Pirani Gauge

The Pirani gauge is a roughing pressure vacuum gauge. It uses the thermal conductivity of gases to measure pressure. The Pirani gauge head is based around a heated wire placed in a vacuum system, the electrical resistance of the wire being proportional to its temperature. At atmospheric pressure, gas molecules collide with the wire and remove heat energy from it (effectively cooling the wire). As gas molecules are removed (when the system is pumped down) there are fewer molecules and therefore fewer collisions. Fewer collisions mean that less heat is removed from the wire and so it heats up. As it heats up, its electrical resistance increases. A simple circuit utilizing the wire detects the change in resistance and, once calibrated, can directly correlate the relationship between pressure and resistance. This effect only works in the pressure region from atmosphere to approximately 10^{– 3} m bar. Therefore, other types of gauge (ion gauge) have to be used to measure pressures lower than this.