affected by the "first sniff" issue.

As the C320 sits idle its sensors constantly interact with the environment and are in

thermodynamic equilibrium with the water in the ambient air. When the pump is turned on, water

begins to desorb from the sensors, which may affect the response of the sensor. Several measures

can be taken to help overcome the first-sniff issue:

• Purge the chamber for six minutes at startup or when the instrument has been idle for 30

minutes or more



• Condition the instrument before collecting training set data. This involves sniffing at

least one sample of any three compound classes in the training set. When running real

samples, make several runs to verify repeatability.

• If the application permits, use a fresh drying tube on a daily basis to remove water from

the ambient air.

14.9 Sampling Considerations

14.9.1 Sample temperature

The temperature of the sample will have a significant effect on the chemical composition and

concentration of the headspace. The composition change and the big concentration change of the

headspace will cause the smellprint to change. Therefore, for measurement repeatability it is

critical to control the sample temperature.

14.9.2 Relative Humidity

The sensors are designed and selected for the identification of organic vapors, but like other

sensors, they will respond to water vapor. The concentration of water found in many samples and

in the ambient environment is often orders of magnitude higher than many of the key sample

constituents. For samples that are relatively dry the use of a drying filter in the baseline purge

flow path should be used to give more consistent results.

14.9.3 Sampling Sequence

To capture the maximum variability within each of the defined sample classes, training

measurements should be taken in random order. This ensures that the sensor array does not give

a fixed output based on the order of exposure of certain compounds.

14.9.4 Substrate Temperature

Measurement sensitivity is affected by temperature - lower temperatures give a stronger sensor

response. Optimally, the training set and subsequent sample runs should be taken at the same

substrate temperature. The substrate temperature should be set above ambient conditions but

should not exceed 42°C. A lower substrate temperature will generally give a longer battery life.



15.0 EXPERIMENTATION AND RESULTS

15.1 Sample Preparation and Sampling Apparatus

The neat contaminants and the hydrocarbon-based solutions were all sampled from 40 mL septa

vials. For a particular sets of tests, the vials were prepared ahead of time and placed in a constant

temperature oven at 35°C (95°F). Unless noted otherwise, the vials were allowed to equilibrate at

least overnight in order to develop a sufficient (and constant) headspace. For all samples, the

sample vial was removed from the oven and immediately placed inside a heated sample holder

(Figure 26) to help maintain the vial temperature during the short sampling procedure. The

sample holder was held at a constant 35°C (95°F) using a hot plate and a thermocouple to monitor

the temperature. Unless otherwise noted, dry air was used to purge the instrument before and

after runs. This was accomplished by connecting a drying tube to the purge inlet as shown in

Figure 27. Connections between the drying tube and instrument were made with Luer-slip

fittings.

Sample Vial Holders

Thermocouple Hole

Figure 26. Sample Vial Holder



Cyranose 320

Drying Agent

Humid Air

Purge Inlet

Figure 27. Dry Air Purge

15.1.1 Neat Compounds

When testing the neat compounds, the vials were sampled according to Figure 28. In this

configuration, only a short sample draw period (approximately 25 seconds at low pump speed) is

permissible because a vacuum quickly develops as the headspace vapor is removed. Each vial

contained 1 mL of the sample to be tested.

Cyranose 320

Sample

Headspace

Needle

Septa Cap

Figure 28. Basic Sampling Technique



15.1.2 Hydrocarbon Solutions

In later experiments, a dry air bubbler, as shown in Figure 29, was used to help regenerate the

headspace as sample vapor was removed. This was used with the hydrocarbon-based solutions so

that a longer sample draw time could be used. The hydrocarbon solutions, regardless of the

sample compound, were prepared as 0.5% solutions in 0.45µm filtered isooctane. Generally, a

0.5% stock solution was prepared and then 5 mL of the solution was placed in the vials. A

minimum of five vials per sample compound was required in order to generate the training matrix

for the Cyranose 320.

Cyranose 320Drying Agent

Humid Air

Dry Air

Figure 29. Sampling With a Dry Air Bubbler

Some differential experiments were also carried out wherein isooctane was used as the purge gas

rather than dry air. The setup for this configuration is shown in Figure 30. The isooctane vial

used a dry air bubbler to keep its headspace constant.



