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TitleObservations on sub-range inconsistency in the SPRT interpolations of ITS-90
Publication TypeJournal Article
Year of Publication2009
AuthorsWhite, D.R., and Strouse G.F.
Pagination101 - 108
Date Published2009
ISSN00261394 (ISSN)
KeywordsAverage values, Fixed points, Intelligent vehicle highway systems, International Temperature Scale of 1990, Interpolation, Interpolation errors, Non uniqueness, Platinum, Quality constraints, Reference resistances, Resistance ratios, Single constraints, Standard deviations, Standard platinum resistance thermometers, Thermometers, Zinc
AbstractThe properties of the sub-range inconsistency (SRI or Type-I non-uniqueness) of the standard platinum resistance thermometer (SPRT) sub-ranges of the International Temperature Scale of 1990 (ITS-90) are investigated. Mathematically, SRI has the form of an interpolation error with zeros at each of the fixed points shared by the two interpolating equations, and a magnitude dependent on the differences in the ratios (Wr - 1)/(W - 1) for each of the fixed points, where W is the resistance ratio R(T)/R(0.01 °C) and Wr is the reference resistance ratio defined by ITS-90. The calibration results for a set of 60 SPRTs were analysed to determine the SRI for the water-zinc and water-aluminium sub-ranges. The maximum SRI occurs near 93.15 °C, and has an average value of 0.12 mK and a standard deviation of 0.48 mK. The reciprocal of the ratio (Wr - 1)/(W - 1), which is proportional to the sensitivity of the SPRT, was found to be 1.0 0.0004 for all 60 SPRTs, and was within 0.00005 for all fixed points for any one SPRT. This suggests that a single constraint on the value of the ratio might be a more useful and discerning SPRT-quality constraint than the current three ITS-90 constraints. © 2009 BIPM and IOP Publishing Ltd.

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