Thermocouple - Aging of Thermocouples

Aging of Thermocouples

Thermoelements are often used at high temperatures and in reactive furnace atmospheres. In this case the practical lifetime is limited by aging. The thermoelectric coefficients of the wires in a thermocouple that is used to measure very high temperatures change with time, and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the joints and the measurement voltage is only correct if each wire is homogeneous. As thermocouples age in a process their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the inhomogeneous section of the thermocouple circuit is exposed to a temperature gradient the measured voltage will differ resulting in error. For this reason, aged thermocouples cannot be taken out of their installed location and recalibrated in a bath or test furnace to determine error. This also explains why error can sometimes be observed when an aged thermocouple is pulled partly out of a furnace -- as the sensor is pulled back, inhomogenous sections may see exposure to increased temperature gradients from hot to cold as the inhomogeneous section now passes through the cooler refractory area, contributing significant error to the measurement. Likewise, an aged thermocouple that is pushed deeper into the furnace might sometimes provide a more accurate reading if being pushed further into the furnace causes the area of inhomogeneity to be located in an area of the furnace where it is no longer exposed to a temperature gradient.