Validation of a Blackbody Comparator-Based System for Thermocouple Calibration

Abstract

A blackbody comparator for thermocouple calibration in the temperature range from $960\,^{\circ }\hbox {C}$ to $1500\,^{\circ }\hbox {C}$ has previously been developed at the Centre for Metrology and Accreditation (MIKES). The calibration system is based on direct comparison of thermocouples and a radiation thermometer. In this article, the blackbody comparator is exploited by comparing an absolute calibrated irradiance mode filter radiometer and a linear pyrometer calibrated according to the International Temperature Scale of 1990 (ITS-90) to each other in the temperature range from $1000\,^{\circ }\hbox {C}$ to $1500\,^{\circ }\hbox {C}$. The results of the comparison are in agreement within uncertainties ($k = 2$). Furthermore, thermal gradients in the blackbody comparator are studied by means of numerical simulation, as the gradients were found to be the major source of uncertainty in previous work. A thermal model was constructed with COMSOL software, and the radial and longitudinal gradients were studied in the comparator. The results of the modeling are in agreement with the uncertainty determination carried out in previous work, but the gradients still remain a significant uncertainty contribution. The validation of the calibration system was completed by comparing calibration results obtained with the system for a Pt/Pd thermocouple to calibration results reported by the National Physical Laboratory (NPL), UK. The results of the comparison agree within the expanded uncertainty ($k = 2$) of the comparison.