1、BRITISH STANDARD BS 1041-4: 1992 Incorporating Amendment No. 1 Temperature measurement Part 4: Guide to the selection and use of thermocouplesBS1041-4:1992 This British Standard, having been prepared under the directionof the Industrial-process Measurementand Control Standards Policy Committee, wasp
2、ublished under the authorityof the Standards Boardand comes into effect on 31January 1992 BSI 04-1999 First published March 1966 Second edition January 1992 The following BSI references relate to the work on this standard: Committee reference PCL/1 Draft for comment 90/21077 DC ISBN 0 580 20071 X Co
3、mmittees responsible for this British Standard The preparation of this British Standard was entrusted by the Industrial-process Measurement and Control Standards Policy Committee (PCL/-) to Technical Committee PCL/1, upon which the following bodies were represented: British Coal Corporation British
4、Gas plc British Pressure Gauge Manufacturers Association Department of Energy (Gas and Oil Measurement Branch) Department of Trade and Industry (National Weights and Measures Laboratory) Energy Industries Council Engineering Equipment and Materials Users Association GAMBICA (BEAMA Ltd.) Health and S
5、afety Executive Institution of Gas Engineers The following bodies were also represented in the drafting of the standard, through subcommittees and panels: British Cable Makers Confederation British Valve and Actuator Manufacturers Association Department of Trade and Industry (National Engineering La
6、boratory) Department of Trade and Industry (National Physical Laboratory) Electricity Industry in United Kingdom Engineering Industries Association Institute of Metals Society of Glass Technology Amendments issued since publication Amd. No. Date Comments 7408 December 1992 Indicated by a sideline in
7、 the marginBS1041-4:1992 BSI 04-1999 i Contents Page Committees responsible Inside front cover Foreword ii 0 Introduction 1 1 Scope 1 2 Definitions 1 3 Thermoelectricity 2 4 Basic thermocouple circuits 3 5 Thermocouple materials and their characteristics 4 6 Durability of thermocouples at high tempe
8、ratures 7 7 Hardware and fabrication 9 8 Electromotive force measurement 14 9 Signal processing and logging 15 10 Thermocouple reference tables, tolerances and calibration 18 Figure 1 Basic circuit diagrams for a thermocouple with conductors a and b 24 Figure 2 Electromotive force characteristics of
9、 the standardized thermocouples 25 Table 1 Approximate e.m.f. output of standardized base metalthermocouples(reference junction at 0C) 22 Table 2 Approximate e.m.f. output of noble metal and refractorymetalthermocouples (reference junction at 0C) 22 Table 3 Recommended maximum operating temperatures
10、 forbareandprotected base metal thermocouple wires operatingcontinuouslyinair without temperature cycling 23 Table 4 Recommended maximum operating temperatures fornoblemetalthermocouple wires operating continuously inairwithouttemperature cycling and intermittently in air 23 Table 5 Alloys commonly
11、used in thermocouple compensating cable 23 Publication(s) referred to Inside back coverBS1041-4:1992 ii BSI 04-1999 Foreword This Part of BS 1041 has been prepared under the direction of the Industrial-Process Measurement and Control Standards Committee. It is a revision of BS 1041-4:1966 which is w
12、ithdrawn. It should be noted that the title has been restyled for consistency with other parts of BS 1041. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British St
13、andard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1to26, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorpor
14、ated. This will be indicated in the amendment table on theinside front cover.BS1041-4:1992 BSI 04-1999 1 0 Introduction Thermocouples are by far the most common temperature sensors in industrial use. They possess the virtues of simplicity, ruggedness, low cost, small physical size, wide temperature
15、range (from about270C up to3000C) and convenient electrical output. These properties make them very suitable for multi-point temperature measurement and monitoring in large and complex process plant, and for an enormous variety of industrial, technological and scientific applications. The thermocoup
16、le has a long history, the original paper by Seebeck having been published in 1822 and the relationship between the three principal thermoelectric effects having been established by William Thomson (later Lord Kelvin) in 1854. The platinum-10% rhodium/platinum thermocouple, which was for a long time
17、 specified as the interpolating instrument in realizing the International Temperature Scales in the range from630C to 1064C, was originally developed by Le Chatelier in 1886. Most of the commonly used base metal thermocouples were developed during the first decade of the twentieth century. For a pro
