Analysis of inhomogeneity in Au/Pt thermocouples to allow intercomparisons at better than 10 millikelvins

In a previous supplementary comparison of Au/Pt thermocouples, APMP.T-S4, it was demonstrated that these artifacts could successfully determine the equivalence of national scales within 40 mK at the Ag point (961.78 °C). However, the uncertainty arising from the measured inhomogeneity of the thermocouples was one of the dominant uncertainty components. To reduce this uncertainty, in the recent comparison APMP.T-S5, piloted by NMIA, the six participating NMIs were asked to provide more detailed data on the temperature profiles of the fixed point furnaces. This data was convolved with the measured thermocouple inhomogeneity to estimate the systematic error in the temperature difference measured between participants. The revised analysis allowed the calculated effect of a ± 0.004% inhomogeneity to be reduced from ± 0.004% of the thermocouple signal to below ± 0.0015%, and allowed Au/Pt thermocouples to determine inter-laboratory temperature differences to better than 16 mK (k = 2) at the Ag point.