Dual-wavelength reflectance-ratio method for emissivity-free temperature measurements applied to electromagnetically levitated liquid Ni

The applicability of the dual-wavelength reflectance-ratio (DWR) method to emissivity-free radiation thermometry of electromagnetically levitated high-temperature liquid metals was investigated. To establish the measurement technique, the DWR method was applied to liquid Ni levitated in a static magnetic field, which suppresses the surface oscillation and the translational motion. In a previous study, temperature of the levitated liquid metals measured by DWR showed deviations of about 95 K to 175 K from the temperature measured by a calibrated pyrometer. Since this discrepancy could be attributed to the imperfect contribution of the auxiliary light, the effect of the optical setup of the auxiliary light on the temperature measurement was investigated in this study. By using a reflecting collimator for the auxiliary optical system and adopting the radiance ratio determined considering the geometrical arrangement of the measurement system, the difference between temperature of liquid Ni kept near its melting temperature (𝑇𝑚 = 1728 K) measured using DWR and the temperature measured by a pyrometer calibrated using the melting point of Ni was 12 K on average and the standard deviation in the temperature measurement was 25 K (𝑛 = 5).