Sensor Performance and Reliability

323 Pages·2005·7.25 MB·English

by  Hashemian

Sensor Performance
and Reliability
H. M. Hashemian
Notice
The information presented in this publication is for the general education of the reader. Because neither
the author nor the publisher have any control over the use of the information by the reader, both the
author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is
expected to exercise sound professional judgment in using any of the information presented in a particular
application.
Additionally, neither the author nor the publisher have investigated or considered the affect of any patents
on the ability of the reader to use any of the information in a particular application.
The reader is responsible for reviewing any possible patents that may affect any particular use of the infor-
mation presented.
Any references to commercial products in the work are cited as examples only. Neither the author nor the
publisher endorses any referenced commercial product. Any trademarks or trade names referenced belong
to the respective owner of the mark or name. Neither the author nor the publisher makes any representa-
tion regarding the availability of any referenced commercial product at any time. The manufacturer’s
instructions on use of any commercial product must be followed at all times, even if in conflict with the
information in this publication.
Copyright © 2005 ISA – The Instrumentation, Systems, and Automation Society
All rights reserved.
Printed in the United States of America.
10 9 8 7 6 5 4 3 2
ISBN 1-55617-897-2 (softbound)
ISBN 1-55617-932-4 (hardbound)
No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by
any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written per-
mission of the publisher.
ISA 67 Alexander Drive
P.O. Box 12277 Research Triangle Park, NC 27709
Library of Congress Cataloging-in-Publication Data in process.
Hashemian, H. M.
Sensor performance and reliability / Hashem M. Hashemian.
p. cm.
ISBN 1-55617-897-2 (softbound)
1. Detectors--Testing. I. Title.
TK6565.D4H38 2005
681'.2--dc22
2004023221
PREFACE
Temperature and pressure sensors (including level and flow sensors) are vital to
process control and safety. Although there have been great advances in process
instrumentation in recent decades, industrial temperature and pressure
measurements are still largely made by conventional sensing devices such as
resistance temperature detectors (RTDs), thermocouples, and a few varieties of
pressure sensing elements such as capacitance cells, bellows, and strain gauges.
This book reviews the operational characteristics of industrial temperature and
pressure sensors and typical problems that the process industry and power plants
have experienced with these sensors over the years. More importantly, this book
describes methods that have been developed in recent years to measure the
performance of process sensors and verify their health and reliability. The significance
of these methods is that they can be used remotely on sensors as installed in
operating processes. They include on-line calibration verification of process
sensors, in-situ response time measurements, detection of blockages and voids in
pressure sensing lines, in-situ testing of cables, and in-situ sensor diagnostics.
XXV
TABLE OF CONTENTS
List of Figures XIII
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acronyms XXI
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preface XXV
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 1 Introduction 1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Process Instrumentation 2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Temperature Sensor Problems 4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Pressure Sensor Problems 7
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Sensing Line Problems 8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Cable Problems 10
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 2 Physical Characteristics of Industrial RTDs 11
. . . . . . . . . . . . . . . . . .
2.1 Construction Details 11
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Direct-Immersion and Thermowell-Mounted RTDs 16
. . . . . .
2.3 Fast Response RTDs 17
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 RTD Instrumentation 20
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 3 Physical Characteristics of Thermocouples 25
. . . . . . . . . . . . . . . . . .
3.1 Principles of Operation 25
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Thermocouple Junction Styles 27
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Standardized Thermocouples 29
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Thermocouple Extension Wires 31
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Colors of Thermocouple Extension Wires 32
. . . . . . . . . . . . . . . . . . . .
3.6 Reference Junction Compensation 34
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7 Thermocouple E-T Curve 36
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.8 Seebeck Theory and Thermocouple Circuit Analysis 37
. . . . . .
Chapter 4 Physical Characteristics of Pressure Sensors 41
. . . . . . . . . . . . . . .
4.1 Principle of Operation 41
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Elastic Sensing Elements 42
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Displacement Sensors for Pressure Measurement 46
. . . . . . . . . . .
4.4 Pressure Transmission 56
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 Sealed Pressure Sensing Systems 60
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 Pressure Damping Systems 62
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IX
SENSOR PERFORMANCE AND RELIABILITY
Chapter 5 Performance Specification of Temperature
and Pressure Sensors 65
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 6 Accuracy of Temperature Sensors 67
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1 Introduction 67
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Calibrating RTDs 68
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 RTD Accuracy 73
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4 Thermocouple Calibration 80
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1 Procedure for Calibration of Thermocouples 81
. . . . .
