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.

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.

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The precision blackbody sources developed at the All-Russian Institute for Optical and Physical Measurements (Moscow, Russia) and their characteristics are analyzed. The precision high-temperature graphite blackbody BB22p, large-area high-temperature pyrolytic graphite blackbody BB3200pg, middle-temperature graphite blackbody BB2000, low-temperature blackbody BB300, and gallium fixed-point blackbody BB29gl and their characteristics are described.

For high-temperature fixed points to be accepted as temperature references, it is of prime importance that their long-term stability is demonstrated. This evaluation is part of the CCT-WG5 high-temperature research project (Machin et al.