FIXED POINT CALIBRATION FOR TYPE N THERMOCOUPLES IN THE 0°C TO 1000°C RANGE

Abstract − Noble metal thermocouples are suitable for fixed point calibration method. Basic metal thermocouples, on the other hand, are usually calibrated by comparison. This happens because of th eir electromotive force (emf) instability in short time. In the range from 0 to 1084°C, according to the Monograph CCT/WG1 – BIPM [1] the typical uncertainty of type N thermocouple is about 0.2 °C (k=2) at the fixed points and 1.0 °C (k=2) in the interpolated values. This paper tries to enlighten this discussion presenting results which confirm that type N thermocouples have the best emf stability among base metal thermocouples up to 1000°C, when it is used by customers for calibrations by comparison, after have being calibrated at fixed points cells in primary laboratories. In this condition, it is possible to show that type N thermocouple permits to achieve results near to mentioned uncertainties. The calibration method, laboratory facilities, results and uncertainty budget are reported in this paper. A sheathed thermocouple having compacted mineral oxide insulation was calibrated by fixed points method at Inmetro and by comparison in Brazilian secondary laboratories along two years. This thermocouple has accumulated more than 200