Investigations of Type 3 non-uniqueness in standard platinum resistance thermometers between 83 K and 353 K

Abstract

A key feature of the International Temperature Scale of 1990 (ITS-90) is Type 3 non-uniqueness which arises from differences between individual Standard Platinum Resistance Thermometers (SPRTs) over a given subrange. This can be significant in the context of the best reported uncertainties of SPRT calibrations, but it is extremely difficult to determine because it is easily obscured by the measurement uncertainties. Hence, there is little good information available, particularly over much of the temperature range commonly encountered by long-stem SPRTs, and the estimates of its magnitude are potentially overstated. This paper presents high precision comparisons at NPL and CEM of two cohorts of 8 and 6 long-stem SPRTs, by measuring the ratio of each SPRT against a common SPRT in stirred liquid baths at temperatures from 178 K to 303 K, and 274 K to 353 K, respectively. Measurements of a third cohort of 8 SPRTs were made at INTiBS in a stirred liquid bath from 198 K to 303 K and in a temperature-controlled cryostat from 84 K to 185 K. The measurements were augmented by calibrations of each SPRT at the triple points of argon and mercury, the melting point of gallium and the freezing point of indium, so extending the overall temperature range down to 84 K and up to 429 K. The three cohorts were linked by two SPRTs which were common to all of them. The comparison differences, which ideally should represent the Type3 non-uniqueness, are presented for the specified ITS-90 interpolations, and for some alternative interpolation schemes. Specifically, it is assessed for interpolations in which the mercury triple point is replaced with the triple point of Xe, CO2 or SF6, which are widely considered to be potential candidates for replacing mercury in a future amendment of the ITS-90. The further alternative of using the melting point of gallium is also investigated.