ITS-90 APPROXIMATION BY MEANS OF NON-STANDARD PLATINUM RESISTANCE THERMOMETERS

摘要

Many laboratories are using Platinum Resistance Thermometers that do not satisfy the requirements of the ITS-90 for its Standard Platinum Resistance Thermometer as working standard for temperature calibrations. Indeed, one of the best procedures for a calibration laboratory is to equip each thermostatic bath or furnace with one working standard thermometer. For this use, the working standard thermometer need be inexpensive and exhibit good stability, but only in the operating range of its bath or furnace. Then, thermometer calibration can be carried out either directly by comparison with the standard reference thermometer when the best accuracy is required, or by comparison with the working standard thermometer when a lower accuracy is sufficient. This lower accuracy depends on the stability of the working standard thermometer between two subsequent calibrations and on the capability of the interpolating equation to approximate the ITS-90. The interpolating equations of the ITS-90 cannot be used, since these thermometers do not satisfy the requirements of the ITS-90. Also the widely-used Callendar-Van Dusen equation results inadequate to describe the behaviour of these thermometers at the level of a few mK. Therefore, several different alternatives have been examined on the basis of calibration data in the temperature range from -80°C to 650°C of many of these working standard thermometers, employed by several laboratories of the Italian Calibration Service (SIT), and adequate equations are presented.