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The optimisation of self-heating corrections in resistance thermometry.

Pearce, J V; Rusby, R L; Harris, P M; Wright, L (2013) The optimisation of self-heating corrections in resistance thermometry. Metrologia, 50 (4). pp. 345-353.

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Corrections for the so-called self-heating effect in measurements with resistance thermometers are made by extrapolating the resistances (or bridge readings) measured at two or more currents to obtain the value at 0 mA. Such corrections are of particular significance when using Standard Platinum Resistance Thermometers with the lowest uncertainties. The purpose of this paper is to establish how to optimise the measurements so as to achieve the minimum uncertainty in the correction for a given total measurement time and maximum measuring current. The assumptions are that the self-heating effect is linear (i.e., proportional to the power dissipated) and that a measurement of resistance is dominated by statistical uncertainties which vary inversely with the current and with the square root of the measurement time. Under these assumptions it can be shown, using analytical or Monte Carlo methods, that there is no benefit in using more than two measuring currents to determine the correction and that the uncertainty is minimised when the lower current is approximately 0.5 times the higher current: it is more advantageous to spend any additional time measuring at the lower of two optimized currents than to introduce new currents. Specifically, it is shown that the optimum solution is to employ two measuring currents in the ratio 1:2 and allocate the measurement times at the lower and higher currents in the ratio 8:1. It is expected that the techniques used here could be applied in other extrapolations, such as for isotherms in the various forms of gas thermometry.

Item Type: Article
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1088/0026-1394/50/4/345
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5869

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