Thermodynamic Temperatures of High-Temperature Fixed Points: Uncertainties Due to Temperature Drop and Emissivity

Abstract

This study forms part of the European Metrology Research Programme project implementing the New Kelvin to assign thermodynamic temperatures to a selected set of high-temperature fixed points (HTFPs), Cu, Co-C, Pt-C, and Re-C. A realistic thermal model of these HTFPs, developed in finite volume software ANSYS FLUENT, was constructed to quantify the uncertainty associated with the temperature drop across the back wall of the cell. In addition, the widely applied software package, STEEP3 was used to investigate the influence of cell emissivity. The temperature drop, T , relates to the temperature difference due to the net loss of heat from the aperture of the cavity between the back wall of the cavity, viewed by the thermometer, defining the radiance temperature, and the solid-liquid interface of the alloy, defining the transition temperature of the HTFP. The actual value of T can be used either as a correction (with associated uncertainty) to thermodynamic temperature evaluations of HTFPs, or as an uncertainty contribution to the overall estimated uncertainty. In addition, the effect of a range of furnace temperature profiles on the temperature drop was calculated and found to be negligible for Cu, Co-C, and Pt-C and small only for Re-C. The effective isothermal emissivity (ε eff ) is calculated over the wavelength range from 450 nm to 850 nm for different assumed values of surface emissivity. Even when furnace temperature profiles are taken into account, the estimated emissivities change only slightly from the effective isothermal emissivity of the bare cell. These emissivity

Thermodynamic temperature assignment to the point of inflection of the melting curve of high-temperature fixed points

“…In effect, it is the temperature drop across the thickness of the cavity (back) wall. As it is impossible to know with precision what that thickness is after cell construction and as this effect is similar for all furnaces and for all cells [16,42,43], no correction has been applied for temperature drop. The estimated uncertainty, taken to be the full value of the calculated temperature drop, is given in table 3.…”

“…It has been observed that use of different furnaces can affect the realized (point of inflection) melting temperature of a HTFP, e.g. [16,18]. Different furnaces were used by the participants.…”

“…First, there is a small effect from the furnace temperature profile on the temperature drop and emissivity of the cavity ( §5). Studies [16,42] considered extreme (theoretical) furnace temperature profiles for different back-wall thicknesses and the maximum difference between furnaces is approximately 40 mK for Re-C, 10 mK for Pt-C and 3 mK for Co-C. Secondly, both the furnace temperature profile and the furnace inertia (which can be characterised by the time the furnace takes to achieve the set point above the melt) can affect the shape of the melt curve, and hence the calculated point-of-inflection temperature [18].…”

“…Emissivity calculations [42] have been made for the blackbody cavities in the cells used in this measurement campaign. The emissivity of a cell depends on its geometry (which is sufficiently similar for all cells here) and on the emissivity of the wall material (graphite).…”

“…The collaborative research that has led to this paper was initiated in 2007 by Working Group 5 of the Consultative Committee for Thermometry (CCT) 1 [10,11]. The aim of the research was to -develop stable, robust cells for Re-C, Pt-C and Co-C; -to perform long-term stability tests on these cells [12,13]; -to provide a 'definitive reference set' of fixed points [14,15]; -to understand uncertainties associated with furnace effects [16][17][18] and cell impurities [19]; and -to perform an initial comparison of filter radiometric techniques [20].…”