6.4.2 Processing Calibration Data 85
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 7 Accuracy of Pressure Transmitters 87
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1 Terms and Definitions 87
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Calibrating Pressure Transmitters 94
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3 Pressure Transmitter Accuracy 97
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 8 Calibration Traceability of Temperature
and Pressure Sensors 101
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 9 Fundamentals of Dynamic Response 105
. . . . . . . . . . . . . . . . . . . . . . . . .
9.1 Dynamic Response of a Simple System 107
. . . . . . . . . . . . . . . . . . . . . .
9.2 Characteristics of First-Order Systems 111
. . . . . . . . . . . . . . . . . . . . . . . .
9.3 Definition of Time Constant 112
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4 Response of Higher-Order Systems 113
. . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 10 Laboratory Measurement of Response
Time of Temperature Sensors 117
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1 Plunge Test 117
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2 Correlation between Response Time and
Process Conditions 119
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 11 Response Time Testing Methods for
Pressure Transmitters 131
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 12 In-Situ Response Time Testing of
Temperature Sensors 137
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.1 Description of LCSR Test 139
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.1.1 LCSR Testing of RTDs 139
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.1.2 LCSR Testing of Thermocouples 142
. . . . . . . . . . . . . . . . .
X
TABLE OF CONTENTS
12.2 Processing LCSR Data 144
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.1 LCSR Test Theory 147
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.2 Heat-Transfer Analysis of a
Temperature Sensor 148
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.3 Derivation of LCSR Equation 151
. . . . . . . . . . . . . . . . . . . .
12.2.4 Derivation of Plunge-Test Equation 152
. . . . . . . . . . . . .
12.3 Procedure for Analyzing LCSR Data 154
. . . . . . . . . . . . . . . . . . . . . . . .
12.4 Self-Heating Test 155
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 13 In-Situ Response Time Testing of
Pressure Transmitters 157
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1 Noise Analysis Technique 157
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2 Power Interrupt (PI) Test 165
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 14 Pressure Sensing Line Problems and Solutions 167
. . . . . . . . . .
14.1 Sensing Line Problems 168
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.1.1 Reference Leg Boil-Off 168
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.1.2 Level Measurement Problems 170
. . . . . . . . . . . . . . . . . . . . .
14.1.3 Voids, Blockages, and Freezing 170
. . . . . . . . . . . . . . . . . . . .
14.1.4 Leakage 171
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.1.5 Common Sensing Lines 171
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.1.6 Noise from Sensing Lines 171
. . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2 Effect of Sensing Lines on Response
Time of Pressure Transmitters 172
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3 On-Line Detection of Sensing Line Problems 176
. . . . . . . . . . . . .
Chapter 15 In-Situ Methods to Verify the Calibration of
Process Instruments 177
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.1 Introduction 177
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.2 Principle of In-Situ Calibration Verification 177
. . . . . . . . . . . . . . . .
15.3 Cross Calibration Test 178
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4 On-Line Calibration Monitoring 180
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5 Application of On-Line Monitoring for
Detection of Venturi Fouling 192
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XI
SENSOR PERFORMANCE AND RELIABILITY
Chapter 16 Aging Effects and Failure Potential of
Process Instrumentation 193
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.1 Introduction 193
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.2 Aging of Temperature Sensors 195
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.2.1 Aging Effects on RTD Calibration 195
. . . . . . . . . . . . . .
16.2.2 Aging Effects on RTD Response Time 198
. . . . . . . . .
16.3 Aging of Pressure Transmitters 200
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.3.1 Stressors of Pressure Transmitters 200
. . . . . . . . . . . . . . . . .
16.3.2 Effects of Aging on Calibration and
Response Time of Pressure Transmitters 203
. . . . . . . .
Chapter 17 In-Situ Testing of Cables 205
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.1 Introduction 205
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2 Components of an Electrical Cable 206
. . . . . . . . . . . . . . . . . . . . . . . . . .
17.3 Cable Testing Techniques 208
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.3.1 Passive Techniques 208
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.3.2 Active Techniques 208
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.4 Description of TDR Test 212
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 18 In-Situ Diagnostics of Temperature Sensors 219
. . . . . . . . . . . . . . .
18.1 Verifying the Attachment of Sensors to
Solid Surfaces 219
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.2 Detecting Secondary Junction in Thermocouples 221
. . . . . . . .
18.3 Detecting Cross-Connected Thermocouples 221
. . . . . . . . . . . . . .
18.4 Verifying Adequate Sensor Insertion in a Thermowell 222
. . .
18.5 Separating RTD Problems from Cable Problems 225
. . . . . . . . .
18.6 Verifying Water Level in Pipes or Vessels 226
. . . . . . . . . . . . . . . . . . .
18.7 Detection of Gross Inhomogeneities in Thermocouples227
Chapter 19 Applications 231
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References 235
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A 237
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix B 241
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Index 297
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XII
1
C H A P T E R
INTRODUCTION
Despite many advances in electronics and computer technologies, industrial
process measurements are still made largely by conventional sensors, such as
thermocouples, resistance temperature detectors (RTDs), and pressure and
differential pressure sensors that were designed more than 50 years ago.
