The transition from ITS-90 to primary thermometry above 1235 K

The international system of units (the SI) is currently defined in terms of fundamental constants with defined values. The kelvin is defined in terms of the Boltzmann constant (k). The new definitions have their associated mise en pratique (MeP) to indicate how the definition of the SI base unit, such as the kelvin, symbol K, may be realized in practice. For the case of temperature, the MeP-K enables the practical implementation of primary thermometry techniques which provide thermodynamic temperature, T. The MeP provides guidelines for the realization and dissemination of the kelvin using thermodynamic thermometry while also recognizing the continued use of International Temperature Scales (ITS-90 and PLTS-2000) as practical references for most temperature measurements, T90. Above the freezing point of silver (1234.93 K ), T₉₀ temperatures, defined by the ITS-90, are determined using the Planck radiation law in a relative manner. The primary method included in the MeP-K for this range is primary radiometric thermometry, which applies the same law in both absolute and relative forms. In this article, a description of the primary methods for measuring T in comparison with T90, above the silver point is presented. A review of the different possibilities using absolute and relative primary radiometric thermometry is provided and the uncertainties and feasibility studied. We illustrate our findings with some examples of primary thermometry measurements performed at the Centro Español de Metrología. These examples provide a comprehensive overview of the advantages or disadvantages of using ITS-90 and the MeP-K primary thermometry above the silver point. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.