Research on experimental methods of the melting curve of MC-C large-area high-temperature fixed-point blackbody

In order to achieve the goal of spectral radiance (SR) and spectral irradiance (SI) calibration with an uncertainty of less than 1.0%, NIM set up a MC-C large-area high-temperature fixed point blackbody(HTFP BB) as the new generation of SR and SI reference source, which is composed of large WC-C fixed point cell and BB3500MP. It can be used directly for the SR and SI realization, thereby further reducing measurement uncertainty. The HTFP BB with WC-C fixed point has excellent stability, reproducibility and repeatability in the high temperature range of more than 3020K, which greatly improves the detection capability in the ultraviolet range (UV). Accurate measurement of melting temperature of HTFP BB is an important source of error, which is determined as the point of inflection (POI) of melting plateau. At the same time, POI is also an important reference point used in comparison experiments. Different calculation methods introduce different degrees of errors, which are critical. This paper studied the current three POI numerical calculation methods, namely “differential+second-order fitting” (DSF), “third-order direct fitting” (TDF), “histogram+Gaussian fitting” (HGF). The numerical calculation of the POI was performed on the data of WC-C14 and Re-C measured at NIM and WC-C14 and WC-C10 measured at VNIIOFI by the above three methods. Combining the fitting results and correlation coefficients to explore the characteristics of each method. Based on the experimental results, a more reasonable calculation method is proposed to reduce the calculation error of the POI of the previous data to less than 5 mK.