Determination of SPRT Realization Uncertainties Between Points
Christopher W. Meyer, Dean C. Ripple
Calibrated Standard Platinum Resistance Thermometers (SPRTs) are used to realize the International Temperature Scale of 1990 (ITS-90) from 13.8033 K to 1234.93 K. The SPRTs are calibrated at a series of fixed points, each assigned a temperature on the ITS 90, by measuring the ratios of the SPRT resistances at those temperatures to that at the triple point of water (TPW). For realizing the scale with a calibrated SPRT, a user measures the resistance ratio at the unknown temperature and uses ITS-90-defined nist-equations to interpolate between fixed points. The uncertainty of the SPRT temperature is therefore largely influenced by the propagation of fixed-point resistance-ratio uncertainties. For properly calculating the total SPRT temperature uncertainty, correlations of uncertainty components must be considered, in particular those involving measurement of the TPW resistance. The calculation method depends on three uncertainty-correlation-related factors involving SPRT use and calibration. The different combinations of these factors result in six different methods for calculating the realization uncertainty. This paper presents these six methods, specifies the conditions of their use, and discusses the relevant uncertainty components for each of them. It also compares the results of these methods with those of two approximations that may be used for calculating the uncertainty and explains the conditions under which the approximations agree with the proper calculations.