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ITS-90 Non-Uniqueness From PRT Subrange Inconsistencies Over the Range 24.56 K to 273.16 K



Christopher W. Meyer, Weston L. Tew


Calculations have been performed to study ITS-90 non-uniqueness over the range 24.5561 K to 273.16 K, where the scale is defined by an interpolating platinum resistance thermometer (PRT) that is calibrated via sets of defined fixed points. Over this range, the definition allows for the PRT to be calibrated over any of four different subranges. Each subrange has its own functional form for interpolation and its own set of calibration fixed points, and all subranges yield calibrations with equally valid realizations of the scale. Because the interpolations for the different subranges are not identical, temperature realizations from a PRT resistance measurement do not result in the same values of the ITS-90 temperature T90 when using calibrations from different subranges; these subrange inconsistencies yield non-uniqueness of the scale in the regions between fixed points. For this work, subrange inconsistency calculations have been performed on eighteen PRTs; fourteen are standard PRTs and four are miniature PRTs. The inconsistency uncertainties, which result from propagation of fixed-point uncertainties, have also been calculated. Statistics on the inconsistencies have been performed for the purpose of helping estimate an uncertainty component to T90 due to the inconsistencies. The calculations show that PRT subrange inconsistencies in the temperature region studied can be as large as 1 mK. Over small temperature ranges in this region, the inconsistencies can result in T90 uncertainties that dominate those propagated from fixed-point uncertainties. We have also studied possible correlations between PRT subrange inconsistencies and other PRT properties/parameters that are simpler to determine; these studies show that there is a correlation


ITS-90, non-uniqueness, platinum resistance thermometry, subrange inconsistency, temperature scale, uncertainty


Meyer, C. and Tew, W. (2006), ITS-90 Non-Uniqueness From PRT Subrange Inconsistencies Over the Range 24.56 K to 273.16 K, Metrologia, [online], (Accessed July 18, 2024)


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Created September 1, 2006, Updated February 19, 2017