nes 605 part 1 guide to selection of sensors for the measurement of system parameters

Guide to Selection of Sensors forMeasurement of System Parameters

Part 1Selection of Liquid Level, Temperature and

Pressure Sensors

Ministry of Defence Defence Standard 02-605 Part 1

Issue 2 Publication Date 4 February 2003

Category 3

AMENDMENTS ISSUED SINCE PUBLICATION

AMD NO DATE OFISSUE

TEXT AFFECTED SIGNATURE &DATE

Revision Note

This Issue of this Standard has been prepared to incorporate changes to text and presentation.The technical content has been updated in line with current practice.

Historical Record

DEF STAN 02-605 Part 1 Issue 1 April 2000DEF STAN 02-605 Part 2 Issue 1 April 2000DEF STAN 02-605 Part 3 Issue 1 April 2000

NES 605 Part 1 Issue 3 April 1989NES 605 Part 1 Issue 2 January 1981NES 605 Part 1 Issue 1 February 1980

NES 605 Part 2 Issue 4 February 1995NES 605 Part 2 Issue 3 April 1989NES 605 Part 2 Issue 2 January 1981NES 605 Part 2 Issue 1

NES 605 Part 1 Issue 3 February 1992NES 605 Part 1 Issue 2 December 1988NES 605 Part 1 Issue 1 January 1981

1

DEFENCE STANDARD 02605

GUIDE TO THE SELECTION OF SENSORS FOR THE

MEASUREMENT OF SYSTEM PARAMETERS

PART 1 ISSUE 2

SELECTION OF LIQUID LEVEL, TEMPERATURE AND

PRESSURE SENSORS

This Defence Standard is

authorized for use in MOD contracts

by the Defence Procurement Agency and

the Defence Logistics Organization

Published by:

Defence Procurement AgencyAn Executive Agency of The Ministry of DefenceUK Defence StandardizationKentigern House65 Brown StreetGlasgow G2 8EX

DEF STAN 02605 PART 1 / ISSUE 2(NES 605 Part 1)

2

SCOPE

1. This Defence Standard (DEF STAN) provides technical guidance for the selection of sensors andtransducers that produce an electrical output signal. These may be suitable for direct readinginstrumentation or for coupling into an electronic automatic control and automation system.

2. This Standard provides guidance on sensors and transducers which are suitable for measuring thelevel, temperature or pressure of a variable.

3. The following components are outside the scope of this Standard:

a. Sensors and transducers, such as pneumatic components, mercury thermometers ormechanical devices which do not produce an electrical output signal;

b. Dipsticks and other portable liquid level sensors;

c. Indicators and thermometers which only produce a local indication of temperature;

d. Local controllers such as level or pressure switches, thermostats, thermal cutout switchesand relief valves.


3

FOREWORD

Sponsorship

1. This Defence Standard (DEF STAN) is sponsored by the Warship Support Agency (WSA),Ministry of Defence (MOD).

2. The complete Standard 02-605 comprises:

Guide to the Selection of Sensors for the Measurement of System Parameters:

Part 1: Selection of Liquid Level, Temperature and Pressure Sensors;

Part 4: Selection of Flow Sensors;

Part 5: Selection of Torsionmeter Systems.

3. Any user of this Standard either within MOD or in industry may propose an amendment to it.Proposals for amendments that are not directly applicable to a particular contract shall be madeto the publishing authority identified on Page (i), and those directly applicable to a particularcontract shall be dealt with using existing departmental procedures.

4. If it is found to be unsuitable for any particular requirement, MOD shall be informed in writingof the circumstances.

5. No alteration shall be made to this Standard except by the issue of an authorized amendment.

6. Unless otherwise stated, reference in this Standard to approval, approved, authorized and similarterms means by the MOD in writing.

7. Any significant amendments that may be made to this Standard at a later date will be indicated bya vertical sideline. Deletions will be indicated by 000 appearing at the end of the line interval.

8. Extracts from British Standards quoted within this Standard have been included with thepermission of the British Standards Institution.

9. This Standard has been re-issued due to a Technical Update and the combining ofDEF STAN 02605 Parts 1, 2 and 3.

Conditions of Release

General

10. This Standard has been devised solely for the use of the MOD, and its contractors in the executionof contracts for the MOD. To the extent permitted by law, the MOD hereby excludes all liabilitywhatsoever and howsoever arising (including but without limitation, liability resulting fromnegligence) for any loss or damage however caused when the Standard is used for any otherpurpose.

11. This document is Crown Copyright and the information herein may be subject to Crown or thirdparty rights. It is not to be released, reproduced or published without written permission of theMOD.

12. The Crown reserves the right to amend or modify the contents of this Standard without consultingor informing any holder.

MOD Tender or Contract Process

13. This Standard is the property of the Crown. Unless otherwise authorized in writing by the MODit must be returned on completion of the contract, or submission of the tender, in connection withwhich it is issued.


4

14. When this Standard is used in connection with a MOD tender or contract, the user shall ensure thathe is in possession of the appropriate version of each document, including related documents,relevant to each particular tender or contract. Enquiries in this connection may be made to theauthority named in the tender or contract.

15. When Defence Standards are incorporated into MOD contracts, users are responsible for theircorrect application and for complying with contractual and any other statutory requirements.Compliance with a Defence Standard does not of itself confer immunity from legal obligations.

Categories of Standard

16. The Category of this Standard has been determined using the following criteria:

a. Category 1. If not applied may have a Critical affect on the following:

Safety of the vessel, its complement or third parties.

Operational performance of the vessel, its systems or equipment.

b. Category 2. If not applied may have a Significant affect on the following:

Safety of the vessel, its complement or third parties.

Operational performance of the vessel, its systems or equipment.

Through life costs and support.

c. Category 3. If not applied may have a Minor affect on the following:

MOD best practice and fleet commonality.

Corporate Experience and Knowledge.

Current support practice.

Related Documents

17. In the tender and procurement processes the related documents listed in each section and AnnexA can be obtained as follows:

a. British Standards British Standards Institution,389 Chiswick High Road,London, W4 4AL.

b. Defence Standards Defence Procurement Agency,An Executive Agency of The Ministry of Defence,UK Defence Standardization,Kentigern House,65 Brown Street,Glasgow, G2 8EX.

c. Other documents Tender or Contract Sponsor to advise.

18. All applications to Ministry Establishments for related documents shall quote the relevant MODInvitation to Tender or Contract number and date, together with the sponsoring Directorate and theTender or Contract Sponsor.

19. Prime Contractors are responsible for supplying their subcontractors with relevant documentation,including specifications, standards and drawings.


5

Health and Safety

Warning

20. This Standard may call for the use of processes, substances and/or procedures that may be injuriousto health if adequate precautions are not taken. It refers only to technical suitability and in no wayabsolves either the supplier or the user from statutory obligations relating to health and safety atany stage of manufacture or use. Where attention is drawn to hazards, those quoted may notnecessarily be exhaustive.

21. This Standard has been written, and shall be used, taking into account the policy stipulated inJSP 430 MOD Ship Safety Management System Handbook.

Additional Information

(There is no relevant information included)


6

CONTENTS

Page No

TITLE PAGE 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SCOPE 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FOREWORD 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sponsorship 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conditions of Release 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Categories of Standard 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Related Documents 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Health and Safety 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Additional Information 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CONTENTS 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 1. PERFORMANCE SPECIFICATION 10. . . . . . . . . . . 1.1 General 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Environmental Conditions 10. . . . . . . . . . . . . . . . . . . . 1.2.1 General 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Operating Conditions 10. . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Environmental Tests 11. . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Calibration and Accuracy 11. . . . . . . . . . . . . . . . . . . . . Figure 1.1 Accuracy Rating 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Range and Span Covered by the Various Types of

Sensor 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Stability 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Symbols and Units 13. . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 2. NATIONAL/INTERNATIONAL REGULATIONS 142.1 Certifying Authority 14. . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Standards Relating to Sensors and Measurement 14. 2.3 Calibration and Accuracy 14. . . . . . . . . . . . . . . . . . . . .

