Non-dispersive infra-red (NDIR) Non-dispersive infra-red (NDIR) analyzersanalyzers

(Working principle)(Working principle)

In the NDIR analyzer the exhaust gas In the NDIR analyzer the exhaust gas species being measured is used to detect species being measured is used to detect itself.itself.This is done by selective absorption.This is done by selective absorption.The infrared energy of a particular The infrared energy of a particular wavelength or frequency is peculiar to a wavelength or frequency is peculiar to a certain gas in that the gas will absorb the certain gas in that the gas will absorb the infrared energy of this wavelength and infrared energy of this wavelength and transmit the infrared energy of other transmit the infrared energy of other wavelengths. wavelengths.

Measurement of CO using Measurement of CO using NDIRNDIR

The absorption band for carbon monoxide is The absorption band for carbon monoxide is between 4.5 and 5 microns. So the energy between 4.5 and 5 microns. So the energy absorbed at this wavelength is an indication of absorbed at this wavelength is an indication of concentration of CO in the exhaust gas.concentration of CO in the exhaust gas.

The NDIR analyzer consists of two infrared The NDIR analyzer consists of two infrared sources, interrupted simultaneously by an optical sources, interrupted simultaneously by an optical chopper. Radiation from these sources pass in chopper. Radiation from these sources pass in parallel paths through a reference cell and a parallel paths through a reference cell and a sample cell.sample cell.

Measurement of CO using NDIR Measurement of CO using NDIR –Contd.–Contd.

Exhaust gas from the tailpipe of the Exhaust gas from the tailpipe of the engine/vehicle is passed through the engine/vehicle is passed through the sample cell during the measurement of sample cell during the measurement of exhaust emissions.exhaust emissions.The reference cell is filled with an inert The reference cell is filled with an inert gas, usually nitrogen, which does not gas, usually nitrogen, which does not absorb the infrared energy for the absorb the infrared energy for the wavelength corresponding to the wavelength corresponding to the compound being measured. compound being measured.

Schematic diagram of NDIRSchematic diagram of NDIR

Measurement of CO using NDIRMeasurement of CO using NDIR—Contd.—Contd.

A closed container filled with only the A closed container filled with only the compound to be measured works as a compound to be measured works as a detector.detector.The detector is divided into two equal The detector is divided into two equal volumes by a thin metallic diaphragm. volumes by a thin metallic diaphragm. When the chopper blocks the radiation, When the chopper blocks the radiation, the pressure in both parts of the detector the pressure in both parts of the detector is same and the diaphragm remains in the is same and the diaphragm remains in the neutral position.neutral position.

Measurement of CO using NDIRMeasurement of CO using NDIR—Contd.—Contd.

As the chopper blocks and unblocks the As the chopper blocks and unblocks the radiations, the radiant energy from one radiations, the radiant energy from one source passes through the reference cell source passes through the reference cell unchanged whereas the sample cell unchanged whereas the sample cell absorbs the infrared energy at the absorbs the infrared energy at the wavelength of the compound in the cell.wavelength of the compound in the cell.The absorption is proportional to the The absorption is proportional to the concentration of the compound to be concentration of the compound to be measured in the sample cell.measured in the sample cell.

Measurement of CO using NDIRMeasurement of CO using NDIR—Contd.—Contd.

Thus unequal amounts of energy are transmitted Thus unequal amounts of energy are transmitted to the two volumes of the detector and the to the two volumes of the detector and the pressure differential so generated causes pressure differential so generated causes movement of the diaphragm of the detector.movement of the diaphragm of the detector.

This changes the capacitance between the This changes the capacitance between the diaphragm and a fixed probe, thereby generating diaphragm and a fixed probe, thereby generating an a.c. signal which is amplified and, after an a.c. signal which is amplified and, after rectification to d.c. displayed on a meter.rectification to d.c. displayed on a meter.

Non-dispersive infra-red (NDIR) Non-dispersive infra-red (NDIR) analyzersanalyzers

The signal is a function of the The signal is a function of the concentration of the compound to be concentration of the compound to be measured.measured.The NDIR can accurately measure CO, The NDIR can accurately measure CO, COCO22 and those hydrocarbons which have and those hydrocarbons which have clear infrared absorption peaks. However, clear infrared absorption peaks. However, usually the exhaust sample to be analyzed usually the exhaust sample to be analyzed contains other species which also absorb contains other species which also absorb infrared energy at the same frequency.infrared energy at the same frequency.

