ISA PRAC THERMO-2012 Practical Thermocouple Thermometry (Second Edition).pdf

1、PRACTICAL THERMOCOUPLE THERMOMETRYSecond EditionPRACTICAL THERMOCOUPLE THERMOMETRYSecond EditionThomas W. KerlinMitchell JohnsonNoticeThe information presented in this publication is for the general education of the reader. Becauseneither the author nor the publisher has any control over the use of

2、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 pre-sented in a particular application.Additionally, neither the author no

3、r the publisher has investigated or considered the effect ofany patents on the ability of the reader to use any of the information in a particular application. Thereader is responsible for reviewing any possible patents that may affect any particular use of theinformation presented.Any references to

4、 commercial products in the work are cited as examples only. Neither theauthor 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 thepublisher makes any representation regar

5、ding the availability of any referenced commercial prod-uct at any time. The manufacturers instructions on use of any commercial product must be fol-lowed at all times, even if in conflict with the information in this publication.Copyright 2012 International Society of Automation (ISA)All rights res

6、erved. Printed in the United States of America. 10 9 8 7 6 5 4 3 2ISBN: 978-1-937560-27-0No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form orby any means, electronic, mechanical, photocopying, recording or otherwise, without the prior writ-ten permissio

7、n of the publisher.ISA67 Alexander DriveP.O. Box 12277Research Triangle Park, NC 27709Library of Congress Cataloging-in-Publication Data in process.AcknowledgmentsThanks to our wives, Nancy and Anna, for their patience with us while we prepared this second edition.We also acknowledge a debt to two o

8、thers whose influence is present throughout the book. The late Bob Shepard, former Oak Ridge National Laboratory scientist and collaborator with Dr. Kerlin on thermometry research and on authorship of an earlier book on industrial thermometry, formulated many clear and concise explanations of basic

9、principles of thermocouple thermometry. Bobs work strongly influenced the style and content of this book. Also Frank Johnson, former President of JMS Southeast, father of Mitch Johnson and collaborator with Dr. Kerlin, was instrumental in defining needs in industry and in formulating effective ways

10、to communicate information to users of temperature measurement systems.Brad Murphy and Barry Oxentine of JMS Southeast prepared most of the new and revised illustrations for this second edition. Their fine work enhances the book.viiTable of ContentsPreface to the Second Edition xiChapter 1Introducti

11、on 11.1 The Thermocouple 11.2 The Competition 21.3 Standards 51.4 Key References 6References 6Chapter 2Fundamentals 9The Main Points 92.1 Temperature Scales 92.2 What Causes the Thermocouple Voltage? 112.3 The Seebeck Coefficient and Thermocouple Loop Analysis 122.4 Thermocouple Types 152.5 Lead Wir

12、e Effects 162.6 Junction Construction Effects on Thermoelectric Performance 192.7 The Differential Thermocouple 202.8 Multiple Thermocouple Circuits 232.9 Thermoelectric Heaters, Coolers and Generators 252.10 The Laws of Thermoelectric Circuits 26References 29Chapter 3Measuring Temperature with a Th

13、ermocouple 31The Main Points 313.1 Converting EMF to Temperature 313.2 Equations for EMF versus Temperature 343.3 Modern Thermocouple Instrumentation 363.4 Zone Boxes 443.5 Installation Effects 463.6 Measuring Transient Temperatures 50References 64Chapter 4Thermocouple Configurations 65The Main Poin

14、ts 654.1 General Considerations 654.2 Make-Your-Own Thermocouples 654.3 Sheathed Thermocouples 664.4 Insulators for Sheathed Thermocouples 734.5 Surface Temperature Measurements 754.6 The Back End of a Sheathed Thermocouple 78viii Practical Thermocouple Thermometry4.7 Thermowells and Protection Tube

15、s 794.8 Special Applications of Thermocouples 85References 88Chapter 5Thermocouple Types 89The Main Points 895.1 The ASTM Thermocouples 895.2 EMF versus Temperature for ASTM Thermocouples 915.3 Chemical Compatibilities 915.4 Tolerances on ASTM Thermocouples 935.5 Seebeck Coefficients for ASTM Thermo

16、couples 955.6 Temperature Limits for ASTM Thermocouples 965.7 Color Coding of ASTM Thermocouples 975.8 Compensating Extension Wires for ASTM Thermocouples 975.9 High-Temperature Thermocouples 995.10 International Standards 100References 104Chapter 6Thermocouple Degradation 105The Main Points 1056.1

