Investigations on the triple point of water by means of a cryogenic current comparator

Thanks to achievable uncertainties down to parts in 109, cryogenic current comparator-based measurement brid- ges are well-established in resistance metrology. Here we consider a demanding application in the context of resistance thermometry, the investigation of the effect of hydrostatic pressure on the temperature measured in a triple point of water cell using a standard platinum resistance thermometer. In this contribution we deal with specific requirements for the proper operation of the chosen measurement bridge which is transportable and commercially available. The estimation of the temperature resolution includes an Allan variance analysis and provides standard uncertainties of a few μK at current levels of order ±100 µA flowing through the thermometer. Measurements of the temperature profile within a triple-point of water cell provided a coefficient for the hydrostatic pressure effect of (-0.876 ± 0.049)·10−3 K·m−1. This result is consistent with previous findings of Nakanishi and Sakurai who reported a value of (-0.854 ± 0.038)·10−3 K·m−1. However, even within the expanded uncertainty, the results of these two independent experiments do not include the agreed correction coefficient of -0.73·10−3 K·m−1 used for ITS-90 calibrations.