Realizing the redefined Kelvin: Extending the life of ITS-90

Following the redefinition of the kelvin, the user is presented with a more nuanced traceability route for temperature through the mise en pratique for the definition of the kelvin (MeP-K-19). Here we describe research to address several present and potential shortcomings with the current main dissemination route, namely the International Temperature Scale of 1990 (ITS-90). The ITS-90 has served the global temperature measurement community well, providing reliable, low uncertainty traceability for over 30 years. However, there are some potentially life-limiting issues for it. Among these are the impact of the main types (1 and 3) of non-uniqueness which represent an important contribution to the uncertainties achievable with the ITS-90, and the possible need to identify an alternative to the mercury triple point, a key fixed point of the ITS-90 whose use could be banned by an international treaty. Significant progress has been made in addressing these problems through a) new determinations of Type 3 non-uniqueness which have been undertaken in the range –189 °C to 156 °C; b) a comprehensive evaluation of Type 1 non-uniqueness on a large number of Standard Platinum Resistance Thermometers (SPRTs) across multiple temperature regions; c) comparison of high temperature SPRTs in pressure-controlled heat pipes to characterize Type 3 non-uniqueness between 660.323 °C and 961.78 °C and d) new designs of CO2 and SF6 cells for use with long-stem SPRTs. The fixed-point cells have been improved compared to previous versions by using purer gases and more stable and uniform temperature-controlled baths, and by the development of a flexible set-up that can accommodate both capsule and long-stem SPRTs. The effect of replacing mercury on the ITS-90 interpolating equations and uncertainty propagation has also been investigated.