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

Temperatures with associated uncertainties are to be assigned to high-temperature fixed-points. It is important that those uncertainties are large enough to cover the sources of error but not so overstated as to unnecessarily reduce the usefulness of the fixed-points. Concerns have been raised about unexplained differences between the same fixed-point measured in different furnaces; here we investigate the effect of using three different furnaces on the freezing and melting behaviour of two Co-C eutectic fixed point blackbody cavities. Each furnace had significantly different temperature profiles. The results indicate that the most uniform furnace yielded a higher melting temperature by over 100 mK. Investigation of possible confounding effects: the effect of furnace uniformity on melting curve shape, the micro-structure of the ingot, the temperature drop across the cavity wall, the cavity emissivity and the size-of-source effect show that these factors together are too small to account for the overall observed difference.