Accurate thermometry by means of industrial platinum resistance thermometers

Abstract

The continuous improvement of the manufacturing techniques of sensing elements for industrial platinum resistance thermometers and assembling techniques makes it possible to use these thermometers for measurements to better than ±0.1°C in the temperature range 0 to 630°C. A selection test and a suitable calibration are necessary to achieve such a goal. The paper addresses both aspects on the grounds of the results obtained in an experiment carried out with the grants of the EC Community Bureau of Reference (BCR) and with the cooperation of most European manufacturers. As regards the calibration, care was exercised to detect hysteresis and to interpolate the results. The conclusion is that elements using wire coils in alumina capillaries do not show appreciable hysteresis when used in the temperature range 0 to 630°C, whereas they do when heated to 850°C. A least-square-fitted quadratic equation is suitable to achieve the goal of an uncertainty of ±0.06°C in the temperature range 0 to 630°C. The uncertainty decreases by more than a factor of two fitting a fourth-degree polynomial to the differences with respect to a reference table. No comparable result was obtained with the IPRTs used up to 850°C. A procedure is outlined to convert the IEC reference table in order to account for the new International Temperature Scale of 1990.