ANALOG PIRANI VACUUM GAUGE
pirani gauges have very wide range of applications where the vacuum measurement needs to be carried between 0.5 mbar to 0.001 mbar.
The sensor tube of the pirani gauge head consists of a high temperature co-efficient resistance filament. The filament is heated with a constant voltage supply. The variation in pressure in vacuum line causes changes concentration of gas molecules present , in turn the variation in the thermal conductivity of the gas.
Hence the heat loss of the electrically heated filament varies with respect to the pressure.
This filament is constructed as an arm of the wheat stone bridge. A slight change in system pressure brings larger change in filament resistivity resulting in an imbalance of the bridge.
this bridge imbalance current is a measure of pressure which is properly conditioned and displayed on the analog meter.