SECTION 3. MILITARY STANDARDS/REQUIREMENTS 14. . . . 3.1 General 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Policy Statement 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Calibration and Accuracy 15. . . . . . . . . . . . . . . . . . . . . 3.4 Output of Sensors 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Power Supply 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Size and Installation of Sensors 16. . . . . . . . . . . . . . . . 3.7 Environmental Conditions 16. . . . . . . . . . . . . . . . . . . . 3.7.1 Shock 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2 Radiation Resistance 17. . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3 Electromagnetic Compatibility 17. . . . . . . . . . . . . . . . . 3.7.4 Acoustic Noise 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


7

Page No

3.8 Hazardous Areas 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.1 Explosive Risk Areas 17. . . . . . . . . . . . . . . . . . . . . . . . . 3.8.2 Dangerous Areas 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.3 High Fire Risk Areas 18. . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 4. DESIGN REQUIREMENTS/GUIDANCE 18. . . . . . . 4.1 Selection of Sensors 18. . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 The Variable 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Level 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Temperature 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Pressure 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 The Application 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Measurement Techniques 20. . . . . . . . . . . . . . . . . . . . . Figure 4.1 Elements of a Transducer 20. . . . . . . . . . . . . . . . . . . . . 4.4.1 Sensing Element Devices/Techniques 21. . . . . . . . . . . . 4.4.1.1 Liquid Level 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1.2 Temperature 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1.3 Pressure 22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Transduction Element Devices/Techniques 22. . . . . . . 4.4.3 Signal Conditioning Element 23. . . . . . . . . . . . . . . . . . 4.5 Range of Various Types of Sensor 23. . . . . . . . . . . . . . . 4.5.1 Liquid Level 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Temperature 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2.1 Temperature Transmitters 23. . . . . . . . . . . . . . . . . . . . . 4.5.3 Pressure 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Calibration of Sensors 24. . . . . . . . . . . . . . . . . . . . . . . . 4.7 Accuracy of Sensors 24. . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1 Temperature 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.2 Pressure 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 Dynamic Characteristics 25. . . . . . . . . . . . . . . . . . . . . . 4.9 Size and Installation of Sensors 25. . . . . . . . . . . . . . . . 4.9.1 Liquid Level 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.2 Temperature 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.2.1 Pockets 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.2.2 Sensing Elements 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.2.3 Depth of Immersion 26. . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.2.4 Electrical Connections 26. . . . . . . . . . . . . . . . . . . . . . . . 4.9.3 Pressure 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.4 Connection Head 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Corrosion and Chemical Attack 27. . . . . . . . . . . . . . . . 4.11 Ingress Protection 27. . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 5. CORPORATE EXPERIENCE & KNOWLEDGE 28. 5.1 General 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Sensing Element Devices/Techniques 28. . . . . . . . . . . . 5.2.1 Pressure Sensors 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . .


8

Page No

5.2.2 Capacitance Sensors 29. . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Conductivity Sensors 30. . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Ultrasonic Sensors 31. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Optical Sensors 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6 Time Domain Reflectrometry 32. . . . . . . . . . . . . . . . . . 5.2.7 Floats 32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8 Displacers 33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9 Thermocouples 33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.10 Resistance Temperature Detectors RTDs 35. . . . . . . 5.2.11 Temperature Pockets 36. . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12 Thermistors 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.13 Semiconductor Temperature Sensors 36. . . . . . . . . . . . 5.2.14 Diaphragm 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.15 Capsule 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.16 Bourdon Tube 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Transduction Element Devices/Techniques 37. . . . . . . 5.3.1 Magnet/Reed Switches 37. . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Tape Float Transducers 38. . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Pressure Sensors 38. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 Capacitance Transducers 38. . . . . . . . . . . . . . . . . . . . . . 5.3.5 Conductivity Transducers 38. . . . . . . . . . . . . . . . . . . . . 5.3.6 Ultrasonic Transducers 38. . . . . . . . . . . . . . . . . . . . . . . 5.3.7 Optical Transducers 39. . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.8 Temperature Transmitters 39. . . . . . . . . . . . . . . . . . . . . 5.3.9 Strain Gauge 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.10 Piezoresistance 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.11 Variable Reluctance 40. . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.12 Capacitance 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.13 Potentiometer 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.14 Force Balance 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.15 Vibrating Wire 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.16 Linear Variable Differential Transformer 41. . . . . . . . 5.3.17 TDR Transducer 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Output of Sensors 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Liquid Level 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Temperature 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2.1 Resistance Temperature Detectors 42. . . . . . . . . . . . . . 5.4.2.2 Thermocouples 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3 Pressure 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Calibration of Sensors 43. . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Liquid Level 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Temperature 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2.1 RTDs 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2.2 Thermocouples 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.3 Pressure 44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Accuracy of Sensors 44. . . . . . . . . . . . . . . . . . . . . . . . . .


9

Page No

5.6.1 Liquid Level 44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2 Temperature 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2.1 Resistance Temperature Detectors 45. . . . . . . . . . . . . . 5.6.2.2 Thermocouples 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Pressure 46. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ANNEX A RELATED DOCUMENTS 47. . . . . . . . . . . . . . . . . . . .

ANNEX B ABBREVIATIONS AND DEFINITIONS 49. . . . . . . . B.1 Abbreviations 49. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2 Definitions 50. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ANNEX C PROCUREMENT CHECK LIST 54. . . . . . . . . . . . . . .

ANNEX D SENSOR PERFORMANCE 57. . . . . . . . . . . . . . . . . . . D.1 Performance of Typical Liquid Level Sensors 57. . . . . D.2 Performance of Typical Temperature Sensors 58. . . . D.3 Performance of Typical Pressure Sensors 59. . . . . . . .

ANNEX E Documents Providing Background Information 61. . . E.1 Documents Providing Background Information 61. . .

ALPHABETICAL INDEX 62. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


10

1. PERFORMANCE SPECIFICATION

Related Documents: BS EN 60068; BS 2011 Part 2; BS 5775; see also Annex A.

1.1 General

a. For many applications, commercially available sensors may meet the requirementsof HM Surface Ships and Submarines. The possibilities of cost reductions byselecting such sensors shall be considered when the System RequirementDocumentation (SRD) is being prepared.

b. Manufacturers data sheets shall be examined to determine whether the claimedperformance of commercially available sensors, transducers and measuringtransmitters meets the requirements for a particular application. Consideration shallbe given to the requirements for normal operation, to the operating limits, and to thetransport and storage limits.

c. Annex D provides performance data for various types of sensor in commercial use.

1.2 Environmental Conditions

1.2.1 General

a. Sensors must be capable of continuous operation with unimpaired performance andadequate reliability. The equipment to which the sensors are attached will besubject to environmental stress, the severity of which will depend upon its locationin the vessel.

1.2.2 Operating Conditions

a. Manufacturers provide information about the performance of their products undervarious operating conditions. Information is also provided about the influence ofenvironmental changes. There are different conditions for:

(1) Reference performance;

(2) Normal operation;

(3) Operating limits;

(4) Transport or storage.

b. The reference performance of a sensor is achieved within upper and lower limits forreference operating conditions, within which the influences on the sensor bychanges in environmental conditions are small enough to disregard.

c. The normal operating conditions is the range of conditions the sensor is designed tooperate within and satisfy specified accuracy limits without any adjustment.

d. The operating limits are the range of conditions to which a sensor may be subjectedwithout permanent impairment of characteristics, adjustment may be required afterexcursion to the operating limits.

e. The transport and storage limits are the range of conditions to which equipment maybe subjected, without operating and remain undamaged, although adjustment maybe required after transport or storage.


11

1.2.3 Environmental Tests

a. It is the responsibility of the procurement authority to ensure that sensors procuredfor Royal Navy service are fit for purpose. Manufacturers shall demonstrate thatcommercially available sensors provide the degree of environmental hardnessrequired for a particular application.

b. It is the responsibility of Marine Domestics Detection, Monitoring, Steering andStabilising Systems (MDMS) Integrated Project Team (IPT) to ensure that the SRDonly includes tests essential for satisfactory operation and to avoid unnecessarygroups of tests.

c. Only a few sensor manufacturers claim compliance with British Standards (BS) forperformance under environmental stress. BS 2011 Part 2 and BS EN 60068 seriesdescribe such procedure, including tests, with guidance notes and backgroundinformation. Most sensor suppliers can provide information on such performance.

1.3 Calibration and Accuracy

a. The SRD shall state the required accuracy rating. A certificate of calibrationtraceability shall be called for only when necessary.

b. The term accuracy rating means the guaranteed limits to inaccuracy for a givensensor. Such inaccuracy stems from the combined effects of a lack of conformity tothe specified characteristic curve (normally linear), of hysteresis, dead band,repeatability errors and of any other features stated in the specification. Thisconcept is illustrated in Figure Figure 1.1.

c. Manufacturers refer to errors in a variety of ways; definitions and terminologyused in this Standard and the SRD are defined in Annex B.


