Development of a Standard Platinum Resistance Thermometer for Use up to the Copper Point

Abstract

The international temperature scale of 1990 defines temperatures in the range from 13.8 K to 1234.93 K ($961.78\,^{\circ }\hbox {C}$) using a standard platinum resistance thermometer (SPRT) as an interpolating instrument. For temperatures above $660\,^{\circ }\hbox {C}$, the current designs of an SPRT require extreme care to avoid contamination, especially by metallic impurities, which can cause rapid and irreversible drift. This study investigates the performance of a new design of a high-temperature SPRT with the aim of improving the stability of the SPRTs and extending their temperature range. The prototype SPRTs have an alumina sheath, a sapphire support for the sensing element, which are aspirated with dry air and operated with a dc bias voltage to suppress the diffusion of metal-ion contaminants. Three prototype thermometers were exposed to temperatures near or above the copper freezing point, $1085\,^{\circ }\hbox {C}$, for total exposure times in excess of 500 h and exhibited drifts in the triple-point resistance of less than 10 mK. The new design eliminates some of the problems associated with fused-silica sheaths and sensor-support structures and is a viable option for a high-accuracy thermometer for temperatures approaching $1100\,^{\circ }\hbox {C}$.