Pirani gauge is a device which is used for the measurement of pressure in vacuum systems or in very low-pressure ranges from 10-1 to 10-3 mm of Hg. The working of this gauge depends upon the variation of thermal conductivity of a gas with pressure. The ability of a material to carry heat by conduction is called thermal conductivity. In other words, we can say that Pirani gauge measures pressure by measuring the change in the ability of a gas to conduct heat. It was invented in 1906 by Marcello Pirani.
Measurement of Pressure
For the measurement of low pressure, both the Pirani gauge chamber and the compensating cell is housed on a Wheatstone bridge circuit as shown in the figure 1. The Pirani gauge chamber encloses a platinum filament. The compensating cell is used to minimize variation caused due to ambient temperature changes.
A conducting wire gets heated when electric current flows through it. The rate at which heat is dissipated from this wire depends on the thermal conductivity of the surrounding medium. The thermal conductivity of the surrounding medium, in turn, depends on the density of the surrounding medium.
If the density of the surrounding medium is high then its thermal conductivity also will be high. This will cause the wire to dissipate heat faster and hence the temperature of the wire is low for a given current flow.
If the density of the surrounding medium is low then its thermal conductivity also will also be low. This will cause the wire to dissipate heat slower and hence the temperature of the wire is high for a given current flow.
In the Wheatstone bridge, a constant current is passed through the platinum filament in the Pirani gauge chamber. Due to this current, the filament gets heated and assumes a resistance which is measured using the bridge. Now the pressure to be measured (i.e. applied pressure) is applied to the Pirani gauge chamber as shown in figure 1. Due to the applied pressure, the density of the surrounding medium (gas in this case) of the Pirani gauge filament changes.
Due to this change in density of the surrounding medium (gas in this case) of the filament, its thermal conductivity changes causing the temperature of the filament to change. When the temperature of the platinum filament changes, the resistance of the filament also changes. The change in the resistance of the filament is determined using the Wheatstone bridge. This change in resistance of the Pirani gauge filament becomes a measure of the applied pressure when calibrated. Alternately, a milliammeter can be calibrated to read pressure directly as a result of the unbalance wheatstone bridge.
Higher pressure ⇒ higher density ⇒ higher conductivity ⇒ reduced filament temperature ⇒ less resistance of filament.
Lower pressure ⇒ lower density ⇒ lower conductivity ⇒ increased filament temperature ⇒ more resistance of filament.
Advantages of Pirani Gauge
1. Pirani gauge gives more accurate results than thermocouple vacuum gauge.
2. It is inexpensive.
3. It is Rugged.
4. The response to the pressure change is good.
Disadvantages of Pirani Gauge
1. Frequent calibration is required.
2. It can not be used for the measurement of very low pressure.