12

Figure 1.1 Accuracy Rating


13

1.4 Range and Span Covered by the Various Types of Sensor

a. The range, within which the specified accuracy of measurement shall be achieved,shall be stated in terms of upper and lower limits and specified in the SRD.

b. If adjustment of span and zero is required, it shall be stated together with anyrequirements for suppressed or elevated zero.

c. Sensors are designed to withstand inputs in excess of their range, up to theoverrange limit. The maximum input that a sensor is required to withstand withoutsuffering damage or permanent change in performance or accuracy shall be stated.Such values are usually expressed in units rather than in multiples of the range.

1.5 Stability

a. In many applications, stability is more important than accuracy. Sensors,transducers and transmitters are designed to retain their performance under theirnormal operating conditions, which will usually include wide variations of ambienttemperature, pressure and humidity. The possibility that the accuracy of a sensormay be impaired by the medium it monitors shall be taken into account.

b. Some means of zero adjustment shall be considered, as an alternative to imposingunusually severe requirements for stability.

c. Preference shall be given to transducers, sensors and transmitters that do not sufferfrom drift over long periods.

d. Sensors that are electrically energised require a finite time (warmup period) beforethe rated characteristics apply. Drift can occur during the warmup period.

1.6 Symbols and Units

a. The SRD shall be written in metric units using the International System of Units(SI), unless the interface requirements make necessary the use of the Imperial andother systems.

b. Temperature is commonly measured in C, and is equal in magnitude to the SI unitKelvin. It is preferential to use the unit C for the SRD.

c. BS 5775 (ISO 31) states the specification for quantities, units and symbols to beused.


14

2. NATIONAL/INTERNATIONAL REGULATIONS

Related Documents: There are no related documents referred to in this section.

2.1 Certifying Authority

a. The Certifying Authority for Britishmade electrical equipment for use inDangerous Areas and High Fire Risk Areas is the Electrical Equipment CertifyingService (EECS), incorporating the British Approvals Service for ElectricalEquipment in Flammable Atmospheres (BASEEFA). There are equivalentoverseas national authorities for nonBritish equipment.

2.2 Standards Relating to Sensors and Measurement

a. Annex E. lists BS which cover terminology, documentation and test procedures formeasuring instruments, process control equipment and electronic measuringequipment, and which could be applied to sensors.


a. Sensors and transducers for applications where the highest accuracy is essentialmay be calibrated using instruments that have been calibrated and certified by theBritish Calibration Service. They are then said to be traceable to the NationalPhysical Laboratory (NPL) standards.

3. MILITARY STANDARDS/REQUIREMENTS

Related Documents: see Annex A.

3.1 General

a. The SRD shall draw attention to applications whose satisfactory performance isvital to the safety or efficiency of the Ship, Submarine or personnel.

b. Typically, these will be found in alarm systems, in closed loop control systems, orwhere the sensor forms part of a critical pressure boundary. In such cases, it may bea requirement to invoke formal Quality Assurance procedures. For Submarines,SSP 25 shall be consulted.

c. Rationalisation of sensors throughout vessels or systems, taking account of benefitsof commonality with existing in-service sensors shall be observed.

3.2 Policy Statement

a. Where procurement of sensors is the responsibility of a Main Shipbuilder or PrimeContractor, the procurement authority shall produce a statement, which will clearlyindicate the procurement policy to be followed.

b. The object of the Policy Statement shall establish limits for procurement, so that acoordinated approach to sensor selection is made across all ship systems andsubsystems.

c. The Policy Statement shall include, but not be limited to:


15

(1) Reference to project papers that shall be consulted;

(2) Rationalisation of proposed sensors against existing in-service sensors shallbe completed;

(3) Statements on standardisation of electrical and mechanical aspects andmeasurement ranges;

(4) Reference to purchasing policy;

(5) Reference to this Standard;

(6) Statements on preference for selection or rejection of particular items;

(7) Statements on defined vessel shock and environmental standards.

d. The Policy Statement shall cover all the major parameters likely to be measured andshall be agreed with MOD prior to implementation. Once agreed, it shall be appliedacross all systems and equipments being procured for the project.


a. DEF STAN 02797 Part 1 requires that where a temperaturesensing device isconnected to an electrical transmitter, an adjacent pocket shall be provided for a testthermometer to ensure the sensing element is operating within the specifiedperformance limit.

3.4 Output of Sensors

a. The sensor converts or transduces the input variable into an output signal suitablefor measurement, as shown in Figure 4.1.

b. Where practical, outputs shall be of the form 420 mA dc in accordance with BS5863, Part 1, or 115 v, 05 v, 010 v or 10 v in accordance withBS 5863, Part 2. In the case of the 420 mA signal, the 0 mA value is reserved forindication of a signal circuit or power supply failure.

c. BS 6174 specifies requirements for dc powered electrical transmitters withanalogue direct current output used in differential pressure measurements.BS 6447 specifies requirements for such transmitters used in absolute and gaugepressure measurement. The scope of these two standards states that they do notapply to marine applications. This does not imply that transmitters conforming tothese standards are unsuitable for marine applications.

d. Limits to the output noise and to the output regulation shall be stated in SRD.

3.5 Power Supply

a. DEF STAN 61-5, Part 4 provides information on the electrical power supplysystems on HM Surface Ships and Submarines.

b. The power supplies and earthing arrangements for sensors, transducers andtransmitters shall be selected in accordance with DEF STAN 08-107.

c. Power supply requirements will usually be determined by the instrumentationsystem of which the sensor forms a part. Where known they shall be stated in theSRD.


16

d. Consideration shall be given to the power consumption which will result from anumber of transducers, feeding into the output load, to ensure an adequate supply isavailable. Where it is necessary to limit the power consumption, the acceptablelimit shall be stipulated on the SRD.

e. Temperature transmitters are available which can transduce the signals from anumber of temperature sensors, all provided from one power supply. The use ofsuch systems shall be considered in applications where many measurements shallbe made in one compartment or on one equipment.

f. Sensors and transducers which are sensitive to voltage spikes may need externalprotection, or may be supplied with builtin protection circuits. The maximumvoltage and duration of spikes likely to be seen shall be stated in the SRD.

3.6 Size and Installation of Sensors

a. The siting of sensors, design of associated pipe work and installation and security offittings shall be in accordance with the requirements of DEF STAN 02-797 Part 1.

b. Reference shall be made to BR 3013(2), to determine the type of pipe connectionscalled for in the SRD.

c. The SRD shall ensure the use of properly resistant materials, finishes and protectionfor sensors and associated items in accordance with DEF STAN 08-107, DEFSTAN 07-224 and DEF STAN 21-5.

d. In general, a separate process connection shall be considered for each instrumentand the locations of tapping points shall take account of the nature of the variableinvolved.

e. Installations of sensors in fuel systems for gas turbines and diesel engines shallcomply with DEF STAN 07-220 and for aviation fuel systems DEF STAN 07-219.

f. Installations of sensors for gasoline systems shall comply with DEF STAN 02-775.

g. The sensing point for any monitoring purpose shall be located at the precise pointfrom which the information is required. The sensing point for an automaticallycontrolled process shall be situated at the point where the controlled condition isrequired and is subject to specific MOD approval. The sensing points for anyautomatic control system shall be independent of those used for otherinstrumentation purposes.

3.7 Environmental Conditions

a. DEF STAN 08-123 states the requirements for equipment design and testing inrespect of environmental conditions. Sensors shall be capable of operating underthe conditions stated in DEF STAN 08-123 and DEF STAN 00-35 Parts 1, 3, 4, 5and 6 as applicable.

b. The Installation Region Reference shall be stated in the SRD.

c. Where the variable to be measured is subjected to temporary environmentalconditions due to operational circumstances, details shall be stipulated in the SRD


17

to ensure that the most appropriate type of sensor is selected. Such changes mayinclude but not be limited to, changes in pressure, temperature or working levels.

3.7.1 Shock

a. The Ship Shock Specification shall be stipulated in the SRD.

b. The requirements for determining the shock strength of equipment are stated inDEF STAN 08-120.

c. The sensors, and parts thereof, must remain captive, i.e. not become a projectilewhen subjected to shock not exceeding the specified level.

3.7.2 Radiation Resistance

a. Sensors attached to equipment intended for operation in the unshielded vicinity ofnuclear reactors may be subject to prolonged exposure to nuclear radiation. Undersuch conditions, the radiation levels shall be stated in the SRD.

