EFFECT OF PURITY OF THE PLATINUM WIRE OF HTPRTS ON THEIR CHARACTERISTICS

摘要

High temperature platinum resistance thermometers (HTPRTs) were introduced in the International Temperature Scale of 1990, replacing thermocouples in the range 660.323°C - 961.78°C. That resulted in increasing the reproducibility of the scale by more than an order of magnitude. In the definition of ITS-90 there is a requirement on the quality of the platinum used in the platinum resistance thermometers in all the temperature sub-ranges above 0°C. The resistance ratio, W(Ga), at the melting point of gallium must not be less than 1.11807. Experience in using HTPRTs showed that different long-term stability of the resistance might be obtained at different temperatures. The reasons for less stability at high temperatures are possibly due to changes in crystal structure of the platinum and mobility of impurities and defects. In this paper the work, carried out at VNIIM on the investigation of HTSPRTs with different values of W(Ga) is analyzed and summarized to discuss the influence of the purity of the platinum wire on the stability of the HTSPRTs. A suggestion is made to tighten the requirement on W(Ga) for platinum resistance thermometers used at high temperature as standard interpolation instruments of the ITS-90.