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Optimisation of SPRT measurements of freezing in a zinc fixed point cell.

Pearce, J V; Veltcheva, R I; Lowe, D H; Malik, Z; Hunt, J D* (2012) Optimisation of SPRT measurements of freezing in a zinc fixed point cell. Metrologia, 49 (3). pp. 359-367.

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Abstract

A numerical model of solute and heat transport in extremely pure materials is described. Its purpose is to characterise the effect of impurities on the freezing curves of metals containing impurities at the level of less than 1 part per million. It is used to simulate experiments performed using a commercially available zinc fixed point cell for SPRT calibrations. The aim is to determine the effect of different vertical temperature gradients on the freezing curve and to find out whether a range of conditions could be determined where there was a good fit between theory and experiment. For this sample agreement between the model and experiment improves as the distribution coefficient k 0. It is found that the model only agrees with the measured freezing curves over the entire freeze for a narrow range of furnace settings where the temperature profile is most uniform. We suggest that this is because if the furnace settings are not optimised, the solid does not grow uniformly, and freezing may continue in regions remote from the SPRT after the material in the vicinity of the SPRT has finished freezing, so distorting the freezing curve. This effect is not present in the model and so the method presented here enables optimisation of the furnace to ensure the SPRT is surrounded by a liquid-solid interface over the entire freezing range. We find that the optimum thermal environment is extremely sensitive to the furnace settings; it is found when the temperature is slightly cooler at the top of the cell as measured in the re-entrant well. We note that optimising the freezing process is a necessary step towards using a thermal analysis to correct for the effects of impurities in the sample.

Item Type: Article
Keywords: fixed point, ITS-90, impurity modelling, Scheil, SPRT, PRT
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1088/0026-1394/49/3/359
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5423

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