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.

Marcarino and Bassani have proposed the sodium liquid-vapour equilibrium as a means to approximate the ITS-90 between 660 C and 962 C with an accuracy of about 10 mK. We have also measured the sodium vapour pressure over this temperature range using a pressure-controlled heat pipe. Our results differ from those of Marcarino and Bassani by approximately 34 mK at 660 C and 21 mK at 962 C with a linear temperature dependence of the difference between these values. This relationship may be explained very satisfactorily by a difference in the impurity content of the two samples. We show that an additional 200 10 -6 mass fraction of potassium in our sample as compared with theirs is sufficient to explain the discrepancy. As sodium of sufficient purity to ensure repeatability of its use as a temperature standard at the millikelvin level is not commercially available, it appears necessary to characterize the pressure/temperature relationship for each sample prior to relying on a measurement of the pressure to determine the temperature of the heat pipe. The alternative is to rely on the heat pipe as an intercomparison device with the temperature given by a calibrated thermometer.

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We have measured the vapour pressure of caesium over the temperature range 370 C to 660 C using a pressure-controlled heat pipe. The equation log( /Pa) log( /K) 2 with the coefficients 34,573234, -4979,5799 K, -9,323 4247, 4,473 3132 10 -3 K -1 and -8,684 092 10 -7 K -2 fits the data to within ±8 mK over the entire range. In addition, the non-uniqueness of the ITS-90 appears to be less than ±1,5 mK over the same temperature range based on the data from three thermometers.

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