18、per understanding of how thermocouples function and how to use them, it is essential to realize that thermoelectricity is a bulk property of metallic conductors in the same sense as thermal conductivity and electrical conductivity. Although thermoelectric effects manifest themselves in circuits comp
19、rising two or more dissimilar conductors, they are not due to any special properties of the junctions between the conductors. The junctions, which for successful measurements have to be at uniform temperatures, are needed only to complete the measuring circuit and are thermoelectrically inactive. In
20、 fact there will be contact potentials at junctions between different metals, but these are not thermoelectric in origin and they are not significantly temperature dependent. Therefore when all contact potentials in a circuit loop are summed, their net result is effectively zero. By contrast, the ch
21、emical state and physical condition, e.g. strained or annealed, of the conductors in regions where they experience temperature gradients, can have a profound effect on the electromotive force (e.m.f.) generated. Great care should be exercised in how the conductors are treated in these regions and us
22、ers should be aware of possible effects due to physical and chemical changes which may occur in use. Before embarking on descriptions of thermocouples and their application a brief account is given of the principal thermoelectric effects, since this should be helpful in achieving an understanding of
23、 good practice in thermocouple thermometry. Reference should be made to textbooks on thermoelectricity and, more generally, on the electrical properties of metals and alloys, for detailed theoretical discussion. Thermocouples are used in so many and varied circumstances that it has only been possibl
24、e to cover the common principles in this standard. It is hoped that it will be a useful aid to understanding the characteristics that are of practical importance so that the most appropriate choices of thermocouple and instrumentation can be made, and their effective application achieved. 1 Scope Th
25、is part of BS1041 provides guidance on the selection and use of thermocouples. It provides an introduction to the operating principles of thermocouples and their application to the measurement of temperature. A brief review of thermoelectricity and basic thermocouple circuits and an overview of the
26、materials commonly used in thermocouples in various temperature ranges, with their strengths and weaknesses are included. The fabrication of thermocouples, associated hardware, measurement techniques, tolerances and calibration are described. 2 Definitions For the purposes of this British Standard t
27、he following definitions apply. 2.1 thermoelectricity 1) Electricity generated in a conductor by virtue of a temperature difference (temperature gradient) within it. 2) The branch of science concerned with electric effects produced in conductors by means of heat. 2.2 thermoelectric e.m.f. the electr
28、omotive force established in a conductor by virtue of a temperature gradient within it (theSeebeck effect) 2.3 thermoelectric power the thermoelectric e.m.f. produced in a conductor per unit temperature difference NOTE 1Thermoelectric power is also known as thermopower or the Seebeck coefficient.BS1
29、041-4:1992 2 BSI 04-1999 NOTE 2Thermopower is the thermoelectric sensitivity, and values are usually given in 4V/C, thus the term “power” is misleading. 2.4 thermocouple a thermoelectric device for measuring temperature, consisting of a pair of dissimilar conductors (thermoelements) connected togeth
30、er at the measuring junction which is maintained at the temperature to be measured, the circuit loop being closed at a reference junction between the two conductors, or at two reference junctions to a third conductor NOTE 1An instrument is connected at a convenient point in the circuit loop so as to
31、 measure the net thermoelectric e.m.f. (orsometimes the thermoelectric current) developed in the circuit. If the thermoelements are connected directly to the measuring instrument, the terminals of the instrument constitute the reference junctions. NOTE 2The e.m.f. produced depends on the thermoeleme
32、nts used and on the temperatures of the measuring and reference junctions. NOTE 3The measuring and reference junctions are often referred to as the “hot” and “cold” junctions respectively, though in many circumstances, especially in measuring temperatures below 0C, the opposite applies. 2.5 thermoel
33、ements the two conductors used in a thermocouple, one of which is designated “positive”, the other “negative”, according to the polarity of the net e.m.f. developed 3 Thermoelectricity 3.1 The Seebeck effect A conductor contains electrons which are continually in motion in all directions. These moti
34、ons are such that in the absence of any external electromagnetic or thermal stimulus there is no net transport of electrons, or current. However, if an electric potential difference is applied, the motions are modified and a current flows. If a temperature gradient is established in the conductor th