3.7.3 Electromagnetic Compatibility

a. Sensors installed in HM Surface Ships and Submarines are subject to a hostileelectromagnetic environment. The SRD shall specify the electromagneticenvironment to which they will be subjected, the quality of the power supply tothem and tests requirements when necessary.

b. Reference shall be made to DEF STAN 59-41, Parts 1 to 4 regardingelectromagnetic compatibility.

c. Reference shall be made to suppliers literature to ascertain what claims (if any) aremade for electromagnetic compatibility of sensors.

3.7.4 Acoustic Noise

a. Sensors for equipment located in aircraft hangars, on flight decks, close to dieselgenerators or gasturbine exhaust outlets, may be required to withstand intenseairborne acoustic noise. This may cause interference or failure through fatigue.

b. Installations in such regions shall be identified in the SRD.

3.8 Hazardous Areas

3.8.1 Explosive Risk Areas

a. Sensors and transducers that shall be fitted in magazines, adjacent compartmentsand designated danger areas, as defined in DEF STAN 07-228 and BS EN60079-14, shall comply with the requirements of the appropriate standard.

b. The SRD may require sensors and associated equipment to satisfy the requirementsfor electrical apparatus in explosive atmospheres. Certificates of compliance issuedby BASEEFA shall be provided to the MOD under such circumstances.

c. Gasoline compartments shall be treated as magazines in respect of precautionsagainst fire.


18

3.8.2 Dangerous Areas

a. Any compartment or space where highly flammable or flammable material isstored, handled or distributed is termed a Dangerous Area. Certain adjacent zonesare similarly classified. A list of compartments and spaces, which shall be regardedas Dangerous Areas, will be stated in the SRD.

b. Reference shall be made to DEF STAN 01-5 for the flash point classification offuels, lubricants and associated products for MOD use.

c. Sensors and transducers within the scope of this Standard are not, except whereessential, to be fitted in Dangerous Areas, this shall prevent serious interferencewith the service for which the compartment is intended. If so fitted, such equipmentshall be certified as intrinsically safe in accordance with BS EN 50020 and inconjunction with BS EN 50014, this equipment shall be installed in accordance withBS EN 60079-14.

d. The cabling, installation, and fittings for sensors, transducers and associatedequipment fitted in Dangerous Areas shall be in accordance with DEF STAN07-228 and DEF STAN 02-775.

3.8.3 High Fire Risk Areas

a. High Fire Risk Areas include Dangerous Areas and also:

(1) Main and auxiliary machinery spaces;

(2) Diesel Generator Compartments;

(3) Gas Turbine Generator Compartments;

(4) Uptakes and Downtakes;

(5) Ventilation trunks from machinery spaces up to the close down flaps;

(6) Machinery removal routes and escape trunks associated with the above listedcompartments;

(7) Fuel pump spaces.

b. Explosive vapour concentrations of Petroleum, Oils and Lubricants Class Cproducts may be present in storage tanks above the liquid surface, in conditions ofhigh ambient temperature and in compartments where system leaks have createdvapour mists. Storage tanks in these conditions shall be treated as containingexplosive vapour.

4. DESIGN REQUIREMENTS/GUIDANCE


4.1 Selection of Sensors

a. The type of sensor required will depend on the following:

(1) Variable which shall be measured;

(2) Application which shall be served;


19

(3) Functional requirements of the system of which it forms a part;

(4) Conditions under which it must operate satisfactorily;

(5) Accessibility for installation and maintenance.

4.2 The Variable

a. The performance characteristic or variable that shall be measured shall be statedprecisely and without ambiguities, to enable a correct choice of sensor to be made.

b. The pressure, viscosity, temperature, density, electrical conductivity, flow rate anddielectric constant of the variable over which the sensor shall perform shall be statedif appropriate.

c. The presence of any corrosive material, additives or inclusions, which could causedamage to sensors shall be declared.

4.2.1 Level

a. For continuous measurements of liquid level, the range, zero and maximum levelsshall be stated.

4.2.2 Temperature

a. Measurements of the temperature of the contents of pipes, tanks and containersshall be distinguishable from measurements of solid objects such as bearings, andfrom measurements of compartment or equipment temperatures.

4.2.3 Pressure

a. Gauge pressure, absolute pressure, differential pressure, vacuum pressure andcompound sensors (i.e. above and below atmospheric) shall be distinguishable.

4.3 The Application

a. The SRD shall state whether it refers to a sensing element, a probe, a transducer, atransmitter, or a complete measuring system. The application description shall statethe equipment on which it shall be used and its mounting arrangements.

b. The SRD should clearly state the location(s), type of display and the unitmeasurements to be employed in each application.

c. Where the same performance characteristics shall be measured in more than oneapplication, concerning one or more substances, reference shall be made to therelated applications. Consideration should be given to using common designs, withcommon ranges and a limited number of mechanical arrangements.

d. The majority of sensors make a negligible physical impact on the quantity orcondition being measured. Where this is not the case, the system design mayrequire this impact to be limited. Any such limitations shall be stated in the SRD.

e. Where sensors are required to measure tank contents or ullage, the SRD shall statewhether a percentage, or unit volume measurement is required and what units shall


20

be employed. Adequate dimensional information for the tank and any inclusionsshall be provided.

f. Temperature sensors, fitted in pockets within pipes containing a flowing fluid, arenot to cause the fluid velocity to exceed the permitted maximum due to obstructionby the pockets.

4.4 Measurement Techniques

a. Sensors and transducers contain three elements in order to carry out the function ofdetecting and measuring a change and generating a signal which can be coupled toan electronic system or indicating equipment. These are illustrated in Figure 4.1.

Changing Variable

Sensing Element

Transduction

SignalElectrical Output

Element

Conditioning

Element

Figure 4.1 Elements of a Transducer

b. The three elements may be combined in a single package, or the signal conditioningcircuitry may be remote from the sensor, and is sometimes designed to serve morethan one sensor.

c. Where an electrical output is required to interface with a platform managementsurveillance system, sufficient technical detail of any such interface shall beprovided in the SRD. The exact nature of each interface shall be agreed by allparties concerned.


21

4.4.1 Sensing Element Devices/Techniques

4.4.1.1 Liquid Level

a. Sensors for detecting changes in liquid level, which are in common use, employ oneof the following eight types of sensing element:

(1) Pressure Sensors;

(2) Capacitance Sensors;

(3) Conductivity Sensors;

(4) Ultrasonic Sensors;

(5) Optical Sensors;

(6) Time Domain Reflectrometry (TDR);

(7) Floats (not recommended);

(8) Displacers (not recommended).

NOTE For more details on these sensors refer to Section 5.

b. Other techniques, which are known to be under investigation or development,include differential temperature devices.

4.4.1.2 Temperature

a. Sensing elements for detecting change in temperature which are commonly usedwith electrical measuring systems are:

(1) Thermocouples;

(2) Resistance Temperature Detectors (RTDs);

(3) Thermistors;

(4) Semiconductors.


b. Of the four types of commonly used elements, thermocouples and RTDs are closelygoverned by internationally acceptable standards. Other types of temperaturesensing device include pyrometers and optical fibre sensors.

c. Flat devices are available for use in situations where a sensor and pocket cannot beinstalled such as a small diameter pipe. There is no standard, which covers theshape, size and fixing method of flat devices.


22

4.4.1.3 Pressure

a. Pressure sensing elements, which are commonly used with electrical measuringsystems are:

(1) Diaphragm;

(2) Capsule;

(3) Bourdon Tube.


4.4.2 Transduction Element Devices/Techniques

a. Conversion of the mechanical displacement of a sensing element into an electricalsignal can be achieved using a variety of techniques, some of which are exclusive toa particular type of sensing element. Further detail is provided at Section 5. Optionscurrently available include:

(1) Magnetic/Reed Switches;

(2) Tape Float Transducers;

(3) Pressure Sensors;

(4) Capacitance Transducers;

(5) Conductivity Transducers;

(6) Ultrasonic Transducers;

(7) Optical Transducers;

(8) Transmitters;

(9) Strain Gauges;

(10) Piezo-Resistance;

(11) Variable Reluctance;

(12) Capacitance;

(13) Potentiometer;

(14) Force Balance;

(15) Vibrating Wire;

(16) Linear Variable Differential Transformer.