Drying Agent

Humid Air

Dry Air

Isooctane

Cyranose 320Drying Agent

Humid Air

Dry Air

Purge Inlet

Sample

Figure 30. Sampling With a Dry Air Bubbler and Isooctane Purge

15.1.3 Gas Samples

Although the Cyranose 320 is designed to handle vapor phase samples, it is not designed to

handle high-pressure systems. In order to measure gas samples, the best approach was to use the

onboard pump to pull the gas sample off of a low-pressure gas stream (Figure 31). Using a

needle valve for tight control, the sample gas contained in the sample cylinder was passed

through a low-pressure gas regulator and exhausted into a fume hood. The needle of the

Cyranose 320 was used to puncture the exhaust hose and draw in sample gas as it flowed by.

Sample Cylinder

NeedleValve

Low PressureRegulator

Exhaust

Cyranose 320

Flow

Figure 31. Sampling from a Gas Cylinder



15.1.4 Instrument Settings

The Cyranose 320 instrument settings that were common to all experiments are tabulated in Table

10. These are based on the manufacturer's recommended settings. The Cyranose 320 is primarily

designed to sample gases at ambient temperatures. However, some of the compounds have low

vapor pressure so heating is required to help generate sufficient headspace gas to properly sample

them. The manufacturer strongly recommends against exposing the Cyranose 320 to high

temperature vapors because the vapors may immediately condense in the cooler pathways inside

the instrument and contaminate future samplings. The instrument utilizes a substrate heater to

prevent this from happening. To minimize risk, we chose to heat the sample vials to 35°C (95°F).

Therefore, the substrate temperature was set to 42°C (107.6°F) or seven degrees higher than the

sample temperature as recommended by the manufacturer (the maximum substrate temperature is

50°C (122°F).

Table 10. Common Instrument Settings

Parameter SettingDigital Filtering OnSubstrate Heater On (42°C)Training Repeat Count 1Identifying Repeat Count 1Algorithm CanonicalPreprocessing Auto-scalingNormalization Normalization 1Identification Quality Medium

15.1.5 Understanding the Results

The following sections contain a series of figures and tables that visually describe the results of

the testing. These plots are best viewed in color. The tables and figures are as follows:

• Table - Training Set

• Figure - PCA Plot

• Figure - Canonical Plot

• Table - Interclass Mahalanobis Distance

Training Set

The training set table lists each compound class and each exposure (measurement). For each

exposure, a distance measure is given which relates how close that sample is to all of the other

exposures in its class. The higher the number, the further away the sample is from the class



center. From this we assume that the sample is different in some way. This information can be

used to find gross outliers - samples that are statistically different from those in its class. The

manufacturer recommends that samples with s distance of 5 or greater be removed or rerun.

PCA Plot

Once a minimum of five samples have been run for a particular class of compound, a principle

component analysis can be performed and the scores (see discussion on PCA) plotted. The PCA

plot is a scores plot and helps to visual how the samples lie together in the sample space. It can

provide a visual indication of outliers or overlap of classes.

Canonical Plot

The canonical plot generally gives similar results to the PCA plot. Once a training set has been

compiled and a model has been built with the data, the results of the model are presented as a

canonical plot, which takes into the account the weighting given to the different samples by the

modeling process. Again, this is just a way to visual the data. Generally, if the data is linearly

separable (i.e. you can draw a line between the classes) then the instrument can easily distinguish

between the different compound classes.

Interclass Mahalanobis Distance

Another output from the modeling process measures how close the compound classes lie relative

to each other. This table gives an indication of how robust a model will be. As a general rule of

thumb suggested by the manufacturer, two classes whose interclass Mahalanobis distance is ≥5

should be distinguishable. Distances less than five indicate that the classes lie relatively close to

one another and should probably be combined into a "superclass," a class containing both

compounds.

The following sections describe the results of the preliminary series of test performed on the

Cyranose 320.

15.2 Experiment #1 - Neat Plasticizers

Five samples each of plasticizer compounds BEHSeb (A), ButylSeb (B), OctPhth (C),

BenButPhth (D), and BEHPhth (E) were prepared and analyzed according to the procedures for

neat compounds described above with the exception that the samples were only allowed to



equilibrate in the oven for 30 minutes. This was a trial run to gain experience with the instrument

and get an idea of how the instrument responds to the plasticizer compounds.

The instrument settings for this test are shown in Table 11.

