Improved Estimates of the Isotopic Correction Constants for the Triple Point of Water

In 2006, the CIPM clarified the definition of the kelvin by specifying the isotopic composition of the water to be used in the realization of the triple point. At the same time, the Consultative Committee for Thermometry gave recommended values for the isotopic correction constants to be used for water departing from the specified composition. However, the uncertainties in the values for the correction constants were undesirably large due to unresolved differences between the data sets from which the values were determined. This paper derives improved values of the constants by considering additional data from isotopic fractionation measurements and the heats of fusion and freezing points of the relevant water isotopologues. Values of the corrections determined from the expanded data are AD = 671(10) μK, A18O = 603(3) μK, and A17O = 60(1) μK. A typical correction made with these values lies just within the expanded uncertainty (k = 2) of the corrections made with the older values, but has about half the uncertainty.