23

4.4.3 Signal Conditioning Element

a. In addition to the sensing and transduction elements described in Clause 4.4.1 andClause 4.4.2, transducers usually incorporate electronic circuitry which carries outsome or all of the following functions:

(1) Amplification of the signal from the transduction element;

(2) Linearization of the signal;

(3) Square root of the signal (for flow measurements);

(4) Compensation for temperature effects;

(5) Zero and span adjustments.

b. Many level sensors are provided with microprocessors and programs, whichcompute the tank contents or ullage from the measurement of liquid level.

4.5 Range of Various Types of Sensor

a. Fixed spans shall be specified for applications in which the measured value areknown from experience. Adjustable spans are useful where calibration depends onindividual site parameters.

b. Adjustable zero is usually expressed as a percentage of span. The span and the zeroselected are not to exceed the manufacturers recommended range.

c. Adjustment to span and zero are usually by means of easily accessible screws, whilerange changes are typically provided for by internal switches.

4.5.1 Liquid Level

a. To measure tank contents, zero will normally correspond to an empty tank, i.e. thelevel of the lowest wetted surface. The upper limit will correspond to the surfacelevel when the tank is full to its designed capacity. An elevated zero set point may berequired for tanks where a constant level or interface is required. A suppressed zeromay be required for tanks that always contain some minimum amount of liquid.

b. Liquid level sensors are designed to withstand levels above their upper limit ofrange.

4.5.2 Temperature

a. Measurements of all the temperatures encountered in RN vessels can be made,either with an RTD or with a thermocouple.

b. The sensors range will depend on the method of construction and on the materialsused; manufacturers literature shall be consulted for details of the range for specificdevices.

4.5.2.1 Temperature Transmitters

a. Transmitters can provide an indication of loss of input, by driving the output currentup-scale if the RTD or thermocouple goes open-circuit. Any consequent impact onthe load limitation data shall be stated.


24

4.5.3 Pressure

a. Consideration shall be given to the burst pressure of transducers, this is sometimesreferred to as the seal pressure.

4.6 Calibration of Sensors

a. The manufacturers check the calibration of sensing elements as part of theproduction process to satisfy the specified requirements at the time of ordering.

b. Factory calibration is carried out at ambient temperature and pressure and atmaximum range. The SRD shall clearly state any requirement for calibration underother conditions.

c. Sensor calibrations tend to drift with time and consideration shall be given topractical calibration intervals. Factors affecting recalibration intervals include:

(1) Properties of the variable to be measured;

(2) Possibility of chemical attack on, or fouling of floats, displacers andelectrodes;

(3) Vibration and shock effects which may cause small movements in mechanicalassemblies;

(4) Ease of access;

(5) The need to maintain accuracy;

(6) Use of a capacitance sensor for a liquid different from that for which it wasoriginally calibrated.

d. To ensure correct in-service measures are completed, suppliers shall specify anycalibration and maintenance procedures necessary, their periodicity and associatedspecial test equipment.

e. For very accurate measurements, recalibration is carried out before everymeasurement, but for practical situations a lowdrift sensor can be expected toperform adequately for 6 months to 2 years.

f. When a sensor is replaced, changes in the orientation and alignment may introducea small error that requires calibration.

g. A calibration curve or other calibration record is sometimes provided with thesensor.

4.7 Accuracy of Sensors

a. For the purpose of specification or selection, comparisons of a combination oflinearity, hysteresis and repeatability errors, temperature effects and drift ofdifferent sensors shall be considered.

b. Stability and repeatability are particularly important. It is also important that shipsmovements should not introduce significant errors, due to the attitude of the sensoror to accelerations imposed upon it. Manufacturers claims relate to land basedapplications, the ability to damp out the effects of ship motion shall be taken intoaccount.


25

c. Drift over a long period may be detected as a zero shift and as a span shift, but theseare normally combined as a single drift figure expressed as a percentage of span.

4.7.1 Temperature

a. Reference shall be made to BS EN 60751 for the tolerances that apply to platinumRTDs, to BS EN 605842 for thermocouples, and to manufacturers literature fordevices fabricated from other materials.

4.7.2 Pressure

a. BS 6174 and BS 6447 require an accuracy test for pressure transmitters to measureerror, dead band and repeatability after preconditioning and under specifiedatmospheric conditions. An accuracy class figure is then calculated by combiningthe modulus of the measured error and repeatability.

4.8 Dynamic Characteristics

a. The sensing element of a transducer has a natural frequency of oscillation. Dampingcan be provided to reduce amplitude of these oscillations following a step change ofmeasurand. In some transducers, adjustment of damping is provided. The speed ofoperation of the transducer is affected by the degree of damping, which is specifiedeither by time constant or by damping factor.

b. In all applications, consideration shall be given to the need to specify the degree ofdamping and frequency response that is required. Such requirements shall be statedin the SRD.

4.9 Size and Installation of Sensors

a. Consideration shall be given when siting sensors to the space required for removalof covers, any adjustments that may be needed, and access for replacement orrecalibration.

b. Restrictions on acceptable size, weight and on means of access for installation,connections and replacement shall be stated in the SRD.

c. The surfaces provided for location of sensors shall be adequately dimensioned forsquareness and fit, and shall meet the manufacturers mounting recommendations.

d. Where pulsation of pressure is expected, fitting pulsation dampers may beconsidered. System designers shall be consulted in the use of such dampers.

e. Sensors offered with electrical connections in the form of flying leads with factorymade seals are preferred to screw terminals, or plugs and sockets. Manufacturersrecommendations on the maximum length and resistance of flying leads shall betaken into account in the selection of sensors.

f. Some transducers incorporate protection against electromagnetic interference andvoltage spikes. In general, twisted-pair conductors are adequate for mosttransducer outputs, but coaxial-screened conductors are sometimes recommended.

g. The dimensions of flange mountings shall comply with BS 1560 or BS 4504-3.3 asappropriate. The dimensions of probes and sensors that are mounted on pipethreads shall comply with BS 21 or BS 2779 as appropriate.


26

h. The electrical and mechanical connections to sensors, probes and transmitters shallbe stated in the SRD as follows:

(1) Number, current and voltage capacity, requirements for earthing, insulation,screening (if any), and coding of electrical connections for power supply,signal output, and local test;

(2) Method of electrical connection, whether by terminal block, flying leads,plug or socket, or other method. Terminal blocks and flying leads are moreresistant to vibration, provided that the cables are adequately secured. Whenflying leads are chosen, their length and method of termination shall bespecified;

(3) Size and method of mechanical connection, including where necessary anyliquid seals, filters, pulsation dampers or other ancillary devices which haveto be allowed for.

4.9.1 Liquid Level

a. In applications where accuracy is important and/or recalibration is frequentlynecessary, level sensors shall be installed in close proximity to sounding tubes.

b. The zero of a diaphragm pressure sensor depends on the attitude of the diaphragm(the span is not affected by change of attitude).

4.9.2 Temperature

4.9.2.1 Pockets

a. Whenever possible, sensors and pockets in accordance with BS 2765 shall beselected.

b. Agreement to dispense with the use of a pocket shall be approved by the appropriateequipment IPT of the equipment being monitored.

4.9.2.2 Sensing Elements

a. Sheaths for sensing elements to BS 2765 are rigid and are not to be bent, since thismay irreparably damage the element. RTDs are particularly prone to bendingdamage.

b. Where it is necessary to make two measurements of temperature at one point, theuse of a duplex probe shall be considered.

4.9.2.3 Depth of Immersion

a. Reference shall be made to BS 1041 Parts 3 and 4, BS 2765 and to manufacturersspecifications to determine the optimum depth of immersion when selectingsensors and pockets.

4.9.2.4 Electrical Connections

a. Reference shall be made to BS 1041 Part 3 for information on the number, type, sizeand routeing of the connections to RTDs.


27

b. The colour coding of the connections to RTDs shall be in accordance with BS EN60751.

c. Reference shall be made to BS 1041 Part 4 for information on the dimensions ofthermocouple leads and on the use of compensating cable and extension leads.

d. When selecting thermocouple compensating leads, extension leads and equipmentwire, preference shall be given to the types listed in DEF STAN 02-512 Part 7, andto types which are colour coded in accordance with BS 4937 Part 30.

4.9.3 Pressure

a. Sizes and shapes of pressure sensors and transducers depend on the manufacturers;there are no standards for interchangeability.

4.9.4 Connection Head

a. Within the connection head are the terminals for the sensor connections. The headmay also house a transmitter.

b. These circuits are separated from the sensor:

(1) When they cannot be accommodated at the measuring point for reasons ofspace, access or environmental stress;

(2) When the electrical power present at the measuring point has to be limited toavoid risk of an explosion.

