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1、BRITISH STANDARD BS 1041-4: 1992 Incorporating Amendment No. 1 Temperature measurement Part 4: Guide to the selection and use of thermocouples Licensed Copy: sheffieldun sheffieldun, na, Fri Nov 24 06:05:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 1041-4:1992 This British Standard, having been
2、prepared under the direction of the Industrial-process Measurement and Control Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on 31 January 1992 BSI 04-1999 First published March 1966 Second edition January 1992 The following BSI references
3、 relate to the work on this standard: Committee reference PCL/1 Draft for comment 90/21077 DC ISBN 0 580 20071 X Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Industrial-process Measurement and Control Standards Policy Committee (PCL/-
4、) to Technical Committee PCL/1, upon which the following bodies were represented: British Coal Corporation British 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
5、) Energy Industries Council Engineering Equipment and Materials Users Association GAMBICA (BEAMA Ltd.) Health and Safety Executive Institution of Gas Engineers The following bodies were also represented in the drafting of the standard, through subcommittees and panels: British Cable Makers Confedera
6、tion British Valve and Actuator Manufacturers Association Department of Trade and Industry (National Engineering Laboratory) Department of Trade and Industry (National Physical Laboratory) Electricity Industry in United Kingdom Engineering Industries Association Institute of Metals Society of Glass
7、Technology Amendments issued since publication Amd. No.DateComments 7408December 1992 Indicated by a sideline in the margin Licensed Copy: sheffieldun sheffieldun, na, Fri Nov 24 06:05:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 1041-4:1992 BSI 04-1999i Contents Page Committees responsibleInsid
8、e front cover Foreword ii 0Introduction1 1Scope1 2Definitions1 3Thermoelectricity2 4Basic thermocouple circuits3 5Thermocouple materials and their characteristics4 6Durability of thermocouples at high temperatures7 7Hardware and fabrication9 8Electromotive force measurement14 9Signal processing and
9、logging15 10Thermocouple reference tables, tolerances and calibration18 Figure 1 Basic circuit diagrams for a thermocouple with conductors a and b24 Figure 2 Electromotive force characteristics of the standardized thermocouples25 Table 1 Approximate e.m.f. output of standardized base metal thermocou
10、ples (reference junction at 0 C)22 Table 2 Approximate e.m.f. output of noble metal and refractory metal thermocouples (reference junction at 0 C)22 Table 3 Recommended maximum operating temperatures for bare and protected base metal thermocouple wires operating continuously in air without temperatu
11、re cycling23 Table 4 Recommended maximum operating temperatures for noble metal thermocouple wires operating continuously in air without temperature cycling and intermittently in air23 Table 5 Alloys commonly used in thermocouple compensating cable23 Publication(s) referred toInside back cover Licen
12、sed Copy: sheffieldun sheffieldun, na, Fri Nov 24 06:05:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 1041-4:1992 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
13、:1966 which is withdrawn. 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 w
14、ith a British Standard 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, pages 1 to 26, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had
15、amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Fri Nov 24 06:05:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 1041-4:1992 BSI 04-19991 0 Introduction Thermocouples are by far the most common temperature
16、 sensors in industrial use. They possess the virtues of simplicity, ruggedness, low cost, small physical size, wide temperature range (from about 270 C up to 3 000 C) and convenient electrical output. These properties make them very suitable for multi-point temperature measurement and monitoring in
17、large and complex process plant, and for an enormous variety of industrial, technological and scientific applications. The thermocouple 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
18、established by William Thomson (later Lord Kelvin) in 1854. The platinum-10 % rhodium/platinum thermocouple, which was for a long time specified as the interpolating instrument in realizing the International Temperature Scales in the range from 630 C to 1 064 C, was originally developed by Le Chatel
19、ier in 1886. Most of the commonly used base metal thermocouples were developed during the first decade of the twentieth century. For a proper understanding of how thermocouples function and how to use them, it is essential to realize that thermoelectricity is a bulk property of metallic conductors i
20、n the same sense as thermal conductivity and electrical conductivity. Although thermoelectric effects manifest themselves in circuits comprising two or more dissimilar conductors, they are not due to any special properties of the junctions between the conductors. The junctions, which for successful
21、measurements have to be at uniform temperatures, are needed only to complete the measuring circuit and are thermoelectrically inactive. In fact there will be contact potentials at junctions between different metals, but these are not thermoelectric in origin and they are not significantly temperatur
