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Investigation of the furnace effect in cobalt-carbon high temperature fixed point cells.

Dong, W; Lowe, D; Machin, G; Bloembergen, P*; Wang, T*; Lu, X* (2017) Investigation of the furnace effect in cobalt-carbon high temperature fixed point cells. Measurement, 106. pp. 88-94.

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To assign thermodynamic temperatures to high temperature fixed points (HTFPs) it is essential to quantify how the furnace affects (the so-called 'furnace effect') their realized temperature This is currently poorly understood (and poorly quantified) but it is apparent that, at least to a small degree, the realised temperature of the HTFPs depend upon the furnace in which they are implemented. In this paper we present a detailed experimental study into the furnace effect on the freezing and melting behavior of three Co-C eutectic fixed point blackbody cavities using three different furnaces. The crucial difference between the furnaces is that they had significantly different temperature profiles. The quantities studied are the point of inflection for melting and the liquidus temperatures obtained for freezing and melting. Three Co-C cells have been used in this investigation, these are designated NIM Co-1#, NIM Co-11-1# and NPL Co-1#. Cell NIM Co-1# and NPL Co-1# are of conventional design, whilst NIM Co-11-1# cell is of the hybrid type.

A number of melt and freeze curves at the same furnace offsets for each Co-C point were measured in the three furnaces using a high performance pyrometer. The results indicate that for all the three cells, the most uniform furnace yielded a higher liquidus temperature, about 120 mK for the conventional and 30 mK for hybrid design cells.

We attribute the 'furnace effect' -at least in part- to the interplay between the horizontal furnace temperature profile and the solid liquid interface of the metal carbon eutectic ingot as it melts. Investigation of possible confounding effects; the microstructure, the temperature drop, the cavity emissivity and the size of source effect show that these are too small to account for the overall observed difference.
It is clear that: (1) the effect of a non-uniform furnace temperature profile, although not fully covering the so-called furnace effect, must be taken into account, at least in the uncertainty budget, when assigning temperatures to high temperature fixed points; (2) the hybrid design should be universally adopted for HTFP fabrication, since it clearly decreases the influence of the furnace.

Item Type: Article
Keywords: High temperature fixed points, uncertainties, furnace effect
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Divisions: Engineering, Materials & Electrical Science
Identification number/DOI: 10.1016/j.measurement.2017.04.005
Last Modified: 02 Feb 2018 13:12
URI: http://eprintspublications.npl.co.uk/id/eprint/7608

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