The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represents 95% coverage. These results are the best consensus estimates obtained from measurements made using various spectroradiometric primary thermometry techniques by nine different national metrology institutes. The good agreement between the institutes suggests that spectroradiometric thermometry techniques are sufficiently mature (at least in those institutes) to allow the direct realization of thermodynamic temperature above 1234 K (rather than the use of a temperature scale) and that metal-carbon eutectics can be used as high-temperature fixed points for thermodynamic temperature dissemination. The results directly support the developing mise en pratique for the definition of the kelvin to include direct measurement of thermodynamic temperature.
The "Mise en pratique for the definition of the kelvin" (MeP-K) was established in April 2006 to be the repository of information required to perform a "practical measurement of temperature in accordance with the International System of Units (SI)." This article describes the progress made by the MeP-K HT (High Temperature Task Group) of CCT-WG5 (radiation thermometry) in drawing together the appropriate methods for accessing thermodynamic temperature above the silver point involving direct radiometric measurements on the one hand and indirect extrapolation, interpolation, and least-squares fitting on the other. An examination of the uncertainties
Metal (carbide)–carbon eutectic fixed points when applied to radiometry should operate at a temperature preferably exceeding 3000 K, have a large aperture, be robust and have long plateau duration. The large-tube-diameter BB3500YY furnace, recently introduced at the National Metrology Institute of Japan (NMIJ), has been tuned for such fixed-point realization. A novel cell structure with an internal insulation of highly purified carbon-composite sheet material was designed and tested. This resulted in improved robustness, immunity to furnace temperature nonuniformity and extended plateau duration. The design was applied to a hyper-eutectic porous ingot cell, and a reproducible plateau was observed. Preliminary results for a large aperture cell with an aperture diameter of 8 mm are also reported.
Thermodynamic temperatures during the melt and the freeze of Co–C, Pd–C, Pt–C and Ru–C metal–carbon fixed-point cells manufactured by LNE–INM/CNAM, NMIJ and NPL were determined by absolutely calibrated filter radiometers traceable to the PTB cryogenic radiometer and a radiance comparison method using an IKE LP3 radiation thermometer. The measurement uncertainties were below 400 mK at temperatures up to 2250 K. The results are in agreement within the combined uncertainties with a study on relative temperature differences of the same set of fixed-point cells. For the fixed-point cells manufactured by NPL the results are compared with a previous thermodynamic temperature measurement.
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