Figure 1: Ionization Gauge.
Working Principle of Ionization Gauge
Extremely low pressures can be measured with ionization gauge (10-3 torr and below). Ionization means process of producing free electron and a positively charged ions by knocking off an electron from an atom. A simple arrangement of hot filament ionization gauge is shown in Fig. 1.
Working and Construction of Ionization Gauge
The gauge consists of triode vacuum tube i.e. cathode, grid and anode plate. Cathode serves as a heated filament. Grid is positive changed. The anode plate is maintained at negative potential with respect to cathode.
Thus the cathode plate is positive ions collector and anode plate is electron collector. This assembly is kept in a vacuum system whose pressure is being measured.
The heated cathode emits the electrons which move past the grid. The positive grid accelerates these electrons where they collide with gas molecules causing its ionization.
The anode is negative so positive ions collect there producing plate current Il the electrons and negative ions are collected by grid, produces grid current I2 in the grid circuit.
This ratio of current Il and I2 gives the vacuum pressure measurement. The vacuum pressure is given by,
\[{{P}_{vaccum}}=\frac{{{I}_{1}}}{{{{I}_{2}}K}}\]
Where k is the proportionality constant known as sensitivity of the gauge.
Advantages of Ionization Gauge
- Very low pressure measurement up to 10-11 torr is possible.
- It can give continuous pressure reading.
- They have good linearity.
- Best suited for the wide pressure range from 10-3 torr to 10-9 torr.
Disadvantages of Ionization Gauge
- Few gases like oxygen carbon dioxide get decomposed by the hot filament.
- Chances of burning the filament if exposed to air.