T − T90 for Radiation Thermometry Realization Above the Copper Point

Abstract

The differences, T − T90, between thermodynamic temperature, T, and temperature, T90, on the International Temperature Scale of 1990 (ITS-90) above the Cu fixed-point temperature, 1357.77 K, were investigated using relative primary radiometric thermometry. The Ag and Cu fixed points were used as reference points for temperature realization by the ITS-90 (n = 1) scheme. The values of T − T90 for the Ag and Cu fixed points have been previously determined as 46.2 mK and 52.1 mK, respectively. Extrapolating these differences based on the sensitivity coefficient used to propagate the uncertainty in the fixed-point temperature to other temperatures results in values of T − T90 of 325 mK or 303 mK at 3000 °C when using the Ag or Cu point T − T90 difference, respectively. These values were confirmed by realizing n = 1 temperature scales using either the thermodynamic values or the ITS-90 values of these fixed points as the reference temperatures and comparing the differences on the two scales. Measurements at the Co–C eutectic point indicated the consistency of thermodynamic temperature realization using the n = 1 scheme, and also demonstrated the equivalence of absolute and relative radiometric thermometry.