Analysis of SPRT Calibration Data Obtained at NPL Since the Introduction of the ITS-90

Abstract

Since the International Temperature Scale of 1990, ITS-90, was introduced, NPL has carried out a large number of calibrations of standard platinum resistance thermometers (SPRTs). Here we present an analysis of 876 calibrations of 435 SPRTs from 12 manufacturers in the various sub-ranges provided for in the ITS-90 between the triple point of argon (or, more usually, at the boiling point of nitrogen) to the freezing point of silver. Over 170 SPRTs have been calibrated more than once, in one case 10 times in 12 years. The calibration records have now been collated into a spreadsheet detailing source and identification data, calibration date, final value of $R(0.01~{^{\circ }}\hbox {C})$, the measured values of resistance ratio $W=R(t/{^{\circ }}\hbox {C})/R(0.01~{^{\circ }}\hbox {C})$ at the fixed points, and the coefficients of the deviation equations, a, b, c, and d. These data will permit investigations of the consistency over 23 years of the manufacture and quality of SPRTs including, inter alia, medium-to-long-term stability, ranges of W-values at each fixed point, correlations between W-values at different points and between the a and b coefficients, and the consistency between interpolations in different ITS-90 sub-ranges. A start has been made on these investigations, and this paper is a first report on the results.