A selective multiple fit method of the point of inflection for the large-area HTFP melting plateau curve

•A large-area WC-C HTFP blackbody was used for spectral irradiance scale realization.•Difference in melting plateau curves of large and small area WC-C HTFP cells.•A new method was proposed for evaluating POI of melting plateau curve of large cell.•The new method can effectively reduce the calculation error ofPOI. National Institute of Metrology (NIM) has set up a new large-area tungsten carbide–carbon (WC-C) high-temperature fixed-point (HTFP) blackbody system as the direct radiation source for measuring spectral irradiance. The point of inflection (POI) of the HTFP melting plateau curve was important to determine the liquidus point and applied as the reference point in temperature calibration. This paper presented a selective multiple fit method. The maximum discrepancy of different factors for the new method and traditional methods were 0.001 K and 0.633 K which led to 0.0003% and 0.20% spectral irradiance errors at 500 nm. In addition, when used to calculate POIs of WC-C and Re-C small-area HTFPs, the maximum deviation between the new method and the average value of traditional methods were −0.004 K and −0.03 K. This method was verified to be insensitive to the influencing factors and more effective for large-area HTFPs applications.