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Impurity and thermal modelling of SPRT fixed-points.

Pearce, J V; Veltcheva, R I; Large, M J (2013) Impurity and thermal modelling of SPRT fixed-points. AIP Conf. Proc., 1552. pp. 283-288.

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Impurities in pure metal fixed points for the calibration of standard platinum resistance thermometers (SPRTs) causes significant variations in the freezing temperature, of the order of sub-mK to several mK. This often represents the largest contribution to the overall uncertainty of the fixed point temperature, and it is therefore of great interest to explore ways of correcting for the effect. The sum of individual estimates (SIE) method, in which the contributions of all the impurities are summed, is the recommended way of determining the correction if one has an accurate knowledge of the impurities present and their low concentration liquidus slopes. However, in practice the limitations of the chemical analysis, the limited knowledge of low concentration liquidus slopes, and whether the analysed sample is representative of the ingot in the fixed point cell makes it challenging to implement reliably. It is thus of interest to investigate the influence of impurities on freezing curves using modeling techniques, and ultimately to apply the model to make corrections to the temperature of the freeze. Some success in analyzing freezing curves has been achieved. For practical purposes, the measured freezing curve will only be well described by impurity models if the sensing element of the SPRT is completely surrounded by the liquid-solid interface. If this is not the case (due to solid or liquid spanning the fixed point cell providing a thermal link between SPRT and furnace, or an irregularly shaped liquid-solid interface), then thermal influences will distort the freezing curve and the use of an impurity model to apply a correction is compromised. We outline some methods for optimisation of the furnace to minimise these spurious thermal influences. As the influence of impurities is always convolved with thermal influences it is instructive to construct a model which takes into account both heat and impurity transport. Such a model can be used to evaluate e.g. the effect of furnace temperature gradients on the shape of the freezing curve. We describe a variety of impurity models, in order of sophistication, comprising of analytical models, metallurgically-based models, finite element models, and phase-field models. Each type of model is aimed at investigating different aspects of the problem. We discuss comparisons between models and experiment.

Item Type: Article
Keywords: SPRT, fixed point, fixed-point, impurity modelling
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1063/1.4819554
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5950

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