NOTE In such applications, the manufacturers recommendations for the type ofinterconnection, and any requirements for a zener barrier to achieve acertified level of safety, shall be strictly adhered to.

c. The head is sealed for protection, typically to IP 42, see clause 4.11. Flameproofvarieties are available for use in explosive atmospheres.

4.10 Corrosion and Chemical Attack

a. Sensors and any connecting pipework and associated transducing electronics aresubject to corrosion and chemical attack caused by the variable to be measured orharmful substances that may be present.

b. Certain types of chemical attack can reduce the working life of thermocouples. BS1041 Part 4 lists the most significant substances that attack the variousthermocouple combinations, and the factors that limit their effective life.

c. Reference shall be made to BR 1326 to ensure that sensors do not incorporate any ofthe metallic or nonmetallic materials listed as unsuitable for Submarines.

4.11 Ingress Protection

a. Sensors may be protected against the ingress of unwanted materials by the provisionof an enclosure. The same enclosure may protect persons against contact withelectrically live or moving parts of the sensor.

b. Ingress Protection, or IP code is represented by a twocharacteristic numeral codeas defined in BS EN 60529. The category of enclosure required shall be stated in the


28

SRD. The degree of protection afforded by the equipment or subassembly uponwhich the sensor shall be mounted shall be taken into account.

5. CORPORATE EXPERIENCE & KNOWLEDGE


5.1 General

a. By themselves, sensors cannot usually be employed in measurement systems. Atransduction element is necessary to quantify the sensing process and produce anoutput signal. This, in turn, may require processing by a signal processing stage.

b. The terms sensor and transducer have become synonymous in commercial usage,and many manufacturers prefer to describe their products as transducers. Where itis required to convey the strict meaning of the term, sensor ambiguity would beavoided by using the term sensing element.

5.2 Sensing Element Devices/Techniques

5.2.1 Pressure Sensors

a. The hydrostatic pressure exerted by a liquid in a tank depends on the height of thecolumn of liquid above the measuring point, on the SG of the liquid and on thepressure of the gas above the surface. A differential pressure sensor may thereforebe used to sense the liquid level. The sensor is usually mounted at a selected datumposition on the side of the tank and close to the bottom, either flush with the wall ofthe tank or with an extension to clear the wall and lining.

b. An alternative design permits the sensor to be fully submerged, mounted either on abracket or attached to the base of the tank, or at the lower end of a vertical rodsuspended from a flange formed on the horizontal cover of the tank, or suspendedby a cable.

c. The highpressure side of the differential sensor is usually connected to the liquid inthe tank. If the tank is vented to atmosphere, the lowpressure side is similarlyvented, otherwise the low pressure side is connected to the top of the tank, above thehighest liquid level. Fully submerged internally mounted sensors are provided witha capillary tube giving access to the lowpressure side of the sensor, which isbrought out to atmosphere.

d. In the case of closed tanks containing volatile liquids, it is necessary to ensure thatliquid does not condense in the reference connection and cause an error inmeasurement. This can be achieved by:

(1) Arranging a downward slope at the top of the reference connection (so thatany condensate flows back into the tank);

(2) Insulating and if necessary heating the reference connection;

(3) Using a condensate pot to provide a constant level of liquid on the highpressure side of the sensor and connecting the low pressure side to the bottomof the tank;


29

(4) Filling the reference pipe with a reference liquid and the zero adjusted tocompensate for the head pressure it exerts.

NOTE These arrangements are known as wet leg.

e. Closed tanks containing liquids which are not volatile in any ambient or operatingconditions do not require these precautions. The arrangement is then known as dryleg.

f. Differential pressure sensors are widely used for measurements of liquid level.They provide greater sensitivity and accuracy than floats or displacers; they benefitfrom modern developments in pressure sensor technology and are available atcompetitive costs. They are compact devices, which perform reliably in hostileconditions and with corrosive liquids. They can be used with viscous fluids andwith liquids which contain suspended solid matter. They are resistant to shock andvibration and their diaphragms are less likely to clog than floats or displacers.

5.2.2 Capacitance Sensors

a. A capacitor consists of two conducting materials separated by an insulator. Itselectrical capacitance is determined by the area of the conductors, their distanceapart, and the dielectric constant of the insulator. Capacitance sensors detect thechange in capacitance which occurs when a liquid replaces the gas in the spacebetween two electrodes, or at a changing interface between two liquids of differentdielectric constants.

b. Capacitance sensors are used for continuous measurement of liquid level, and fordetecting level at a set point. They are suitable for use with fuel and lubricating oils,water, hydraulic fluid, glycol and many other liquids, both conducting andnonconducting. They are also used with granular and other solid materials.

c. The dielectric constant of a liquid changes with temperature. Sensors required tooperate over a wide temperature range therefore require compensation.

d. Capacitance sensors generally take the form of a cylindrical probe or rod, mountedvertically for continuous measurement, or horizontally for set point detection. Thehead of the probe or electrode contains the transducer electronics and provides asuitable fitting to the tank. Probes are supplied to the required length.

e. They are made of steel, aluminium or stainless steel. The sensors are usuallylocated inside the tank containing the liquid. Alternatively, the electrode may beinstalled inside an external pipe connected to the tank.

f. Bare metal probes are used to measure the level of nonconductive liquids. Forconductive liquids, an insulated probe is required.

g. The probe head insulates the probe from the tank. If the tank wall is electricallyconductive, it acts as the earth electrode of the sensor. If the tank is nonconductivean earth electrode must be provided by either:

(1) Fitting a concentric tube electrode around the probe and connecting it toearth; or

(2) Installing a metal strap vertically within the tank and connecting it to theprobe head earth.


30

h. The span of a capacitance levelmeasuring system depends on the differentialcapacitance of the probe over the range of levels to be measured.

i. The selection of a capacitance probe for a specific application is to be made inconsultation with the manufacturer concerned.

j. Capacitance probes are simple devices with no moving parts and can be designed towithstand shock, vibration and corrosion. They may be used at various operatingpressures and temperatures. They perform satisfactorily in ships, aircraft and roadvehicles.

k. Measurements of liquid level are subject to error if:

(1) The probe becomes coated with a conductive material;

(2) The dielectric constant of the liquid changes;

(3) The tank to be measured is filled with a liquid of different dielectric constant.

5.2.3 Conductivity Sensors

a. The level of a liquid which conducts electricity can be sensed by conductivitysensors. Conductivity sensors can also detect interfaces between liquids ofdifferent conductivity.

b. A conductivity sensor consists of a conducting electrode fitted to an insulatedelectrode holder. The electrode is installed vertically from above the surface of theliquid to provide a point measurement at its lower tip. Multiple electrodes ofdifferent lengths can be arranged to give a succession of measurements at differentlevels.

c. Electrodes for horizontal mounting are also supplied, but are subject to error if theliquid film bridging the probe to earth does not run off when the contents fall belowthe set point.

d. Monel and other corrosion resistant alloys are available for use with aggressiveliquids. Electrodes may be covered (to within a cm or so from the tip) with a plasticinsulator to prevent bridging of adjacent multiple electrode assemblies and toreduce errors due to leakage in humid conditions.

e. The conductivity sensor is supplied with a low voltage, one pole is connected to theprobe electrode, and the other to the container wall. (A second electrode is used incontainers that are nonconductive.) When the liquid level is below the electrodetip, the circuit resistance is high. When the level reaches the probe, a low resistancepath is formed. The change in resistance is thus an indication of liquid level.

f. The conductivity sensing circuit is designed so that the ac current flow is lowenough to avoid electrolytic decomposition. The resistance change which triggersthe output signal is adjustable in range. Sometimes the response time is alsoadjustable.

g. Conductivity level sensors are simple, low cost devices, for point, or multipoint,measurement and control of conductive liquids. Errors will arise if the electrodesare coated by nonconductive materials.


31

h. The selection of a conductivity sensor for a specific application is to be made inconsultation with the manufacturer concerned.

