Realization of ITS-90 Radiance Temperature Scale from 961.78 °C to 3000 °C at CSIR-NPL

The traceable temperature measurements using the radiation thermometers and thermal imagers above 1000 degrees C require blackbody fixed points and extrapolation or interpolation of Planck's radiation law. For the realization of the ITS-90 radiance temperature scale, a radiation thermometer needs to be calibrated by using the pure metal fixed-point blackbodies (Ag or Au or Cu) and Planck's radiation law in ratio form. At CSIR-NPL, the high-temperature radiance scale above the silver fixed-point is realized using a silicon photodiode detector-based linear pyrometer (LP4) with a 650 nm nominal wavelength. This paper presents the realization of high-temperature blackbodies of Ag (961.78 degrees C) and Cu (1084.62 degrees C), and a detailed investigation of their melting and freezing plateaus measured using LP4. The mean effective wavelength of the LP4 was evaluated using Wien's approximation to Ag and Cu photocurrent outputs, computed to be 650.09 nm, and used in Planck's equation to determine T-90 temperatures. The characteristics of these blackbody sources and radiation thermometer, such as size-of-source effect (SSE) and distance effect (DE) were measured, and their contributions in measurement uncertainty were evaluated. Expanded uncertainties of fixed-point realization for Ag and Cu are estimated as 0.25 and 0.30 degrees C (k = 2), respectively. Further, photocurrents calculated using Ag fixed-point blackbody for corresponding temperatures by ratio method were employed to solve the coefficients of the Sukuma-Hattori equation for extrapolation of temperatures and measurement uncertainties up to 3000 degrees C. The uncertainty of scale realization is estimated to be 3.72 degrees C (k = 2) at 3000 degrees C.