Approximation of the ITS-90 with high temperature thermometers up to 1085 °C

The upper limit of the ITS-90 in the contact thermometry part of the scale is the silver freezing point at 961.78 degreesC. This limit is justified by the decrease of the stability and reliability of high-temperature platinum resistance thermometers above the silver point. It was shown in several papers before that some High Temperature Platinum Resistance Thermometers (HTPRTs) were able to work up to the gold and copper points with accuracy better than that for thermocouples or pyrometers. So, HTPRTs can be successfully used in some applications in the high-temperature range, such as calibrations of standard thermocouples or pyrometers by comparison with a high-temperature thermometer. To calculate the temperature measured with an HTPRT above the silver point, it is necessary to develop an interpolation function. Some methods for the interpolation have already been suggested in several papers. In the work on developing the new reference functions for noble metal thermocouples (Burns et al. 1992) one interpolation model was applied to a Russian-made HTPRT. In this paper we investigate eight interpolation equations, for which three to five calibration points are used. The functions were applied to 13 Russian HTPRTs calibrated up to 1064.18 degreesC and seven HTPRTs calibrated up to 1084.62 degreesC. The uncertainty of the approximation was found to vary from 3 mK to 25 mK depending on the thermometer and interpolation model.