Thermodynamic Temperature Measurement to the Indium Point Based on Radiance Comparison

Abstract

A multi-national project (the EMRP InK project) was completed recently, which successfully determined the thermodynamic temperatures of several of the high-temperature fixed points above the copper point. The National Metrology Institute of Japan contributed to this project with its newly established absolute spectral radiance calibration capability. In the current study, we have extended the range of thermodynamic temperature measurement to below the copper point and measured the thermodynamic temperatures of the indium point (\(T_{90} =\) 429.748 5 \(\hbox {K}\)), tin point (505.078 K), zinc point (692.677 K), aluminum point (933.473 K) and the silver point (1 234.93 K) by radiance comparison against the copper point, with a set of radiation thermometers having center wavelengths ranging from \(0.65\,\upmu \hbox {m}\) to \(1.6\,\upmu \hbox {m}\). The copper-point temperature was measured by the absolute radiation thermometer which was calibrated by radiance method traceable to the electrical substitution cryogenic radiometer. The radiance of the fixed-point blackbodies was measured by standard radiation thermometers whose spectral responsivity and nonlinearity are precisely evaluated, and then the thermodynamic temperatures were determined from radiance ratios to the copper point. The values of \(T-T_{90}\) for the silver-, aluminum-, zinc-, tin- and indium-point cells were determined as −4 mK (\(U = 104\,\hbox {mK}, k=2\)), −99 mK (88 mK), −76 mK (76 mK), −68 mK (163 mK) and −42 mK (279 mK), respectively.