Final report on key comparison CCT-K4 of local realizations of aluminium and silver freezing-point temperatures

This report describes key comparison CCT-K4 held under the auspices of the Consultative Committee for Thermometry (CCT). For the first time, local realizations of ITS-90 temperatures were compared by directly circulating aluminium and silver freezing-point cells. Twelve national metrology institutes (BNM/INM, IMGC, KRISS, NIM, NIST, NMIJ, NMi/VSL, NML, NPL, NRC, PTB, VNIIM), organized into three subgroups, American, European and Asian/Australian, contributed to the results. The American subgroup used an exemplary bilateral and the other subgroups a round-robin procedure. Key comparison reference values (KCRV) for both freezing points were calculated as the weighted average temperatures using the squared sum of the combined standard uncertainties of the laboratories and an uncertainty representing the stability of the circulated cells to set the weights. For the aluminium freezing point, the absolute temperature differences from the corresponding KCRV varied between 0.13 mK and 4.35 mK with expanded uncertainties between 1.08 mK and 4.16 mK (95 % level of confidence). For the silver freezing point, the absolute differences ranged from 0.74 mK to 13.03 mK and the expanded uncertainties were between 1.01 mK and 12.89 mK.