Pirani gauge
The Pirani gauge is a robust thermal conductivity gauge used for the measurement of the pressures in vacuum systems.[1] It was invented in 1906 by Marcello Pirani.[2]
Construction
The Pirani gauge consists of a metal filament (usually platinum) suspended in a tube which is connected to the system whose vacuum is to be measured. Connection is usually made either by a ground glass joint or a flanged metal connector, sealed with an o-ring. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken. If the gas is at high pressure, gas molecules collide frequently with the filament and absorb energy from the filament which results in cooling of the filament. As the pressure of the gas molecules decreases the number of gas molecules inside the chamber also goes down resulting in fewer collisions with the filament. As a result the temperature of the filament increases because of decreased cooling. Electrical resistance of a wire varies with temperature. Hence by studying the variation of the resistance of the wire we can predict the vacuum surrounding the wire. This is the principle of Pirani Gauge. This gauge is used to measure the pressure between 0.5 torr to 10-4 torr.
Mode of operation
A heated metal filament suspended in a gas will lose heat to the gas as its molecules collide with the wire removing heat, and accelerating in the process. If the gas pressure is reduced the number of molecules present will fall proportionately and the wire will rise in temperature due to the reduced cooling effect.
The electrical resistance of a wire varies with its temperature. This resistance, and hence the pressure of the gas, can therefore be used to measure the vacuum surrounding the wire. In many systems the wire is maintained at a constant temperature, the current required to achieve this is therefore a measure of the vacuum being studied.
The gauge may be used for pressures between 0.5 Torr to 10−4 Torr. The thermal conductivity of the gas may affect the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For higher vacuum measurement other instruments such as a Penning gauge are used.
References
- ^ "Phys. Rev. 37 (1931): A. Ellett and R. M. Zabel - The Pirani Gauge for the Measurement of Small Changes of Pressure".
- ^ vonPirani, M (1906). Deutch. Phys. Gesell., Verh. 8: 24.
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