Optimizing the implementation of high-temperature fixed points through the use of thermal modeling

High-temperature fixed points (HTFP) have the potential to make a step-change improvement in high-temperature metrology, significantly reducing the uncertainty of scale realization of the current ITS-90 and improving dissemination of high-temperature scales to industry. However, in a practical implementation, the performance of HTFP could be limited, by, for example, injudicious use of insulation in the vicinity of the fixed point, furnace gradients, or incomplete filling. This article investigates some of these aspects for a selection of HTFP. Steady-state modeling of the influence of insulation on the radiance temperature was performed for Co-C (1,324 degrees C), Pd-C (1,492 degrees C), Pt-C (1,738 degrees C), Ru-C (1,953 degrees C), and Re-C (2,474 degrees C) fixed points. This included studying mitigation scenarios through the insertion of different types and designs of insulation. The optimum design was identified to minimize the temperature drop in a particular furnace. It was found that, for the furnace and fixed-point combination modeled, the actual effect of the insulation was almost insignificant. Transient modeling was performed for a Re-C fixed point, to track the evolution of the radiance temperature through the melting transition. The starting point of the model was the beginning of the melt. The evolution of radiance temperature with time in "perfectly" filled cells was modeled with a range of linear temperature gradients across the eutectic cell. The gradient had a significant effect on the duration of the transition and on the structure of the melt itself. Despite the model's simplicity, it qualitatively demonstrated that the melt transition temperature, as identified by the point of inflection, could be significantly affected by the presence of furnace gradients.