Thermocouples are the most widely used of all temperature sensors. They can be low cost, have very wide operating ranges and have reasonable accuracy, they rely on the Seebeck effect which states that when heat flows in a conductor, an emf is generated.
If the thermocouple wire is not uniform or metallurgically consistent over its entire length, for example due to a contaminated section of wire, then the wire would be referred to as ‘inhomogeneous’ and we can expect problems due to ‘inhomogeneity’. For example an industrial thermocouple may give different readings when immersed to different positions, and perhaps one value when used in a process and a different reading when placed into a calibration furnace.
A calibration laboratory should consider this inhomogeneity effect during calibration, it is referred to in the Euramet Guide, “Guidelines on the Calibration of Thermocouples EURAMET Calibration Guide No. 8 Version 3.” – but it has been difficult to do..
Isotech have a thermocouple homogeneity scanner that can automatically scan a thermocouple to determine the performance and discover hidden faults. NPL in the UK have recently and found our commercial device is consistent with a different type of non-commercial scanner used at the NPL.