Characterisation of furnace thermal effects on the reproducibility of high-temperature fixed-point cells. The INRiM contribution in the framework of the WP1 of the EMPIR project 18SIB02 "Realising the redefined kelvin” – (Real-K)

The primary scope of the EMPIR project 18SIB02 Real-K – “Realising the redefined kelvin”, has been those to turn the new kelvin redefinition and its associated Mise en Pratique into a real practice through different measurement techniques. The aim of the Work Package 1 of this project was that of prepare, demonstrate and establish the future dissemination of the redefined kelvin, in the temperature range from 1300 K to 3000 K, by radiation thermometry. This has been achieved by developing a number of high-temperature fixed points (HTFPs) to cover the whole temperature range above the freezing temperature of copper (the current highest fixed point of the International Temperature Scale (ITS-90). Specifically, the thermodynamic temperatures of four new HTFPs, namely, Fe-C (1426 K), Pd-C (1765 K), Ru-C (2226 K) and WC-C (3020 K) had to be established with low uncertainty. The main and more important aspect regarding the temperature uncertainties is those related to the sensitivity of such cells to thermal conditions during their implementation and the effect of changing these thermal conditions on the uncertainty in the determination of the temperature of their phase transition. The INRiM radiation thermometry laboratory performed the characterization of the thermal effects on the reproducibility of Fe-C and Pd-C cells using its primary standard radiation thermometer and a three-zone high-temperature furnace equipped with a specific and characterised internal arrangement in order to obtain the needed longitudinal temperature gradients along the cells. The output of this work, coordinated by NPL, is described in the following technical report. The report compares the results of the INRiM measurements with those of the other NMIs participating in this WP and provides a final uncertainty budget, related to the possible thermal effects induced by the different arrangements of the furnaces used, for all four HTFP cells studied.