Figure 1: Hot-Cathode Ionization Gauge.
Working Principle of Ionization Gauge
The basic principle of an ionization gauge is the electrons emitted from a cathode strikes a gas molecule and knocks out an electron from it. Thus the gas molecule remains as a positively charged ion and carries a current called ion current. This current become a measure of the applied pressure when calibrated.
Construction of Ionization Gauge
The ionization gauge contains a pressure chamber in which a cathode. grid (ion collector) and anode (electron collector) are placed. The pressure chamber is open at one side to receive the pressure to be measured (vacuum). The arrangement of ionization gauge for the measurement of low pressure or vacuum is shown in figure 1.
Working of Ionization Gauge
The pressure to be measured is admitted into a chamber. The cathode present in the chamber is heated to produce electrons. These electrons collide with the gas molecules and knocks out electrons from them and thus leaves the gas molecules as positively charged ions. The positive charged ions are then attracted to an electron collector plate which is negatively charged and thus produce a current flow. This current is then measured and calibrated become a measure of the applied pressure.
It is observed that the pressure of the gas present in the chamber is proportional to the ratio of electron current (anode current) to ion current (Grid current).
\[{{\text{P}}_{i}}\propto \frac{{{\text{i}}_{e}}}{{{\text{i}}_{i}}}\]
\[{{\text{P}}_{i}}=\frac{1}{K}\left( \frac{{{\text{i}}_{e}}}{{{\text{i}}_{i}}} \right)\]
Where,
K = Gauge sensitivity (constant)
The ‘K’ value for nitrogen is 20 Torr-1. But the appropriate value of K is obtained by the gauge calibration because the sensitivity depends upon the chamber dimensions and filled gas.
Advantages of Ionization Gauge
- It has good linearity.
- It responds very quickly to pressure changes.
- Readings can be taken from a distance.
- It gives accurate results.
Disadvantages of Ionization Gauge
- When exposed to air, the hot filament burns out.
- Decomposition of gases by the hot filament.
- Contamination of measured gas by gases which are forced out of the hot filament.
Applications of Ionization Gauge
An ionization gauge is used to measure pressure in the range of 10-10 to 1 ton.