17、Basics 1056.2 Decalibration Tendencies of ASTM Thermocouples 1126.3 Insulation Resistance 1146.4 Mechanical Problems with Thermocouples 1156.5 Thermocouple Diagnostics 1176.6 Self-Validating Sensors 122References 125Chapter 7Thermocouple Selection and Performance 127The Main Points 1277.1 The Factor

18、s in Selecting a Thermocouple 1277.2 Costs 1287.3 Thermocouple Accuracy 1307.4 Thermocouple Durability 1327.5 Options for Achieving Performance Requirements 1337.6 The Future 135References 136Appendix AHypothetical Thermocouple Problems and Solutions 137Appendix BCircuit Analysis for Parallel Thermo

19、couples 145Appendix CThermocouple Tables (Temperature in C) 147Table of Contents ixAppendix DPolynomial Equation Coefficients 155Appendix EThermowell Selection 167Appendix FCase Studies 171Appendix GQuick Reference Thermocouple Guide 177Index 179xiPreface to the Second EditionMitchell Johnson, Presi

20、dent of JMS-Southeast, joined Dr. Kerlin in preparing this second edition. He brings a wealth of knowledge about real-world applications of thermocouples.The descriptions of thermocouple principles, the tools needed to analyze thermocouple performance, the causes of thermocouple errors, and the char

21、acteristics of the commonly-used thermocouples in the 1999 edition of this book are still as pertinent and correct as they were in 1999. The second edition updates the book with increased coverage of topics related to thermocouple applications. It provides new solved sample problems that include ill

22、ustrations of the use of the thermocouple loop analysis method. It includes new or revised sections to discuss new developments and to expand treatments of important technologies. It includes case studies of real-world problems and their solutions. Part of the motivation for preparing this second ed

23、ition is the apparent lack of widespread use of thermocouple loop analysis to characterize thermocouple performance and problems. We contend that this method is an essential tool for those who are responsible for measuring temperature with thermocouples.One might argue that internet information now

24、makes a book on thermocouples unnecessary. Certainly, almost everything found in this book can be found on the internet. However, the book eliminates the need to search through, evaluate, and digest a huge information resource. The book is intended as an easy-to-use reference that organizes and expl

25、ains the subject in a concise fashion and is convenient to access.11Introduction1.1 The ThermocoupleThe thermocouple must surely be one of the simplest measuring devices ever conceived. What could be simpler than two different wires joined at one end? With this arrangement, a voltage is produced alo

26、ng the wires that increases in magnitude as the temperature difference between the joined end and the open end increases. All that is needed to determine the temperature at the junction of the wires is to measure the voltage at the open end, make adjustments to compensate for differences between the

27、 open-end temperature and the open-end temperature used in calibration, and convert this compensated voltage into temperature using the calibration for the wire types.This approach is a proven technology for temperature measurement in industry. Thermocouples account for more temperature measurements

28、 in U.S. industry than any other sensor type. Thermocouples are rugged, inexpensive, and easy to use. However, they have significant inherent inaccuracies and a tendency to degrade with use. Users should understand these phenomena so they can properly assess the accuracy of their measurements, selec

29、t the proper thermocouple for a given application, and install and operate the thermocouple in the most advantageous way.This short book focuses on the practical aspects of thermocouple thermometry: how thermocouples work; how they go bad; how to assess measurement accuracy; and how to select, insta

30、ll, and operate them. In this book, a thermocouple will usually be shown schematically, as in Figure 1-1. In practical applications, however, the arrangement is often as shown in Figure 1-2. In the case illustrated in Figure 1-2, the wires are contained in a metallic sheath where the junction is for

31、med. The wires come in three categories: base metal (such as copper, nickel, and iron and are cheapest and most common), refractory metal (such as tungsten and rhenium and used for very high temperatures) and noble metals (such as platinum and rhodium and used for high accuracy and high temperature)

32、. The open end is connected to a readout that automatically measures the voltage, corrects for effects caused by the temperature at the open end, and then computes and displays the 2 Practical Thermocouple Thermometrytemperature. This simplicity of implementation is both a blessing and a curse. On t

33、he one hand, it is very easy to obtain a measurement: just turn the system on and the result appears. On the other hand, this ease of use often discourages users from expending enough time to understand what is happening, and the unfortunate result may be undetected and unnecessary measurement error