35、e motions are again modified, this time with the result that heat is conducted and a gradient in electron density is set up. Since electrons are charged it follows that an electric potential difference will be established, which may be positive or negative depending on the details of the electronic
36、structure of the conductor. As it is difficult to demonstrate the existence of this potential difference in an isolated conductor, the circuit has to be completed with a second conductor which will necessarily experience the same temperature gradient. In order not to counterbalance the effect, this
37、has to be of a different material, i.e. one forms a thermocouple and observes the difference in the thermoelectric e.m.f.s 1)generated in the two conductors. This is the basic thermoelectric effect which was discovered by Seebeck and which bears his name. The magnitude of the e.m.f generated depends
38、 on the thermoelectric powers of the two conductors and on the temperature gradient to which they are exposed. For the case where the conductors are connected to a high impedance voltmeter as shown in Figure 1 a) the e.m.f., E (in 4V) may be written as follows: where S aand S bare the thermoelectric
39、 powers of conductors a and b (in 4V/C) t 1and t 2are the junction temperatures (in C). Equation 1 is more strictly correct than the following alternative equations: Equation 1 shows that E is the sum in the circuit loop of the e.m.f.s built up in the two separate conductors, the junctions exist onl
40、y to connect them together. Equations 2 and 3 on the other hand suggest that E is the difference between junction e.m.f.s, E ab , between the two conductors at temperatures t 1and t 2 . Circuit analysis can proceed as if this is the case, but those concerned with making, calibrating, installing and
41、using thermocouples will need to bear in mind the source of the e.m.f.s, and to exercise care in how they treat conductors in regions of temperature gradient. Junctions, being the points of measurement (orinterms of equation 1, the initial and final limits of the integrations) should always be isoth
42、ermal and therefore should not themselves contribute to the e.m.f. As a consequence, the junctions may be formed in any manner that is electrically, mechanically and chemically effective and appropriate. 1) Since a thermocouple is, like a battery, an active generator of electric potential difference
43、, its output is more properly termed an electromotive force, or e.m.f., than a voltage. (1) E = E ab (t 1 )E ab (t 2 ); or (2) E = E ab (t 1 )+E ba (t 2 ). (3) ES a dtS b dt t 1 t 2 + t 2 t 1 =BS1041-4:1992 BSI 04-1999 3 Since thermopower is a property of bulk conductors and not of junctions, it fol
44、lows that the conductors of a thermocouple should be homogeneous. The thermoelectric power can be very sensitive to chemical composition and physical condition. If either of these varies along the length of a conductor of a thermocouple, the output may be dependent on the temperature profile, i.e. o
45、n exactly where the temperature gradient is. This has obvious implications for the manufacture and correct use of thermocouples. 3.2 The Thomson and Peltier effects Two other thermoelectric effects arise in the conductors of circuits in which a current is caused to flow. If a portion of a conductor
46、in a temperature gradient along which an electric current is flowing is considered, the electrons enter with a certain energy and pass on to the next portion of the conductor with a different energy by virtue of the temperature change. The energy which they have either lost or gained appears as heat
47、 liberated or absorbed. This is known as the Thomson heat after William Thomson (Lord Kelvin) who first postulated its existence. It is often referred to as the specific heat (heat capacity) of electricity. The Thomson coefficient, usually written , is the heat gain per unit volume per unit current
48、per unit temperature gradient (in 4V/C) and it is related to the thermoelectric power by the equation: where T is the temperature (in K); s is the thermoelectric power of the conductor (in4V/ C). Although the effect is generally quite small, it is through the Thomson effect that absolute thermoelect
49、ric power is best measured. The current is passed through the conductor, first in one direction and then in the opposite direction, in order that the Thomson heat, which is reversible, can be distinguished from the Joule heating which, being proportional to the current squared, is irreversible. The calorimetric measurements are not easy and in consequence absolute thermoelectric powers are not accurately known, but once measurements have been made for one material other thermoelectric powers can readily be measured relative to this reference
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