Applying Isotopic Effect in ITS-90 SPRT Calibrations

The International Temperature Scale of 1990 (ITS-90) defines exact values for all fixed-point temperatures. For example, for the standard platinum resistance thermometers (SPRT), at each fixed point, the measured SPRT resistance and the temperature defined in the ITS-90 are used as input data into the correction equations of the ITS-90. Starting from 2006, formal equations were added to the Technical Annex for the ITS-90 for computing the fixed-point temperatures of the substances of different isotopic compositions, presently the triple and vapor-pressure points, Ne triple point, and triple point. This paper addresses the issue of the method required to apply the procedure defined in the ITS-90 for the calibration of a SPRT, according to the new requirements. The required procedure does not involve a correction of the fixed-point temperatures, since they are defined exactly by the ITS-90, but requires instead, the re-computing of the measured resistances at the relevant fixed points. In those cases where resistance ratios with respect to the triple point of water are required, the re-computation must be first applied separately to the specific fixed points and to the triple point of water. In case the re-computation is not possible because of insufficient information on the isotopic composition of the sample used, an additional component must be added to the total uncertainty budget.