Change of pressure in vacuum systems brings about a rise or fall in number of gas molecules present
Change of pressure in vacuum systems brings about a rise or fall in number of gas molecules present and hence a rise (or) fall in the thermal conductivity of the gas. Thus the heat loss of constant voltage electrically heated filament in the system varies with the pressure.
The pirani gauge head filament has high temperature co-efficient of resistance. So a slight change in system pressure brings about useful change in filament resistance resulting in an out of balance voltage which can be read as a pressure on meter. The pirani gauge head indicates the total pressure of combined gases and other vapors in the system on the heated wire, which can loss heat to both gas and vapor molecules. The change in resistance due to temperature build-up, of a coil filament, which is supplied with constant voltage placed in Wheatstone bridge configuration, consequent to the pressure and thermal conductivity which is directly read by the gauge in terms of m.bar.
The gauge consists of the stabilized power supply and the indicating meter with the arrangement to select any of two gauge heads. Each gauge head consists of bridge and calibration elements. These heads are factory calibrated with full interchangeability. Each gauge head is provided with `O' ring coupling, to braze to customer's vacuum system.