Table 11. Experiment #1. Instrument Settings

Parameter Time (sec) Pump SpeedBaseline Purge 30 MediumSample Draw 25 Low1st Sample Gas Purge 0 High1st Air Intake Purge 5 High2nd Sample Gas Purge 30 High2nd Air Intake Purge 30 High

Sensors Removed 5, 6, 23, 31

When all classes were included in the model, significant overlap was seen between compounds

ButylSeb (B), BenButPhth (D), and BEHPhth (E). BEHSeb (A) and OctPhth (C) were clearly

distinguishable. Please refer to the following tables and figures:

• Table 12. Experiment #1. Training Set

• Figure 32. Experiment #1. PCA Plot, All Classes

• Figure 33. Experiment #1. Canonical Plot, All Classes

• Table 13. Experiment #1. Interclass Mahalanobis Distance, All Classes

When any two of the three overlapping classes were removed, the remaining classes were more

easily distinguishable. Please refer to the following tables and figures:

ButylSeb (B) and BEHPhth (E) Removed

• Figure 34. Experiment #1. Canonical Plot, Classes B and E Removed

• Table 14. Experiment #1. Interclass Mahalanobis Distance, Classes B and E Removed

ButylSeb (B) and BenButPhth (D) Removed

• Figure 35. Experiment #1. Canonical Plot, Classes B and D Removed

• Table 15. Experiment #1. Interclass Mahalanobis Distance, Classes B and D Removed

BenButPhth (D) and BEHPhth (E) Removed

• Figure 36. Experiment #1. Canonical Plot, Classes D and E Removed

• Table 16. Experiment #1. Interclass Mahalanobis Distance, Classes D and E Removed



Table 12. Experiment #1. Training Set



Figure 32. Experiment #1. PCA Plot, All Classes



Figure 33. Experiment #1. Canonical Plot, All Classes

Table 13. Experiment #1. Interclass Mahalanobis Distance, All Classes



Figure 34. Experiment #1. Canonical Plot, Classes B and E Removed

Table 14. Experiment #1. Interclass Mahalanobis Distance, Classes B and E Removed



Figure 35. Experiment #1. Canonical Plot, Classes B and D Removed

Table 15. Experiment #1. Interclass Mahalanobis Distance, Classes B and D Removed



Figure 36. Experiment #1. Canonical Plot, Classes D and E Removed

Table 16. Experiment #1. Interclass Mahalanobis Distance, Classes D and E Removed



15.3 Experiment #2 – Plasticizers Revisited

Having gained some experience in operating the instrument, we chose to repeat Experiment #1.

Since several of the classes from Experiment #1 showed significant overlap, we decided to

increase the equilibration time in hopes that the headspace would be more concentrated. The

samples were allowed to stay in the oven overnight to maximize the equilibration period.




Sensors Removed 5, 6, 23, 31

The initial results from these tests showed significant improvements. Although the compound

classes were still laying close together, compounds ButylSeb (B), BenButPhth (D), and BEHPhth

(E) were better resolved with less overlap. The PCA plot showed that some potential outliers

were present (e.g. BEHSeb (A) #7 and BenButPhth (D) #1). Please refer to the following tables

and figures:





An outlier analysis was performed and samples with a Euclidean distance greater than 4.5 were

removed from the training set. The model was then rebuilt using the new training set. After

removing the gross outliers, the class separation improved somewhat but the Mahalanobis

distance measurement suggested that compounds ButylSeb (B), OctPhth (C), and BenButPhth (E)

were questionably close. It's likely that the sensor array responds to these compounds in a similar

manner and may not be completely separable. Please refer to the following tables and figures:



• Table 20. Experiment #2. Training Set, Outliers Removed

• Figure 39. Experiment #2. PCA Plot, All Classes, Outliers Removed

• Figure 40. Experiment #2. Canonical Plot, All Classes, Outliers Removed

• Table 21. Experiment #2. Interclass Mahalanobis Distance, All Classes, Outliers

Removed

To go one step further, we performed two more analyses by eliminating either compound

ButylSeb (B) or BenButPhth (D). In each case, we started with the original training set, removed

one of the compound classes, performed an outlier analysis, and them rebuilt the model.

Statistically, the Mahalanobis distance for the two new models suggest that the compound classes

should be separable but the classes lie so close together that frequent misidentification may be a

problem. Please refer to the following tables and figures:

• Table 22. Experiment #2. Training Set, Classes A, B, C, and E, Outliers Removed

• Figure 41. Experiment #2. Canonical Plot, Classes A, B, C, and E, Outliers Removed

• Table 23. Experiment #2. Interclass Mahalanobis Distance, Classes A, B, C, and E,

Outliers Removed

• Table 24. Experiment #2. Training Set, Classes A, C, D, and E, Outliers Removed

• Figure 42. Experiment #2. Canonical Plot, Classes A, C, D, and E, Outliers Removed

• Table 25. Experiment #2. Interclass Mahalanobis Distance, Classes A, C, D, and E

Outliers Removed



Table 20. Experiment #2. Training Set, Outliers Removed



Figure 39. Experiment #2. PCA Plot, All Classes, Outliers Removed



Figure 40. Experiment #2. Canonical Plot, All Classes, Outliers Removed

Table 21. Experiment #2. Interclass Mahalanobis Distance, All Classes, Outliers Removed



