In low pressure gas, the number of positive ions generated by the ionization of gas molecules is proportional to the pressure of the gas. The ionization vacuum gauge is a vacuum measuring instrument based on the principle that the pressure of the gas to be measured is proportional to the ionized flow produced by the ionization of gases under certain conditions.
According to the different methods of ion production, a vacuum gauge for ionizing the gas by a hot cathode emits electrons is called a hot cathode ionization vacuum gauge, and the hot cathode ionization gauge is consists of a hot cathode gauge tube and a measuring instrument. The measuring instrument consists of the gauge tube power supply and transmit current regulator, ion current measurement amplifier. The hot cathode ionization gauge connects with the measured vacuum system, which is a triode with a cathode, a gate and a collector. The collector electric potential is relative to cathode negative potential, while the grid is relative to the cathode positive potential. When the ionization gauge is electrified and heated, the cathode emits electrons, which collide with the gas molecules in the process of reaching the grid, resulting in the ionization of positive ions and electrons. When the emission current is fixed, the number of positive ions is proportional to the pressure of the gas under test. After the positive ions are collected, the positive ion is amplified by the measuring circuit, and the vacuum degree to be measured can be read out by the approval meter.
Hot cathode ionization gauge is similar to the triode gauge, as shown in figure a, which is composed of the cylinder shaped plate (ion collector) C, the grid G and located in the center of the grid cathode filament network composed of F, the cylinder shaped plate is outside of the anode grid.
Figure b is the external control circuit, the grid potential is between +100 ~ +300 V, and the plate potential is between 0 ~ –50V. To emits electrons of cathode filament F after electric heating. As the anode grid G is a positive voltage, the emitted electrons are accelerated, the electrons collide with the internal gas molecules, so that the gas molecules ionization, the greater the gas pressure, the greater the gas density, the more opportunities for collision, the resulting of the more positive ions. The positive ions in the cylindrical plate negative voltage under the action of positive ions to attract the formation of plate current, the greater the molecular gas density (i.e., the greater the pressure), the greater the plate current, on the contrary is small. The plate current in the measurement range is proportional to the measured pressure.
When the vacuum system exposes the atmosphere, it will absorb a lot of gases on the surface of the ionization gauge glass bubble and the electrode surface. In the vacuum environment, the gas will be released again, which will affect the accuracy of the measurement. In order to eliminate this effect, the gauge tube must be degassed before measuring. The ionization gauge uses the baking method to degassing, that is, the filament and the grid can be energized and heated respectively, while the plate electrode using high-frequency induction heating or electron bombardment, the gas was released before the measurement. The general ionization vacuum gauge has a degassing function, when the degree of vacuum is more than 1 × 10-2 Pa, to degassing the ionization gauge according to the specification.