An investigation of the thermoelectric properties of type S thermocouples from different manufacturers

An investigation into the differences between commercial thermocouple wires from five different manufacturers was carried out, including fixed-point emf, reproducibility, high temperature durability, cyclic properties, and interchangeability. All the thermocouples met the ASTM criteria, but, dependent on the wire source, a maximum temperature difference of 2degreesC was found at the Cu fixed-point temperature. The reproducibility of fresh type S thermocouples was within 40 mK at the Ag fixed-point temperature, regardless of the wire source. On exposure to high temperatures of 1400degreesC, the fixed-point emf values and immersion behaviours were changed, in that the emf decreased and the inhomogeneity increased. After about 300 thermal cycles between 600 and 1100degreesC, the fixed-point emf at the Ag fixed-point did not change, but the inhomogeneity increased by about 4 times. From the emf measurements on 19 thermocouples from a single manufacturer, the interchangeability was estimated to be about 0.16degreesC. This work can provide information either on the selection of a thermocouple and its proper usage for the precise measurement of temperature in industry, or for calibration. (C) 2002 Published by Elsevier Science Ltd.