Non-dispersive infra-red (NDIR) Non-dispersive infra-red (NDIR) analyzers Contd.analyzers Contd.

For example, an NDIR analyzer sensitized For example, an NDIR analyzer sensitized to n-hexane for detection of HC responds to n-hexane for detection of HC responds equally well to other paraffinic HC but not equally well to other paraffinic HC but not to olefins, acetylenes or aromatics.to olefins, acetylenes or aromatics.

Therefore, the reading given by such Therefore, the reading given by such analyzer is multiplied by 1.8 to correct it to analyzer is multiplied by 1.8 to correct it to the total UBHC as measured by an FID the total UBHC as measured by an FID analyzer in the same exhaust stream.analyzer in the same exhaust stream.

Gas chromatographyGas chromatography

In gas chromatography first the individual In gas chromatography first the individual constituents are separated from the constituents are separated from the mixture of gas.mixture of gas.

After separation, each compound can be After separation, each compound can be separately analyzed for concentration.separately analyzed for concentration.

This is the only method by which each This is the only method by which each component existing in an exhaust sample component existing in an exhaust sample can be identified and analyzed.can be identified and analyzed.

Gas chromatography –Contd.Gas chromatography –Contd.

However, it is very time consuming and However, it is very time consuming and the samples can be taken only in batches.the samples can be taken only in batches.Gas chromatograph is primarily a Gas chromatograph is primarily a laboratory tool.laboratory tool.In addition to the above methods such as In addition to the above methods such as mass spectroscopy, chemiluminescent mass spectroscopy, chemiluminescent analyzers, FID, smokemeters and analyzers, FID, smokemeters and electrochemical analyzers are also used electrochemical analyzers are also used for measuring exhaust emissions.for measuring exhaust emissions.

Flame ionization detector (FID) Flame ionization detector (FID) (Working principle)(Working principle)

A hydrogen – air flame contains a negligible A hydrogen – air flame contains a negligible amount of ions. However even if a trace amount amount of ions. However even if a trace amount of organic compound such as HC is introduced of organic compound such as HC is introduced into the flame, a large number of ions are into the flame, a large number of ions are produced.produced.

If a polarised voltage is applied across the If a polarised voltage is applied across the burner jet and an adjacent collector, an ion burner jet and an adjacent collector, an ion migration will produce a current proportional to migration will produce a current proportional to number of ions and thus HC concentration number of ions and thus HC concentration present in the flame. present in the flame.

Flame ionization detector (FID)Flame ionization detector (FID)

Flame ionization detector (FID) – Flame ionization detector (FID) – Contd.Contd.

The output of FID depends on the number The output of FID depends on the number of carbon atoms passing through the flame of carbon atoms passing through the flame in unit time.in unit time.

Doubling the flow velocity would double Doubling the flow velocity would double the output.the output.

Hexane (CHexane (C66HH1414) would double the output of ) would double the output of propane (Cpropane (C33HH88). ).

Chemiluminescent AnalyzerChemiluminescent Analyzer

Chemiluminescent is the emission of light Chemiluminescent is the emission of light energy resulting from a chemical reaction.energy resulting from a chemical reaction.

If NO and OIf NO and O33 are brought together a are brought together a chemical reaction takes place which chemical reaction takes place which produces NOproduces NO22 in activated state. in activated state.

Activated NOActivated NO22 emits light emits light

NO + ONO + O33 → → NONO22 + O + O22

NONO22 →→ NO NO22 + light emissions + light emissions

Schematic diagram of Schematic diagram of chemiluminescent analyzerchemiluminescent analyzer

Chemiluminescent Analyzer – Chemiluminescent Analyzer – Contd.Contd.

To measure NO concentration the gas sample to To measure NO concentration the gas sample to be analysed is send to a reaction chamber, be analysed is send to a reaction chamber, where it combines with ozone.where it combines with ozone.The resulting chemiluminescent is monitored The resulting chemiluminescent is monitored through an optical filter by a high sensitivity through an optical filter by a high sensitivity photomultiplier positioned at one end of the photomultiplier positioned at one end of the reactor.reactor.The filter photo multiplier combination responds The filter photo multiplier combination responds to light in a narrow wavelength band unique to to light in a narrow wavelength band unique to the above reaction. the above reaction.