34、s.1.2 The CompetitionThermocouples are used routinely for temperature measurements ranging from 270C to 2320C. Other sensor types are available for use over portions of this range.1-3Specifically, the sensors that are alternatives to thermocouples (and their range of application) are as follows:Figu

35、re 1-1. Schematic View of a ThermocoupleFigure 1-2. Thermocouple in Practical ApplicationsIntroduction 3Sensor Useful Temperature RangeTypical resistance temperature 196C to 661Cdetectors (RTDs)1Thermistors 55C to 100CIntegrated circuit sensors 55C to 150CResistance temperature detectors and thermis

36、tors (the latter for a narrow range of temperatures near ambient) are the only serious competitors for use as immersion sensors in process environments that require a sheath or protection tube to isolate the sensor from the process. Integrated circuit sensors are used in benign environments such as

37、for heating, ventilating, and air conditioning systems or as components of electronic instrumentation systems.The competitors to thermocouples for process measurements have different relative advantages, mainly with respect to measures of suitability for a given application. These measures are allow

38、able temperature range, accuracy, and measurement system affordability (the measurement system consists of the three components needed to make a measurement: the sensor, wiring and instrumentation). For a number of years, thermocouples have been losing market share to RTDs in total temperature senso

39、r sales. This trend is likely to continue. RTDs have evolved from fragile, expensive laboratory sensors to quite rugged and inexpensive industrial sensorslargely due to improvements in the quality of thin film RTD elementsthough they are still not as rugged as thermocouples. RTDs have lower decalibr

40、ation tendencies and lower costs for wiring between the sensor and its transmitter or readout. Greater achievable accuracy is an advantage for RTDs over any type of thermocouple up to around 460oC. Beyond this temperature, RTDs still have lower limits of error than base-metal thermocouples, but larg

41、er limits of error than noble-metal thermocouples.Thermocouples remain the least expensive sensor for many applications, their accuracy and decalibration tendency are improving as the subtleties of the underlying principles of thermocouple thermometry are understood better and improvements arise in

42、composition control and sensor fabrication procedures. They are suitable for use in unusual configurations, they are rugged, and they are able to operate at high temperatures. These advantages guarantee that thermocouples will continue to be very important sensors for industry.4 Practical Thermocoup

43、le ThermometryTable 1-1 summarizes the relative advantages and disadvantages of thermocouples and RTDs.Table 1-1. Comparison of Thermocouples and RTDsNoncontact temperature sensors are also available. They provide measurement capability that includes situations where measurements with thermocouples

44、are not possible. Infrared temperature sensors and optical pyrometers can measure temperatures that far exceed those possible by means of any contact temperature sensors. These sensors work by measuring the electromagnetic radiation emitted from an object. They are useful for monitoring surface temp

45、eratures. Disadvantages of non-contact sensors include high cost, error caused by emissivity uncertainties, the inability to take an internal temperature and the fragility of the measuring device itself.Thermocouple RTDAccuracyLimits of error wider than for RTDs (except for noble metal thermocouples

46、 above roughly 460C)Limits of error smaller than base-metal thermocouples at all temperatures and noble metal thermocouples below roughly 460CRuggednessExcellent Relatively sensitive to temperature-induced strain, thermal or mechanical shock and pressureRange270C to 2320C 196C to 661C (typical) (som

47、ewhat lower and higher limits in special designs)SizeCan be as small as .01“ and may be tip sensitiveLimited to 1/16“, temperature sensitive for length of bulbDriftShould be checked periodically for driftLess drift than thermocouples (typically 0.01 to 0.1C / year)ResolutionMust resolve fractions of

48、 millivolts per degree, lower signal-to-noise ratio Must resolve fractions of ohms per degree, higher signal-to-noise ratioCold JunctionRequired Not RequiredLead WireMust match lead wire calibration to thermocouple calibrationCan use copper wire for extension wireResponseCan be made small enough for

49、 millisecond response timeThermal mass restricts time to seconds in most casesCostLow Higher than thermocouplesIntroduction 51.3 StandardsStandards serve to define the acceptable performance levels of products such as thermocouples. In the United States, consensus standards are prepared by professional societies and are then approved and promulgated by the American National Standards Institute (ANSI). The American Society for Testing and Materials (ASTM) maintains Committee E.20 to address the needs of thermometry standards in the United States. The In