Platinum Metals Rev., 1978, 22, (3), 89
Thermocouple Reference Tables
An International Standard Accepted
The accurate measurement of high temperatures made possible by the use of rhodium-platinum thermocouples depends uponthe use of reference tables which are themselves not only accurate and reliable but are accepted throughout the world ofscience and technology. For many years differences were to be found between the tables issued respectively by British and American standardising bodies, but a move to eliminate this confusion and to provide internationally agreed tables wasmade jointly by Johnson Matthey and Engelhard Industries in 1965 in an approach to the British Standards Institution and the National Physical Laboratory. This led to a collaborative investigation by the National Bureau of Standards in Washington, the National Research Council of Canada and the N.P.L. and to the re-calibration of couples supplied by seven leading manufacturers. An account of this work was published in this journal in 1972 (1).
The resulting reference tables, based upon the 1968 revision of the International Practical Temperature Scale (IPTS–1968), were adopted and published in 1973 by the British Standards Institution as B.S. 4937 and in identical form by the National Bureau of Standards as Monograph 125, these replacing the older sets of tables (B.S. 1826 of 1962 and N.B.S. Circular 561 of 1955) which showed substantial differences, more particularly between N.B.S. 561 and the new tables(2).
The next move was to secure wider international agreement and so to promote the interchangeability of thermocouples and to eliminate a common source of error and confusion, but there has been some reluctance on the part of many of the major industrialised nations to amend their own standards until such international agreement could be achieved. This was a matter for the International Electrotechnical Commission (IEC) based in Geneva and in 1974 a committee was formed for this purpose. Now an IEC Standard—584–1 “Thermocouples, Part I Reference Tables” has been issued. These tables are in agreement with both B.S. 4937 and N.B.S. 125, and the letter designation employed in those standards is confirmed:
Type S: 10 per cent Rhodium-Platinum : Platinum
Type R: 13 per cent Rhodium-Platinum : Platinum
Type B: 30 per cent Rhodium-Platinum : 6 per cent Rhodium-Platinum
The remainder of the standards cover the base metal combinations iron: copper-nickel, copper: copper-nickel, nickel-chromium: copper-nickel and nickel-chromium: nickel-aluminium.
The following countries represented on the IEC committee voted in favour of publication of this new international standard:
Republic of South Africa
United States of America
This step forward towards international unification is of great importance, and it is to be hoped that all national bodies will now amend their standards—particularly those based upon the old N.B.S. 561—to fall into line with IEC 584–1. The general adoption of these tables will be of great value in eliminating a common source of error and of confusion between workers in different countries. A further advantage is that the new values show a much smoother progression, providing an increased accuracy of interpolation.
Further work is being carried out by an IEC Technical Committee (65B) on a set of internationally acceptable tolerances for all seven thermocouple combinations, and these should eventually be issued as Part II of the 584–1 standard.
- 1 T. J. Quinn and T. R. D. Chandler, Platinum Metals Rev., 1972, 16, (1), 2 – 9
- 2 P. H. Wells, Platinum Metals Rev., 1973, 17, (3), 96 – 97