Melting and freezing plateaus were radiometrically observed for the metal-carbon eutectics Ni-C, Pd-C, Pt-C, and Ru-C, using graphite crucibles and black-body cavities. The graphite crucibles were able to withstand heat cycles without breaking. Since the crucible material is a component of the eutectic fixed-point material, the latter is inherently free of contamination from the crucible. The temperature differences between the melting and freezing plateaus were generally less than 0.1 C. The method seems to be a promising way of realizing a series of practical reference points above the copper point.

The melting and freezing plateaus of three metal-carbon eutectic alloys, cobalt-carbon, iridium-carbon and rhenium-carbon, were observed by radiation thermometry for the first time, and their repeatability evaluated. The observed melting temperatures were 1324 °C, 2290 °C and 2474 °C, respectively. A small dependence of the freezing temperature on the cooling rate was observed for the cobalt-carbon eutectic. Six metal-carbon eutectic points have previously been reported by the present authors: these, with their melting temperatures, are iron-carbon (1153 °C), nickel-carbon (1329 °C), palladium-carbon (1492 °C), rhodium-carbon (1657 °C), platinum-carbon (1738 °C), and ruthenium-carbon (1953 °C). These nine fixed-point temperatures cover the range from the copper point to 2500 °C. The evaluated melting-point repeatabilities (standard deviations) were generally better than 0.1 °C. The metal-carbon eutectic points seem to be a promising way of realizing fixed points for practical calibration of radiation thermometers. The two fixed points above 2500 K in particular are potentially useful for thermodynamic temperature determination based on thermal radiation, as well as for radiometry and photometry.

Round-robin measurements with a transfer standard radiation thermometer were organized by the NRLM in the framework of a three-year joint research agreement with the NIST, the IMGC and the PTB: the NPL also took part in this exercise. The aim of the study was to assess the mutual traceability of the ITS-90 temperature scales established by the different laboratories in the high-temperature range (above 1000 °C). The thermometer was a monochromatic (0,65 µm) silicon-detector thermometer belonging to the NRLM. It was circulated in the period from May to July 1993 and was calibrated by all the participants against their local reference thermometers. The temperature interval from 1000 °C to 2000 °C was covered by all the participants, but some extended the range down to 800 °C or up to 2700 °C. The results indicate that all the calibrations agree to within 0,5 °C at 1000 °C and to within 2 °C at 2000 °C.

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