A basic pirani gauge consists of a fine wire of tungsten or platinum of about 0.002 cm in diameter. This wire is mounted in a tube and then connected to the system whose vacuum is to be measured. The temperature range is around (7-400) degree Celsius and the heating current is between (10-100) mA. A bridge circuit is also used for greater accuracy. The pirani gauge is connected as one arm of the bridge circuit. The figure is shown below. Vacuum measurement is usually taken in three ways.
- When the pressure changes, there will be a change in current. For this, the voltage V has to be kept constant.
- The resistance R2 of the gauge is measured, by keeping the gauge current constant.
- The null balance of the bridge circuit is maintained by adjusting the voltage or current. This change is made with the help of a potentiometer and the change brought will be a measure of the pressure produced.
An additional reference gauge can also be used in the adjacent arm of another pirani gauge, in the bridge circuit. The additional gauge is evacuated and sealed, which helps in the compensaton for variation in ambient temperature. For commercial use, the range of the instrument can be extended from 10-3 Torr to 1 Torr.