A Challenge to Improve High-Temperature Platinum Resistance Thermometer

Abstract

High-temperature standard platinum resistance thermometers (HTSPRTs) are used to interpolate the international temperature scale of 1990 (ITS-90), especially for temperatures between the aluminum and the silver points. For this, long-term stability of the HTSPRT is essential. CHINO R800-3L type SPRT, which has a nominal resistance at the triple point of water (TPW) around 0.25 $\Omega$, is the one developed earlier for the interpolation of the ITS-90 at this temperature range. Further development to this previous model has been carried out for the purpose of improving the thermal stability. The improvement was focused on reducing the effect coming from the difference in thermal expansion between platinum wire and the quartz frame on which the platinum wire is installed. New HTSPRTs were made by CHINO Corporation. Some series of tests were carried out at CHINO and at NMIJ. Initial tests after the HTSPRT fabrication were done at CHINO, where thermal cycles between $500\,{^{\circ }}\hbox {C}$ and $980\,{ ^{\circ }}\hbox {C}$ were applied to the HTSPRTs to see change in the resistances at the TPW $(R_{\mathrm{TPW}})$ and at the gallium point $(R_{\mathrm{Ga}})$. Repeated resistance measurements at the silver point $(R_{\mathrm{Ag}})$ were performed after completing the thermal cycling test. Before and after every measurement at silver point, $R_{\mathrm{TPW}}$ was measured, while before and after every two silver point realization $R_{\mathrm{Ga}}$ were measured. After completing this test, the HTSPRTs were transported to NMIJ, where the same repeated measurements at the silver point were done at NMIJ. These were then repeated at CHINO and at NMIJ upon repeated transportation among the institutes, to evaluate some effect due to transportation. This paper reports the details of the above-mentioned tests, the results and the analysis.