Thermal conduction errors of manganin-constantan thermocouple arrays

Thermocouples are commonly employed for invasive temperature measurement in clinical hyperthermia. The effect of thermal conduction along the leads on the accuracy of thermocouples in an inhomogeneous temperature field has been examined. These effects are modelled as a convolution with a suitable point spread function and in most cases the errors produced are equivalent to a mispositioning of the thermocouple by less than 1 mm. The point spread functions of T-type thermocouples have been measured, and agree with the theoretical predictions that conduction errors are reduced if the copper leg is replaced by manganin. A temperature gradient across a junction has been shown to cause errors in junctions sharing the same common wire, but for compact junctions this error only becomes significant for temperature gradients much higher than those likely to be encountered in a clinical situation. It can be concluded that neither the errors due to thermal conduction nor those due to temperature gradients across junctions are sufficiently large to impair their clinical use.