No abstract

Standard platinum resistance thermometers that are constructed with transparent sheaths of fused silica or pyrex glass are subject to thermal losses arising from losses of visible and near-infrared radiation up (piping) and through these sheaths. This heat loss may introduce substantial errors in temperature determinations; e.g. 0.084, 0.015, and 0.001 1 °C at the Sb, Zn, and Sn fixed-temperature points for a fused-silica sheath, or 0.000 2 °C at the Zn and Sn points for a pyrex sheath. This effect has been investigated in detail for thermometers of various types at the Sb, Zn, and Sn points, and it has been shown that simple stem radiation-traps, consisting of a blackening or roughening of the smooth outer surface of the thermometer sheaths, will eliminate these errors and greatly improve the immersion characteristics of the thermometers.Suitable radiation-trapping should be incorporated on the stems of every transparent-sheathed standard platinum resistance thermometer; without strong evidence to the contrary, radiation loss errors of the orders of magnitude cited above should be suspected in reported measurements involving resistance thermometers having unprotected transparent fused-silica sheaths.

The thermoelectric voltage of the gold/platinum thermocouple has been measured over the range 0–962 °C by comparison with calibrated platinum resistance thermometers. From 0 to 500 °C, the measurements were carried out in stirred liquid baths; from 660 to 964 °C, they were carried out in a pressure-controlled sodium-filled heat pipe furnace that provided an isothermal intercomparison environment. Measurements were also made in the metal freezing points of indium, tin, zinc, antimony, and silver, at the melting point of gallium, and at the liquidus point of the silver-copper eutectic. By fitting the measured thermoelectric voltages to a single eighth-degree polynomial in temperature by the method of least squares, a reference function is obtained for the Au/Pt thermocouple that provides emf as a function of temperature (ITS-90) to within ±10 mK from 0 to 962 °C. The Au/Pt thermocouple merits serious consideration for precise temperature measurements as its stability approaches that of the high-temperature platinum resistance thermometer.

Extensive thermal analyses have been made on 10 samples (suppliers' analyzed impurity contents <0.2 to