Traceable Calibration of a Radiation Thermometer in the Range 100 °C to 300 °C by Model Fitting

The Norwegian Metrology Service (JV) offers calibration of blackbodies, thermal imagers, and radiation thermometers to the national clients. The temperature measurements are traceable to the ITS-90 with a set of reference blackbodies covering the range from 10 °C to 1700 °C. However, between 100 °C and 300 °C we do not have a direct measurement of the cavity temperature from a traceable sensor, and rely instead on a pyrometer to provide the reference temperature. The pyrometer is regularly calibrated externally at a handful of predefined temperatures. In this work we present a calibration scheme for the pyrometer which allows calibration at the JV premises: the pyrometer is set to record the measured radiation level at predefined temperatures below 100 °C and just above 300 °C. The calibration data are used to fit a Sakuma–Hattori model, and subsequent readings of the radiation level can be input to the model to extract the corresponding temperature. We present uncertainty budgets for the calibration data, which is subsequently used to estimate a combined uncertainty at arbitrary measured temperatures between 100 °C and 300 °C. Finally, temperatures obtained with the described scheme are compared with recent calibration values obtained externally, and we show that this is a reasonable way to achieve traceable calibration of the pyrometer with adequate precision and low uncertainty. The model fitting has the added benefit of a continuous calibration curve throughout the relevant temperature range rather than at a handful of arbitrary points.