5.2.4 Ultrasonic Sensors

a. Ultrasonic liquid level sensors employ a piezoelectric crystal to detect thepresence or attenuation of a continuous or pulsed sound pressure signal at afrequency above the upper range of hearing. Such sensors are employed as leveland interface detectors located at the set point level to be detected. They are alsoused as noncontacting continuous level sensors, located above the liquid anddetect the echoes of pulses directed at that surface.

b. Ultrasonic sensors are available for use with clean, dirty, aerated and viscousliquids, and for liquids which are contaminated with scale and solids. Continuous orsetpoint measurements can be made. Manufacturers should be consulted toidentify the suitability of these sensors with regard to the application.

c. Ultrasonic gap sensors for point liquid level detection are available in numerousconfigurations and materials. These sensors contain a crystal which transmits abeam of ultrasonic energy across a gap to a receiving crystal. Multigap sensorsexist which provide more than one setpoint. Sensors may be located verticallyfrom above the liquid surface, horizontally through the container wall, or inclined atabout 10 to the horizontal to detect a liquid/liquid interface.

d. Ultrasonic sensors for continuous liquid level measurement are usually mountedabove the surface of the liquid. (Submerged bottom mounted sensors are alsoavailable.) They transmit a pulse of ultrasonic sound to the surface, and receive acorresponding echo. The time interval between the outward and return signals is ameasure of the distance travelled and hence of the distance between sensor andsurface. The measuring span in many configurations is less than the range, becausethere is a minimum range below which the time interval cannot be accuratelymeasured. The vertical dead space can be reduced if a deflector or concentrator isused to turn the beam through 90, using a horizontally arranged sensor. Anothertechnique is to use a receiver that is separate from the transmitter and contains itsown crystal.

e. The timing circuitry allows for adjustment of the measuring span. Changes oftemperature in the air space above the liquid introduce an error and many ultrasoniclevel measurement systems include an automatic compensation for temperature.

f. Internal pipes or stays, and liquid entering the tank from one side or above the beam,may cause attenuation or interference. Provision for counteracting such effects issometimes made in the procedures for calibrating the sensor and for processing itsoutput signal.

g. Ultrasonic gap sensors are widely used in marine applications. Ultrasonic sensorshave no moving parts, and can make accurate continuous measurements atrelatively long range without making contact with the liquid in question. Thecomposition, cleanliness and electrical properties of the liquid do not affect themeasurement.

5.2.5 Optical Sensors

a. An optical level sensor is a short cylindrical probe with a conical point. The probecontains a light emitting diode, transmitting an infrared light to the tip. With no


32

liquid surrounding the tip, the beam is reflected internally and returns along theprobe to a phototransistor detector. When the tip is immersed in liquid, light escapesthrough the interface and the reduction in internal reflection is detected.

b. Positive switching circuits within the sensor provide a full on or off switch.Versions are available to switch on when covered, or vice versa. The sensor maybe installed vertically above the liquid, or horizontally either way up, in the tankwall. These devices can detect hydraulic oils, lubricants, fuels, coolant liquids, andother chemicals, and are immune to ambient light.

c. Sensors providing a time delay are available to dampen the effects of surfaceagitation or severe vibration.

d. Changes in the viscosity, density, conductivity or dielectric constant do not affectthe measurement. Optical sensors are lowcost devices with no moving parts andcan be very compact.

5.2.6 Time Domain Reflectrometry

a. Electromagnetic pulses are transmitted down a cable or rod and reflected off thesurface of the variable being measured. The reflected pulse is registered by thesignal source. Level is determined by measuring the travel time (time of flight) ofthe pulse or the delay in the receipt of the reflected echo. This delay is directlycorrelated to a distance if the propagation velocity is known. It is unaffected bychanges in product characteristics such as density, pressure, temperature anddielectric constant.

b. The measurement possibilities include level measurement in liquids and solids plusinterface measurements between liquids of diverse dielectric constant values.

c. For liquid applications dual rod or concentric tube/rod designs are preferred. Theadvantage of these designs is that the electric field around the probe is concentratedin the tube or between the rods and there is little external influence from the vesselstructure.

d. These designs are sometimes susceptible to a build up of foreign matter between therods or within the tube. This can in turn lead to measurement inaccuracies due tofalse echos being registered. They are therefore best suited to relatively cleanliquids.

e. TDR can also experience some difficulty in producing accurate measurements if thevariable is in very close proximity to the sender/receiver because the time of flight isso small. This is, however. being addressed and a number of manufacturers claim tohave overcome this problem.

5.2.7 Floats (Not Recommended)

a. Floats are cylindrical or spherical vessels, sealed against ingress of liquid. Normallymade of a suitably resistant material, partially submerged and supported by thebuoyancy force exerted by the liquid they displace. Floats may be restrained by avertical tube, or they may be attached to a pivoted arm. In both cases they rise andfall to follow the level of the liquid surface. Floats could be mounted to the top,bottom or sides of the tank which contains the liquid, within the tank itself, or inexternal chambers with valves to permit isolation and maintenance.

b. Liquids with a very low Specific Gravity (SG) may exert insufficient buoyant forceon a float for proper action.


33

c. Float rods would have been used to extend the vertical distance between a low-levelfloat and a top mounted switch. Clearance would be necessary above the float at itshighest indicating level, and below it at its lowest indicating level. Manufacturersliterature would be examined to ascertain the clearances required.

d. Floats and their associated arms and guides are comparatively bulky, and requiredadequate clearance for satisfactory operation. Vertical float guides requiredadequate headroom for withdrawal, and if long would require support at both ends.

e. Float sensors are widely used and are inexpensive. In high pressure or corrosiveapplications, the necessary seals may require frequent maintenance. Float sensorsare suitable for clean liquids of low viscosity. They would not be suitable for liquidsthat can clog the moving parts or can coat the float and change its buoyancy.

5.2.8 Displacers (Not Recommended)

a. Displacers are cylinders mounted in a vertical cage, and supported by a spring.Displacers can be mounted inside a tank containing liquid or the cage can be fittedexternally. To measure the level of a free liquid surface the displacer is partiallysubmerged in the liquid. A displacer may also be used to measure the interfacebetween two immiscible liquids, provided they are of different densities. Thevariable buoyant force acting on the displacer provides a measure of the requiredlevel in both applications. The SG of a displacer is higher than that of the liquid itdisplaces.

b. The transduction element, which converts buoyant force to electrical signal,imposes limits on the span of the buoyant force. The length of the displacer has to beslightly greater than the measurement span required, and hence its material,diameter and SG have to be selected from the manufacturers available designs sothat the above limits are observed. This selection also takes account of the SG of theliquid(s) in question.

c. The SG of the measured liquid changes with temperature. If the error thus caused isunacceptable, a temperaturecompensated displacer can be used. It contains aliquid of equal expansion coefficient and is fitted with a bellows end to permitexpansion and contraction.

d. Displacers are in general more accurate, but more expensive than floats. They areless bulky, work best in clean liquids of low viscosity and are not suitable for liquidsthat can coat their surface or cause excessive friction between displacer and cage.

5.2.9 Thermocouples

a. A thermocouple consists of two dissimilar conductors that may be pure metals,alloys or nonmetals. At one end, the two conductors are formed into a junction atwhich the conductors are in good electrical and thermal contact. The other ends ofthe conductors are made available for connection to the terminals of an emfmeasuring system which provides a reference junction. If the conductors areinsulated from each other (except at the measuring junction) and if a temperaturedifference exists between the measuring and reference junctions an electromotiveforce will be developed.

b. BS EN 60584 Part 1 contains international thermocouple reference tables, whichare used to determine the measuring junction temperature that corresponds to a


34

detected emf, for a specified combination of conductors. Tables are provided foreight combinations of conductors, designated Type R, Type S, Type B, Type J, TypeT, Type E, Type K and Type N respectively.

c. The materials most widely used for thermocouples may be divided into two groups;base metals and rare metals. The thermocouple combination must be chosen so as toensure that the physical and chemical conditions to which the device is exposed willnot result in a change in calibration. The emf/temperature relationships for basemetal thermocouples only apply up to about 1200 C. From 1110 C to 1800 C raremetal devices are used. In both groups the low temperature limit of working isusually decided by the limiting minimum emf which can be employed withmeasuring instruments. In general, base thermocouples develop larger emf thanrare metal types.

d. The thermocouple materials must be chosen in order to ensure that the device willnot suffer a significant change in calibration due to the effects of the operatingenvironment. The calibration may change as a result of oxidation, reduction, otherchemical attack or the preferential evaporation of constituents of an alloy. BS 1041,Part 4 describes the limiting characteristics of the types listed above.

e. The thermocouple measuring junction is constructed in one of three ways:

(1) An exposed junction is used, where a fast response is required, for themeasurement of noncorrosive gas temperatures. It is unlikely that thisjunction will be suitable for use in HM Surface Ships and Submarines;

(2) The insulated, or ungrounded, junction is used for corrosive gas and liquidtemperatures;

(3) An earthed junction, also referred to as grounded or bottomed, is also usedfor corrosive gas and liquid, and high pressure applications. The speed ofresponse will be faster than that obtained from the insulated junction.

f. The thermoelectric properties of thermocouple and compensating cable can bechanged by flexing and cold working.

g. The thermocouple conductors are insulated from each other, except at themeasuring junction, using conventional electrical insulation materials chosen towithstand the operating temperature range of the device.

h. The termination of a sheathed mineral insulated thermocouple at the referencejunction end, usually comprises a small pot filled with sealant, called the cold seal.It houses the connections to the thermocouple wires. Such mineral insulatedthermocouples shall be handled in accordance with manufacturers instructions.The installation is to be designed so that the thermocouples are not cut to length, asthis may affect accuracy. Provision is to be made for coiling and supporting anyexcess length near to the termination.

i. Mineral insulated thermocouples are manufactured by embedding the twoconductors in a mineral insulant, for example, magnesium oxide, within a metallicsheath. These thermocouples are resistant to thermal shock, operate at highertemperatures than other thermocouples, and are of smaller diameter than thoseinsulated by other means resulting in a rapid response and high sensitivity. Bothbase metal and rare metal types are available.