Chemiluminescent Analyzer – Chemiluminescent Analyzer – Contd.Contd.

The output from the photo multiplier is linearly The output from the photo multiplier is linearly proportional to the NO concentration.proportional to the NO concentration.

To measure NOTo measure NOxx concentration (i.e. NO plus concentration (i.e. NO plus NONO22) the sample gas flow is diverted through an ) the sample gas flow is diverted through an NONO22-to-NO converter.-to-NO converter.

The chemiluminescent response in the flow The chemiluminescent response in the flow reactor to the converter effluent is linearly reactor to the converter effluent is linearly proportional to the NOproportional to the NOxx concentration entering concentration entering the converter. the converter.

Chemiluminescent Chemiluminescent Analyzer – Contd.Analyzer – Contd.

The photo multiplier output The photo multiplier output is amplified and given to an is amplified and given to an indicator for measuring NOindicator for measuring NOxx concentration directly. concentration directly.

Measurement of SmokeMeasurement of Smoke

There are two basic types of smoke meters There are two basic types of smoke meters which are used to measure smoke density. which are used to measure smoke density. (i) Filter darkening types(i) Filter darkening types(ii) Light extinction type.(ii) Light extinction type.The light extinction type of meters can measure The light extinction type of meters can measure both white and black smoke whereas the filter both white and black smoke whereas the filter paper darkening type meters can give only black paper darkening type meters can give only black smoke.smoke.The light extinction meter can be used for The light extinction meter can be used for continuous measurements while the filter type continuous measurements while the filter type can be used only under steady state conditions.can be used only under steady state conditions.

Measurement of Smoke-Contd.Measurement of Smoke-Contd.(Bosch smoke meter)(Bosch smoke meter)

Bosch smoke meter is filter darkening type.Bosch smoke meter is filter darkening type.A measured volume of exhaust gas is drawn A measured volume of exhaust gas is drawn though a filter paper which is blackened to though a filter paper which is blackened to various degrees depending upon the amount of various degrees depending upon the amount of carbon present in the exhaust.carbon present in the exhaust.The density of soot is measured by determining The density of soot is measured by determining the amount of light reflected from the sooted the amount of light reflected from the sooted paper.paper.The diameter of the filter paper, the sample The diameter of the filter paper, the sample volume etc., all are well defined.volume etc., all are well defined.

Schematic diagram Hartridge Schematic diagram Hartridge smoke meter smoke meter

Measurement of Smoke- Contd.Measurement of Smoke- Contd. (Hartridge smoker meter) (Hartridge smoker meter)

This smoke meter works on the light extinction principle. This smoke meter works on the light extinction principle. A continuously taken exhaust sample is passed through A continuously taken exhaust sample is passed through a tube of about 46 cm length which has a light source at a tube of about 46 cm length which has a light source at one end and photocell or solar cell at the other end.one end and photocell or solar cell at the other end.The amount of light passed through this smoke column The amount of light passed through this smoke column is used as an indication of smoke level.is used as an indication of smoke level.This smoke density is defined as the ratio of electric This smoke density is defined as the ratio of electric output from the photocell or solar cell when sample is output from the photocell or solar cell when sample is passed through this smoke column to the electric output passed through this smoke column to the electric output when clean air is passed through it. when clean air is passed through it.

Measurement of Smoke- Contd.Measurement of Smoke- Contd.

The fraction of the light transmitted through the The fraction of the light transmitted through the smoke (T) and the length of the light path (L) are smoke (T) and the length of the light path (L) are related y the Beer-Lambert law.related y the Beer-Lambert law.T = e T = e –KL–KL

K = n A K = n A θθWhere K is the optional absorption coefficient of Where K is the optional absorption coefficient of the obscuring matter per unit length, n the the obscuring matter per unit length, n the number of soot particles per unit volume; A the number of soot particles per unit volume; A the average projected are of each particles; and average projected are of each particles; and θθ the specific absorbance per particle.the specific absorbance per particle.