Today, there are smart sensors, fiber optic sensors, ultrasonic sensors, and
wireless sensors on the market, contributing significantly to recent advances in
instrumentation. Yet many of these new sensors still depend on conventional
sensing technologies to measure a process parameter. For example, smart
temperature sensors often use RTDs or thermocouples to measure temperature,
and smart pressure sensors use conventional capacitance sensing cells, bellows,
and other traditional sensors to measure pressure. The smart components are
mostly in the sensor electronics and memory and in the sensor’s ability to adjust
its output remotely using digital technology.
The same is true for wireless sensors. They usually use a conventional
sensing device to measure a process parameter and wireless technology to
transmit the information to a remote location. Therefore, many of today’s instru-
mentation or sensor problems are similar to those familiar to industry over the
years. For example, sensor drift is almost as much of a problem today as it was
three decades ago. There is no new sensor technology on the horizon to make
possible any significantly new drift-free, sturdy sensors that can readily tolerate
the temperature, humidity, and vibration environments that exist in industrial
processes. Great advances have been made in producing essentially drift-free
electronics for sensors, but the sensors themselves have not changed much over
the years. Also, questions still linger over how to objectively assess the accuracy,
response time, residual life, and other characteristics of installed instrumentation.
No consensus has been established in these areas even among professionals in
the process instrumentation field. This book is intended to provide readers with
an understanding of some of these problems and to offer practical means to
identify them, assess their consequences, and help resolve them.
1
SENSOR PERFORMANCE AND RELIABILITY
1.1 Process Instrumentation
Process instrumentation usually involves temperature and pressure sensors.
Temperature sensors include RTDsand thermocouples. Other temperature sensors
such as thermistors are also found in industrial processes, but most industrial
temperature measurements are made with RTDs and thermocouples. Figure 1.1
shows the relative output of the three most commonly used temperature sensors
as a function of temperature. This figure makes clear that thermocouples have the
highest temperature range, RTDs have the best linearity, and thermistors have
the best sensitivity (for low temperature measurements).
T THERMISTOR
U
P
T
U
O
E
V
TI RTD
A
L
E
R THERMOCOUPLE

0 1000 2000
TEMPERATURE (˚C)
Figure 1.1. Comparison of Industrial Temperature Sensors
Today, RTDs are used in about 30 to 40 percent of all industrial applications,
thermocouples in about 50 to 60 percent, and thermistors and other temperature
sensors in the remaining applications. Table 1.1 compares the main characteristics
of RTDs and thermocouples. Both RTDs and thermocouples are simple devices,
but problems such as calibration drift and degradation of response time are still
encountered in their application in industrial processes. These problems as well
as how to test for and resolve them are the subject of this book, which also
includes a general description of the physical characteristics of sensors and how
to establish and verify their performance.
2
INTRODUCTION
Table 1.1. RTDs and Thermocouples Compared
• RTDs
More accurate than thermocouples, but not as effective in poor heat
transfer media. Also, not as good for vibration environments, but better
than thermocouples in noisy environments.
• Thermocouples
Wider temperature range than RTDs, but less accurate and cannot be
calibrated after use. Survive better than RTDs in vibration environ-
ments, but not as good in noisy environments.
Preferred Sensor
Performance Indicator RTDs Thermocouples
✔
Accuracy
✔
Air/Gas Temperature Measurement
✔
Vibration Environment
✔
Noisy Environment
✔
High Temperature Range
Post-Use Calibration ✔
Pressure sensors (including differential pressure sensors, which are used to
measure level and flow) are not as simple as RTDs and thermocouples. As such,
they present more opportunities for problems and failure than do temperature
sensors. Pressure sensors are electromechanical devices, so problems can occur in
both the mechanical and the electrical components of such systems. In addition,
pressure sensors are connected to small-diameter tubes, called sensing lines or
impulse lines, which are used to transfer the pressure information from the process
to the sensor. Some sensing lines are filled with air or gas; others are filled with
the process fluid or oil. Fluid-sensing lines can contribute to pressure sensor
problems because they develop anomalies such as blockages and resonances
caused by voids and standing waves. Blockages reduce the dynamic response of
the pressure-sensing system, while voids in fluid-sensing lines can lead to noisy
pressure signals, measurement errors, and sluggish dynamic response.
Both blockages and voids in fluid-sensing lines can be detected remotely while
the plant is on line by using the noise analysis technique. The noise analysis
technique is also used to test the response time of pressure sensors in-situ as
installed in an operating process. The details are covered in Chapters 13 and 14.
3