Abstract
The long-term stability of standard platinum resistance thermometers (SPRTs) in a temperature range between 0.01 °C and 419.527 °C was investigated from 352 samples of data accumulated from fixed-point calibration conducted during a period of eleven years. The rates of change of resistance RTPW at the triple point of water and the resistance ratio at the freezing point of Sn (WSn) and Zn (WZn) were analysed. For 75% of the samples investigated, the rate of change of RTPW was within 5.17 mK/year in magnitude. For the same portion of SPRTs, the rates of change in WSn and WZn were within 0.82 mK/year and 1.69 mK/year, respectively. Further statistical investigation showed that measurement at the triple point of water only was not a sufficient test to identify the change in WSn or WZn. However, due to the strong correlation between the changes in WSn and WZn, measurement of the resistance ratio at one of the fixed points in the range of usage is adequate to identify possible change in the entire range of usage.
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