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Type 3 Non-uniqueness in Interpolations Using Standard Platinum Resistance Thermometers Between − 196 °C and 100 °C

Rusby, R L; Stemp, H; Pearce, J V; Veltcheva, R I (2019) Type 3 Non-uniqueness in Interpolations Using Standard Platinum Resistance Thermometers Between − 196 °C and 100 °C. International Journal of Thermophysics, 40 (11). 103 ISSN 0195-928X

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Abstract

This paper reports on precise measurements of the non-uniqueness (interpolation errors) associated with using Standard Platinum Resistance Thermometers (SPRTs) in the range from -196 °C to 100 °C. Six long-stem SPRTs were compared with unprecedented (~0.1 m°C) precision in a stirred oil bath over the range from 100 °C down to -95 °C, with an additional comparison measurement in a bath of liquid nitrogen at -196 °C. The measurements resulted in a highly self-consistent set of resistance ratios W(t), where W(t) = R(t) / R(0 °C). Selecting one of the SPRTs as the reference, the differences (Wx - Wref), x = 1 to 6, were calculated, in the manner of the International Temperature Scale of 1990, ITS-90, and then used to investigate the consistency of interpolations between the SPRTs in various temperature ranges. With some minor exceptions the data suggest that the interpolation consistency (taken to be an upper limit for the Type 3 non-uniqueness between the SPRTs) was ±0.03 m°C over the range from -38 °C to 30 °C (representing the mercury to gallium subrange in the ITS-90), ±0.1 m°C over the range -95 °C to 80 °C, and ±0.2 m°C at -95 °C for interpolations extending from -196 °C to 0 °C (no data could be taken between -196 °C and -95 °C). Various alternative interpolation schemes are investigated, especially to examine ways of avoiding the use of the mercury triple point in the ITS-90. One scheme of interest for long-stem SPRTs would be simply to omit the mercury point and extend the subrange from the argon point up to the gallium point or, with suitably-chosen interpolation equations, to higher fixed points. These possibilities remain to be fully investigated.

Item Type: Article
Subjects: Engineering Measurements > Thermal
Divisions: Engineering
Identification number/DOI: 10.1007/s10765-019-2560-y
Last Modified: 19 Feb 2020 15:50
URI: http://eprintspublications.npl.co.uk/id/eprint/8643

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