Resistance thermometer Self-heating
A resistance thermometer is a passive resistance sensor; it requires a measuring current to produce a useful signal.
Because this measuring current heats the element wire above the true ambient temperature, errors will result unless the extra heat is dissipated.
Self-heating is most often expressed in mW/ºC, which is the power in mill watts (1000 I2 R) required to raise the thermometers internal temperature by 1ºC. The higher the mW/ºC figure, the lower the Self Heating.
As an example, assume a 5 mA measuring current is driven through 100 platinum RTD at 100ºC.
Self-heating is specified as 50 mW/ºC in water moving at 3 ft/sec. The amount of heat generated is:
1000 mW x (0.005 A)2 x (138.5) = 3.5 mW
The self-heating error is:
(3.5 mW) / (50 mW/ºC) = 0.07ºC
The generated heat increases with higher sensor element resistance (when a constant current measurement device is used), or with increasing measuring current.
The resulting error is inversely proportional to the ability of the thermometer to shed extra heat; which in turn depends on thermometer materials, construction, and environment.
The worst self-heating occurs when a high resistance is packed into a small body. Thin film elements, with little surface area to dissipate heat, are an example.
Self-heating also depends on the medium in which the thermometer is immersed. Error in still air may be over 100 times greater than in moving water.