This paper describes a thorough investigation of the NPL Vega BB3500 furnace to improve the quality and duration of the melt plateaux of a Re-C eutectic cell. The results of this investigation have shown that the position of the cell within the furnace is the most critical factor for optimizing fixed-point performance. It was found that better performance could be obtained from a furnace with uniform ring resistance than with highly non-uniform resistance profiles, but this effect was relatively small. When high resistance (hot) rings were deliberately introduced on either side of the cell, at a distance corresponding to several rings beyond the position of the cell, considerable improvement in the plateaux was observed. Nevertheless, the cell (or the cell performance or even plateaux) is very sensitive to the positioning of these hot rings. A new cell holder was designed and introduced. Raising the cell on supporting rings provided a radiative gap that increased the thermal uniformity of the cell itself. Carbon-carbon composite material (CC sheet) wrapped loosely around the cell further improved the uniformity of the cell wall and hence the plateau quality. Following these changes, the plateau duration was increased by a factor of 5.
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