22、e dependent. Therefore when all contact potentials in a circuit loop are summed, their net result is effectively zero. By contrast, the chemical state and physical condition, e.g. strained or annealed, of the conductors in regions where they experience temperature gradients, can have a profound effe
23、ct on the electromotive force (e.m.f.) generated. Great care should be exercised in how the conductors are treated in these regions and users 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 a
24、pplication a brief account is given of the principal thermoelectric effects, since this should be helpful in achieving an understanding of good practice in thermocouple thermometry. Reference should be made to textbooks on thermoelectricity and, more generally, on the electrical properties of metals
25、 and alloys, for detailed theoretical discussion. Thermocouples are used in so many and varied circumstances that it has only been possible 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
26、so that the most appropriate choices of thermocouple and instrumentation can be made, and their effective application achieved. 1 Scope This part of BS 1041 provides guidance on the selection and use of thermocouples. It provides an introduction to the operating principles of thermocouples and their
27、 application to the measurement of temperature. A brief review of thermoelectricity and basic thermocouple circuits and an overview of the materials commonly used in thermocouples in various temperature ranges, with their strengths and weaknesses are included. The fabrication of thermocouples, assoc
28、iated hardware, measurement techniques, tolerances and calibration are described. 2 Definitions For the purposes of this British Standard the following definitions apply. 2.1 thermoelectricity 1) Electricity generated in a conductor by virtue of a temperature difference (temperature gradient) within
29、 it. 2) The branch of science concerned with electric effects produced in conductors by means of heat. 2.2 thermoelectric e.m.f. the electromotive force established in a conductor by virtue of a temperature gradient within it (the Seebeck effect) 2.3 thermoelectric power the thermoelectric e.m.f. pr
30、oduced in a conductor per unit temperature difference NOTE 1Thermoelectric power is also known as thermopower or the Seebeck coefficient. Licensed Copy: sheffieldun sheffieldun, na, Fri Nov 24 06:05:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 1041-4:1992 2 BSI 04-1999 NOTE 2Thermopower is the t
31、hermoelectric 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 together at the measuring junction which is maintained at
32、 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 measure the net thermoelectric e.m.f. (or sometime
33、s 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 thermoelements used and on the temperatures of the measuring
34、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 0 C, the opposite applies. 2.5 thermoelements the two conductors used in a thermocouple,
35、 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 motions are such that in the absence of any external
36、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 the motions are again modified, this time with the
37、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 structure of the conductor. As it is difficult to
38、 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 has to be of a different material, i.e. one forms
39、 a thermocouple and observes the difference in the thermoelectric e.m.f.s1) 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 on the thermoelectric powers of the two conducto
40、rs 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 Sa and Sb are the thermoelectric powers of conductors a and b (in 4V/C) t1 and t2
41、 are 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 only to connect them together. Equations 2 and 3 on
42、the other hand suggest that E is the difference between junction e.m.f.s, Eab, between the two conductors at temperatures t1 and t2. Circuit analysis can proceed as if this is the case, but those concerned with making, calibrating, installing and using thermocouples will need to bear in mind the sou
43、rce 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 (or in terms of equation 1, the initial and final limits of the integrations) should always be isothermal and therefore should not themselves contribut
44、e 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, its output is more properly termed an electromoti
45、ve force, or e.m.f., than a voltage. (1) E = Eab(t1) Eab(t2); or(2) E = Eab(t1) + Eba(t2).(3) ESadtSbdt t1 t2 + t2 t1 = Licensed Copy: sheffieldun sheffieldun, na, Fri Nov 24 06:05:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 1041-4:1992 BSI 04-19993 Since thermopower is a property of bulk condu
46、ctors and not of junctions, it follows 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
47、on the temperature profile, i.e. on 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
48、flow. If a portion of a conductor 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 eit
49、her lost or gained appears as heat 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 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 (in 4V/C). Although the ef