Thermocouples are no longer a part of the ITS-90 scale. However they are so important to industry that every Primary Laboratory should consider having a thermocouple calibration system.

Isotech offers a selection of fine thermocouples designed specifically for the Primary Laboratory.

The Isothermal range of Thermocouple Standards are the result of 5 years development. The type R and S standards will cover the range from 0°C to 1600°C.

The measuring assembly comprises a 7mm x 300mm or 600mm gas tight 99.7% recrystallized alumina sheath inside which is a 2.5mm diameter twin bore tube holding the thermocouple.

The inner 2.5mm assembly is removable since some calibration laboratories will only accept fine bore tubed thermocouples and some applications require fine bore tubing.

1.7 meters of covered noble metal thermocouple wire connect the measuring sheath to the reference sheath which is a 4.5mm x 250mm stainless steel sheath suitable for referencing in a Zeref 0°C reference system. Two thermo electrically free multistrand copper wires (teflon coated) connect the thermocouple to the voltage measuring device.

The thermocouple material is continuous from the hot or measuring junction to the cold, or referencing junction.

Calibration
The 1600 is supplied with a certificate giving the error between the ideal value and the actual emf of the thermocouple at the gold point. For types R and S thermocouples, manufacturing tolerances are small and, therefore, the use of a standard reference table is particularly apt. A few calibration points, only, are required to determine the small differences between the characteristics of an individual thermocouple and the standard reference table. As an example of consistency, 48 Isotech thermocouples calibrated at NPL, had a standard deviation of the differences from the reference table value at the gold point (11, 364µV) of only 7µV, equivalent to about 0.5°C.

Thermocouple characteristics are sufficiently smooth to allow interpolation of deviations from the reference table to be carried out over fairly wide temperature spans without introducing unacceptable errors.