Pt-sheathed platinum resistance thermometer at the freezing point of copper

The present status of a platinum sheath type high-temperature platinum resistance thermometer (Pt-sheathed HTPRT) is reported. The resistances of Pt-sheathed HTPRTs were measured at the freezing point of copper, which is higher than the upper temperature limit for a standard PRT defined in the ITS-90. The result showed that the resistance of the Pt-sheathed HTPRT was reproducible with a standard deviation of 4 mK at that temperature. This indicates that the thermometers developed are NOT contaminated by metal vapor, which is considered as the most serious problem for an SPRT for use at high temperatures.