Table 22. Experiment #2. Training Set, Classes A, B, C, and E, Outliers Removed



Figure 41. Experiment #2. Canonical Plot, Classes A, B, C, and E, Outliers Removed

Table 23. Experiment #2. Interclass Mahalanobis Distance, Classes A, B, C, and E,Outliers Removed



Table 24. Experiment #2. Training Set, Classes A, C, D, and E, Outliers Removed



Figure 42. Experiment #2. Canonical Plot, Classes A, C, D, and E, Outliers Removed

Table 25. Experiment #2. Interclass Mahalanobis Distance, Classes A, C, D, and E OutliersRemoved



15.4 Experiment #3 - Contaminant Comparison

To expand the capability of our model, we added several additional compounds. Specifically,

hexadecane, isooctane, acetone, water, and methanol were added. Each of these compounds are

potential contaminants in LPG. Hexadecane was chosen to represent a “heavy” hydrocarbon and

is the heaviest hydrocarbon that could be worked with practically (heavier hydrocarbons are

solids at ambient temperature). Isooctane is a lighter, branched hydrocarbon. To form the

“plasticizer” class, each of the five plasticizer compounds were utlilized. So the ten plasticizer

samples consisted of compounds A, A, B, C, D, E, A, A, B, C. Finally, the experiment was run in

two parts, six samples one day and four the next.




Sensors Removed None

Ten samples of each compound were analyzed, an outlier analysis performed, and a model built.

The PCA and canonical plots gave a clear indication that vapor pressure plays a significant role in

the ability of the instrument to detect certain compounds. Isooctane (K), acetone (L), and

methanol (N) were well separated and these are the most volatile compounds in the training set.

The plasticizer (Pl), hexadecane (I), and water (M) samples were grouped fairly tightly and their

ability to be distinguished is questionable. Please refer to the following tables and figures:







To determine if the plasticizers (Pl), hexadecane (I), and water (M) classes could be better

resolved, a new model consisting of just those classes was built. With outliers removed, the

model indicates that these classes could be sufficiently separated to create a robust model. Please

refer to the following tables and figures:

• Table 29. Experiment #3. Training Set (Plasticizers, Hexadecane, Water), Outliers

Removed.

• Figure 45. Experiment #3. PCA Plot (Plasticizers, Hexadecane, Water), Outliers

Removed

• Figure 46. Experiment #3. Canonical Plot (Plasticizers, Hexadecane, Water), Outliers

Removed

From the plots, the only apparent manifestation resulting from the samples being run over two

days is a small clustering in the water samples (Figure 45). Since this is not evident in the other

compound classes, it is probable that this clustering is a result of ambient humidity differences

from one day to the next. Although the instrument purge uses a drying tube to dry the purge air,

the instrument is very sensitive and may be responsive to very small changes in ambient

humidity.

This experiment also provides a good example of an approach known as tiered-modeling. This

technique relies on a series of models each with finer grain than the last. For example, given an

unknown sample, its smellprint is first classified according to a high level model. This model

may consist of many classes that overlap or whose classes consist of several types of compounds

that have been joined to create a “superclass,” like our plasticizer class in this experiment. Once

the smellprint is associated with a certain group of classes it is then subjected to a lower level

model consisting of just those classes. The smellprint is continually reclassified until at some

point the sample can be identified as belonging to a single class. In this experiment, a sample

classified as a plasticizer, hexadecane, or water sample in our first model might be a further

verified by applying the second model containing just those classes. If that classification were to

identify the sample as a plasticizer then a model from our previous experiment might be used to

determine which specific plasticizer it is. Overall, the tiered modeling approach allows

continuous refinement of the sample classification until the sample is uniqieuly identified.



Table 29. Experiment #3. Training Set (Plasticizers, Hexadecane, Water), OutliersRemoved.



Figure 45. Experiment #3. PCA Plot (Plasticizers, Hexadecane, Water), Outliers Removed



Figure 46. Experiment #3. Canonical Plot (Plasticizers, Hexadecane, Water), OutliersRemoved

Table 30. Experiment #1. Interclass Mahalanobis Distance (Plasticizers, Hexadecane,Water), Outliers Removed



15.5 Experiment #4 - HC Solutions

Our next series of tests were aimed at determining whether a single, low concentration

contaminant could be detected in a hydrocarbon matrix. We chose isooctane as the hydrocarbon

because it is relatively volatile and would generate a headspace easily but not so volatile that

evaporation would cause the sample composition to change. These tests were conceived of to

mimic what we will see in LPG but are easier to work with at ambient temperature and pressure.

