Platinum Metals Rev., 1976, 20, (1), 10
Platinum and Its Alloys in Temperature Measurement
Temperature Measurement 1975: Papers from the European Conference on Temperature Measurement Edited By B. F. Billing and T. J. Quinn, Institute of Physics, London, 454 pp. £20 ($50)
It is nearly five years since the last major conference on temperature measurement was held in Washington, and during this period there has been considerable activity over the whole field of standardisation and the formulation of temperature scales. The present volume contains the seven invited and 44 contributed papers given before an international conference held at the National Physical Laboratory, Teddington, England, in April last year, sponsored by The Institute of Physics, The Institute of Measurement and Control, The Institution of Electrical Engineers and the European Physical Society.
An introductory chapter by T. J. Quinn of the N.P.L. reviews the whole range of temperature standards; this is followed by six chapters covering specific methods, each containing a review paper and individual papers on techniques.
In the opening review paper on resistance thermometry J. S. Johnston of Rosemount Engineering deals with the construction, accuracy, reliability and application of the platinum resistance thermometer—a device now produced and used in industry in numbers of the order of several million a year—and refers to the newer technique of employing a screen-printed film of platinum instead of a fine wire. Detectors made by this process have a performance over the range —50 to 500°C comparable with that of wire-wound glazed thermometers and may replace them in a number of industrial applications, although for many years the more conventional construction will continue to be used for their wider temperature range and good stability. One of the main problems in resistance thermometers remains, however, the need for a detector capable of withstanding the vibrations and shock associated with some industrial uses. Here a construction in which platinum wire is fully encapsulated offers the greatest reliability.
Work at the National Research Council of Canada has recently shown that oxygen-activated thermal cycling effects can occur in platinum resistance thermometers, causing some instability, and a paper by R. J. Berry tentatively identifies this effect—first the oxidation of the platinum surface in the low temperature region and the subsequent dissociation of this oxide at around 450°C. The change is, however, measured only in parts per million and the usefulness of resistance thermometers in normal industrial practices is not likely to be affected.
A contribution by P. Marcarino and L. Crovini of the Istituto di Metrologia ‘G. Colonnetti’ of Turin proposes the use of the platinum resistance thermometer to define a provisional temperature scale up to the silver point, 962°C.
Low temperature resistance thermometry, using as the element the 0.5 atomic per cent iron-rhodium alloy introduced by Professor Brian Coles of Imperial College is dealt with in a paper by R. L. Rusby of the N.P.L. Its excellent long-term stability makes this device very suitable for precision work below 20K.
The chapter on thermocouple thermometry opens with a review paper from G. W. Burns and W. S. Hurst of the National Bureau of Standards, Washington. This deals at some length with the seven types of couples that have become standardised internationally, including of course the three principal platinum alloy combinations chosen for their resistance to oxidation, high melting points, reproducibility and long life up to as high as 1500°C. The newer combination—6 per cent rhodium-platinum against 30 per cent rhodium-platinum—has now been standardised for some years, primarily for use in the range 1250 to 1750°C, where it exhibits better mechanical strength and stability by comparison with the old established 10 and 13 per cent rhodium-platinum alloys against high purity platinum. These authors also draw attention to some modern applications of thermocouples to illustrate the severe requirements imposed by today’s technology, including their use in nuclear environments, in glass manufacturing plants where temperatures of 1570°C are reached, and in high temperature tunnel furnaces used in the production of ceramics and ferrites. The 6:30 rhodium alloy couple has also been used for measuring gas-stream temperatures as high as 1750°C in the development of jet engines, while it is finding applications in the expendable thermocouple devices for instantaneous molten metal temperature measurements in the steel industry.
Many of the other papers in this volume have to do with the finer points of precision thermometry and the further development of the International Temperature Scale, an amended version of IPTS-68 being promised at the turn of the year. Precision thermometry has now become exceedingly complex and may seem somewhat remote from the practical needs of temperature measurement in industry, but much of the work described in these papers should none the less be of interest to both manufacturers and users of resistance thermometers and thermocouples.