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.
Extending the application of metal (carbide)-carbon eutectic fixed-point cells to radiometry, e.g., for measurements in irradiance mode, requires fixed-point cells with large apertures. In order to make large-aperture cells more readily usable in furnace systems with smaller furnace tubes commonly used for small-aperture fixed-point cells, a novel cell design was developed. For each of Cu, Pt-C, and Re-C fixed points, two types of fixed-point cells were manufactured, the small-and largeaperture cell. For Pt-C and Re-C, the large-aperture cells were filled with a hypereutectic metal-carbon mixture; for the small cells, a hypo-eutectic mixture was used for filling. For each material, the small and large cells were compared with respect to radiometric differences. Whereas plateau shape and melting temperature are in good agreement for the small-and large-aperture Cu cells, a larger difference was observed between small-and large-aperture cells of Pt-C and Re-C, respectively. The origin of these observations, attributed to the temperature distribution inside the furnace, ingot contamination during manufacture, and non-uniform ingot formation for the larger cells, is discussed. The comparison of measurements by a radiation thermometer and filter radiometer of the Re-C and Pt-C large-aperture cells showed large differences that could be explained only by a strong radiance distribution across the cavity bottom. Further investigations are envisaged to clarify the cause.123 970 Int J Thermophys (2008) 29:969-983
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