The triple point of xenon

The triple point of xenon is shown to be a suitable fixed point for incorporation into the next revision of the International Temperature Scale (ITS) as a means of reducing the non-uniqueness in the important 84 K to 273 K range. Isotopic effects, once thought to be a limiting factor, are shown to be negligible. Purity remains the overriding concern, but high-purity xenon gas is obtainable, giving a very flat plateau with a melting range within ±10 µK from 50% to 90% of the melted sample fraction. The triple point temperature of xenon is shown to be 161.405 96 K ± 0.32 mK ( k = 1) on the ITS-90. The propagated calibration uncertainty of the platinum resistance thermometers and the component attributed to the non-uniqueness of the ITS-90, as evidenced by the differences among the seven calibrated thermometers, dominate the overall uncertainty of the estimated triple point temperature. The xenon triple point itself is highly reproducible, with a standard deviation of 48 µK for the eight melts of this study and a total realization uncertainty of just 76 µK.