Uncertainty analysis of MSL’s CCT-K7.2021 key comparison measurements taking account of correlations

Abstract The National Research Council of Canada (NRC) recently piloted the second international key comparison of triple point of water cells, CCT-K7.2021, where participants submitted a transfer cell and a report of the temperature difference between their transfer cell and their respective national reference. By comparing the differences between each transfer cell, NRC could calculate the differences between each participant’s national reference. While there was good agreement among all the participants, there was a large variation among the submitted uncertainties. This was possibly due to the complexity of the uncertainty analysis, which typically has many correlated components between the transfer cell and the national reference that partially cancel when the difference is taken. The Measurement Standards Laboratory of New Zealand (MSL) carried out its analysis using a detailed measurement model that included all known correlations and applied an algorithmic approach fully compliant with the Guide to the Expression of Uncertainty in Measurement . As a consequence, MSL submitted an uncertainty value less than half the uncertainty values submitted by almost all of the other participants. This paper describes the measurement model implemented by MSL and demonstrates how accounting correctly for correlations leads to a significantly lower uncertainty. The paper also shows how applying the same method to the previously published NRC analysis results in a reduction in the NRC uncertainty by a factor of about 2.5, making it smaller than the MSL uncertainty.