What is Pirani gauge – Working ,Application , Advantages

The Pirani gauge is a robust thermal conductivity gauge used for the measurement of the low pressures in vacuum systems

Construction and Working Of Pirani Gauges : 

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.
A conducting wire (platinum filament) gets heated when electric current flows through it. This wire suspended in a gas will lose heat to the gas as its molecules collide with the wire and remove heat. As the gas pressure is reduced (by the vacuum pumps) the number of molecules present will fall proportionately, the conductivity of the surrounding media will fall and the wire will lose heat more slowly. Measuring the heat loss is an indirect indication of pressure.

pirani vacuum gauges
pirani vacuum gauges

The electrical resistance of the wire varies with its temperature, so the measurement of resistance also indicates the temperature of wire. Now the change in resistance of the filament is determined using the bridge. This change in resistance of the pirani gauge filament becomes a measure of the applied pressure
when calibrated.

In many systems, the wire is maintained at a constant resistance R by controlling the current I through the wire. The resistance can be set using a bridge circuit. The power delivered to the wire is I2R, and the same power is transferred to the gas. The current required to achieve this balance is therefore a measure of the
vacuum.
The gauge may be used for pressures between 0.5 Torr to 10−3 Torr. The thermal conductivity and heat capacity of the gas may affect the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For lower pressure measurement other instruments such as a Penning gauge are used.

Applications of Pirani gauge
Used to measure low vacuum and ultra high vacuum pressures.

Advantages of Pirani gauge

  • They are rugged and inexpensive
  • Give accurate results
  • Good response to pressure changes.
  • Relation between pressure and resistance is linear for the range of use.
  • Readings can be taken from a distance.

Limitations of Pirani gauge

  • Pirani gauge must be checked frequently.
  • Pirani gauge must be calibrated from different gases.
  • Electric power is a must for its operation.
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