About Pirani gauge
Pirani gauge is similar to a thermocouple gauge but has a heated element which consists of four coiled tungsten wires connected in parallel. Two identical tubes are normally used, connected in a bridge circuit with one containing the gas at unknown pressure and the other evacuated to a very low pressure. Current is passed through the tungsten element, which attains a certain temperature according to the thermal conductivity of the gas. The resistance of the element changes with temperature and causes an imbalance of the measurement bridge. Thus the Pirani gauge avoids the use of a thermocouple to measure temperature (as in the thermocouple gauge) by effectively using a resistance thermometer as the heated element. Such gauges cover the pressure range 10-5 mbar to 1 mbar.