The compounds chosen for this study were BEHSeb (A), BEHPhth (E), and methanol (N). Six

vials of each solution (0.5% in isooctane) were prepared as described above and the smellprints

were collected in the same manner as previous experiments. The solutions are identified in the

tables and plots as isooctane (Iso), BEHSeb (IsoA), BEHPhth (IsoE), and methanol (IsoN).





The results from these tests were generally poor and reinforced the fact that compound volatility

is a critical factor in determining whether a compound will be detected or not. Only methanol

(N), the most volatile compound used in these tests was differentiated by the instrument. This

happened because the headspace for that sample was enriched with methanol due to its volatility.

In the other samples, the plasticizers have so little volatility that they remain in the liquid phase.

The vapor phase of the plasticizer samples could not be distinguished from the reference

isooctane (Iso) sample. No additional analysis was performed on these samples. Please refer to

the following tables and figures:







15.6 Experiment #5 - HC Solutions with Dry Air Bubbler

After the failure of the previous experiment to detect the plasticizers in the isooctane solutions,

we opted to repeat the experiment using an air bubbler to help enrich the headspace gases and

allow longer sample draw times.



Parameter Time (sec) Pump SpeedBaseline Purge 30 MediumSample Draw 60 Medium1st Sample Gas Purge 0 High1st Air Intake Purge 0 High2nd Sample Gas Purge 70 High2nd Air Intake Purge 70 High


The results of these tests were essentially identical to the previous ones. Although the bubbler

appeared to have some effect (the spread of the samples increased), the plasticizer (IsoA and

IsoE) samples are still indistinguishable from the isooctane (Iso) reference. No further analysis

was performed on these samples. Please refer to the following tables and figures:







15.7 Experiment #6 - HC Solutions and Differential Measurements

In a final effort to detect the plasticizer compounds in the isooctane matrix, we attempted to

perform differential measurements on the samples. Rather than purging with dry air, a vial

containing isooctane and a dry air bubbler was connected to the purge inlet of the instrument. At

the end of the purge cycle when the baseline resistance is taken, the instrument would be

saturated with isooctane thus making all subsequent measurements relative to that baseline.

Conceptually, this should help amplify the response from the low concentration species that

coexist in the headspace vapor with the isooctane. One of the plasticizer samples, IsoA, was

replaced with a 0.5% water/isooctane solution (IsoM). As before, a bubbler was used with the

sample to allow extended sample draw times.



Parameter Time (sec) Pump SpeedBaseline Purge 40 MediumSample Draw 60 Medium1st Sample Gas Purge 0 High1st Air Intake Purge 0 High2nd Sample Gas Purge 90 High2nd Air Intake Purge 90 High


Once again the plasticizer compound (IsoE) was indistinguishable from the isooctane (Iso)

reference sample. The water sample did show signs of separation from the isooctane (Iso) and it

would likely be distinguishable in a refined model. Please refer to the following tables and

figures:







15.8 Experiment #7 - Propane Trials

Having met with some failure in detecting low concentration, low volatility compounds in a

hydrocarbon matrix, we shifted our focus for the propane samples to something more suited to the

instruments purpose. One possible application for this instrument with respect to LPG is in

determination of gross contamination or changes in composition. To test this concept and work

the bugs out of our apparatus, we began by doing a simple comparison between two distinctly

different products: pure research grade propane and Coleman propane. Although we were unable

to verify the exact composition of the Coleman propane, we know its composition is uniquely

different because of the obvious presence of the odorant.





Besides the obvious fact that the instrument was able to distinguish the two samples, an important

observation was that the samples appeared to vary in a systematic way. We determined that this

resulted from the cooling of the gas cylinders during the test. This reduced the pressure and

therefore concentration of the gas from the cylinder. Please refer to the following tables and

figures:







15.9 Experiment #8 - Propane Comparison

In our final experiment, we expanded our training set to include two other sample types. One of

the samples was an n-butane (Butane in the tables and figures) gas standard that we happened to

have on-hand in the lab. The other sample was a 0.57% methanol/propane solution that we

prepared from the pure research grade propane (identified as "Meth" in the tables and figures).

The concentration was estimated from the volume of added methanol and propane. To solve our

cooling problems, we placed heat lamps near the sample cylinders to help keep them at a constant

temperature throughout the sampling procedure.





Having solved the cooling problem and reduced the systematic variation from the previous tests,

we were able to achieve better success in these experiments. Each of the four classes are clearly

distinguishable and the interclass Mahalanobis distances verify that. Please refer to the following

tables and figures:







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