Stability of K-, N- and S-type thermocouples in the temperature range from 0°C to 1060°C

Abstract

The behaviour in air of K and N thermocouples of the bare-wire and metal-sheath types is examined in the 0°C to 850°C range. Considered are emf variations due to short-term ordering phenomena, which occur mostly in K thermocouples, as well as hysteresis. Suitable stabilization cycles can achieve improvement of the reproducibility. The paper gives also data concerning the S-type thermocouples used in industrial laboratories and SIT (Italian Calibration Service) calibration centres. In fact, although the platinum resistance thermometer and monochromatic pyrometer are the primary standards in the 600°C to 1060°C range according to ITS-90, the S-type thermocouple is still used as a reference in calibrations at the industrial level considering its low cost and easy use in these measurements. A calibration from time to time is necessary to verify the conformity to the reference tables and reproducibility of the S-type thermocouples to achieve a total uncertainty better than ± 0.2°C.