Resistance Thermometers

The hot wire probe referred above can be used in the constant current mode as a resistance thermometer to determine the local fluid temperature. Here we exploit the fact that the wire resistance changes with temperature. This variation is close to linear and is of the form

A typical value of for commonly used metals such as aluminum, gold, platinum and tungsten is C. To measure resistance a small current of around 1 mA is passed through the wire and the instantaneous voltage drop across the wire is measured. This method permits the tracking of thermal transients and fluctuations in the fluid. However it suffers from an error that occurs due to Joule heating of the wire itself. For this reason resistance thermometers are more commonly used with circuits that employ a null method to measure resistance. The current flow in such circuits can be made extremely small.