Estimation of the degree of equivalence of the ITS-90 above the silver point between NPL, NIM and CEM using high temperature fixed points

Abstract

Formal pair-wise temperature differences, a comparison reference value (CRV) and degree of equivalence, between the UK (NPL), Chinese (NIM) and Spanish (CEM) national measurement institutes (NMIs), are calculated from the results of a prototype key comparison of the ITS-90 above the silver point using high temperature fixed points (HTFPs) of Co–C (1324 °C), Pt–C (1738 °C) and Re–C (2474 °C). The local realizations of ITS-90 temperatures were assigned by NPL, NIM and CEM to their individual sets of HTFPs. NIM and CEM then transported their cells to NPL and the temperature was realized by all the cells determined in quasi-identical measurement conditions using a linear pyrometer. From these results the pair-wise temperature differences between the three institutes were established. In addition a non-weighted and a weighted CRV were calculated for the comparison and degree of equivalence estimated. The results show very close agreement of the ITS-90 as realized by the three institutes. It is argued that the ease of the comparison and the low uncertainties attained strongly suggest that HTFPs should be the scale comparison artefacts of choice when comparing primary realizations either of the ITS-90 or thermodynamic temperature.