Large- and Small-Aperture Fixed-Point Cells of Cu, Pt–C, and Re–C

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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 large-aperture cell. For Pt–C and Re–C, the large-aperture cells were filled with a hyper-eutectic 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.

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