The Pirani Gauge is a type of Thermal Conductivity Gauge.
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.
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 I
2
R, 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