35

j. It may be possible to house the temperature transmitter directly above thetemperature sensing element. Frequently the available space and environmentalconditions will dictate that the transmitter is installed at some distance from thesensing element. When this is the case the thermocouple must be connected to thetransmitter using either extension cables or compensating cables.

k. Extension cables are used in situations where a high accuracy is required. They aremade of the same materials as the thermocouple combination in order to ensure thatthe measurement is not affected by the use of materials with differenttemperature/emf characteristics being used to complete the circuit betweenmeasuring and reference junctions. This will inevitably be expensive when thethermocouple is composed of rare metals.

l. Compensating cables are used to connect the thermocouple to the measuring systemin situations where a lower accuracy is acceptable. They are made of materialswhich resemble the thermoelectric properties of the thermocouple conductors.

m. Thermocouples generate an emf when there is a difference in temperature betweenthe measuring junction and the reference junction. For the most accuratemeasurement the temperature of the reference junction must remain static, forexample, by immersion in water at the ice point. In practice the reference junction ishoused within the measuring system and corrections for temperature variations areproduced electronically.

5.2.10 Resistance Temperature Detectors RTDs

a. RTDs are based on the principle of the variation of resistivity with temperature ofvarious electrical conductors. Suitable materials have a high temperaturecoefficient of resistance, are stable and have a high resistivity to permit theconstruction of small sensors. Both platinum and nickel are extensively usedbecause they meet the above criteria and are relatively easy to obtain in a pure state.Platinum has the additional advantage of a temperature coefficient of resistance,which is linear over a larger temperature range.

b. BS EN 60751 specifies requirements for industrial platinum RTDs whose electricalresistance is a defined function of temperature.

c. Wire wound resistance elements can be constructed so that the surface area is largein relation to the volume in order to provide a fast response or, if thin wire is used,the winding can be made compact for measuring temperature at a point. Theplatinum coil may be wound within a ceramic tube, which enables the sensor to beused over a wide temperature range. In applications where vibration is a problemthe platinum may be wound round a ceramic former and secured to the ceramic by aglass coating. For surface temperature measurement, or for applications where atipsensitive sensor is required a small coil may be mounted on a flat ceramic body.

d. Resistance elements may also be constructed of platinum foil, and are particularlysuitable for surface measurement.

e. Thick film devices, in which a platinum track is printed on to an alumina substrateand coated with a protective ceramic glaze, are manufactured both for surfacetemperature measurement and, in a rodshaped configuration, as a direct


36

replacement for wirewound elements. The high thermal conductivity of thealumina ensures a fast thermal response. These devices can be manufactured inaccordance with the resistance/temperature relationship of BS EN 60751. Therodshaped devices may be in accordance with the physical sizes specified in BS2765.

f. The resistance of the detecting element is determined by comparison with knownresistances in a Wheatstone Bridge circuit housed in the transmitter. Various bridgeconfigurations can be employed requiring twowire, three wire, or four wireconnections to the detector. The fourwire circuit achieves greatest accuracy sinceit satisfactorily minimises the effect of the resistance value of the lead wires. BS1041 shows a number of bridge circuits.

5.2.11 Temperature Pockets

a. Pockets are tubular receptacles, closed at one end, for insertion in a pipe or vessel bymeans of a pressuretight joint. The standard allows for parallel and taperedpockets, secured either by a parallel or tapered thread, or by a flange.

b. Standard insertion lengths are specified, there being six lengths for flanged pockets(115 mm to 600 mm) and seven lengths for threaded pockets (75 mm to 625 mm).There are seven standard bore diameters, from 3 mm to 20 mm. Any one of thediameters can be combined with any one of the lengths.

c. Pocket designs are to be suitable for the environmental conditions in use. Parallelstems are suitable for conditions of low vibration and flow. Tapered stems aresuitable for conditions that are more arduous. Flanged fittings are to be used forhighpressure systems.

d. The standard mating dimensions of pockets and elements have been chosen toobtain a small radial air gap and hence a quick thermal response whilst permittingeasy insertion and withdrawal.

5.2.12 Thermistors

a. Thermistors are semiconductors which behave as thermal resistors and have ahigh temperature coefficient of resistance which is usually negative. These devicesare made from the sintered compounds of metallic oxides of copper, manganese,nickel and cobalt, formed into beads, rings or discs. The resistance/temperaturerelationship is normally nonlinear, although modified thermistors are availablewhich are linear over a restricted range.

b. Unlike thermocouples and resistance thermometers, the performance ofthermistors is not closely governed by international standards. Temperaturesensing accuracy and stability vary from one manufacturer to another. Due to theproblems which therefore arise in sensor replacement, thermistors are unlikely to besuitable for temperature measurement in HM Surface Ships and Submarines.

c. Positive temperature coefficient thermistors are used for thermal protectionpurposes as described in DEF STAN 22-48.

5.2.13 Semiconductor Temperature Sensors

a. In addition to thermistors, other types of semiconductor temperature sensors basedon integrated circuit arrangements are available. Some devices give a linear voltage


37

output. These are sometimes used to provide reference junction compensation inthermocouple instruments. Other types provide an output current proportional totemperature.

b. These devices have not yet found widespread acceptance in industrial situations.They operate over a very limited range and are not covered by widely acceptedstandards. They are unlikely to be suitable for temperature measurement in HMSurface Ships and Submarines.

5.2.14 Diaphragm

a. A metallic or silicon diaphragm is sealed within the body of the sensor and pressuredifference applied across the diaphragm causes deflection. This is the main form ofpressure sensor in common use and it has a number of advantages. A single designcan be used for absolute, gauge and differential pressures. The pressurised volumescan be very small, allowing miniaturisation and fast response. Pressurised volumesare usually equal, enabling transient differential pressures to be measured. Thediaphragm can also be readily protected from comparatively high overpressures.

b. There are no major disadvantages and the performance differences between metaland silicon types are not significant. The silicon types tend to be superior at constanttemperature because of their high resistance and sensitivity, but the highertemperature coefficient of silicon is a disadvantage for measurements at varyingtemperatures. Metal diaphragms tend to be large and often require to be mountedwith the diaphragm in the vertical plane.

5.2.15 Capsule

a. A flexible capsule, usually of metal, is incorporated in a pressure chamber andapplication of pressure distorts the capsule. The main advantage of thisarrangement is a relatively high sensitivity at low pressures.

b. Disadvantages are that it is usually more expensive than a diaphragm and that formeasurement of differential pressure, two capsules are normally needed. These areeither located in separate pressure chambers or arranged as a nesting pair. This typeis difficult to protect against overpressure.

5.2.16 Bourdon Tube

a. The elastic element consists of a flattened metal tube bent into a circular arc or helixand closed at one end. Pressure is applied to the open end and the tube tends tostraighten. This type is relatively inexpensive and is often employed in dial gauges,but is becoming rejected in sensors because it is relatively inaccurate, particularly atlow pressures. It is difficult to protect against overpressure and has a relativelyslow response.

5.3 Transduction Element Devices/Techniques

5.3.1 Magnet/Reed Switches

a. Floats and displacers can be magnetically coupled to visual indicators and arrays ofelectrical switches. In many designs the magnetic coupling provides a snapact

nes 605 part 1 guide to selection of sensors for the measurement of system parameters

Documents

defence standard def

selection of liquid

ministry of defence

complete standard

selection of flow sensorspart

exdef stan

historical recorddef

portable liquid level