When an electron passes through a rarefied gas in a vacuum vessel, it collides with the gas molecules. When the electron energy is small, the collision only changes the velocity and direction of the two. When the energy of the electron is higher than the ionization potential of the gas, the high-speed movement of the electron and the collision of the gas molecules will cause the gas molecules to ionize. The number of positive ions produced is directly proportional to the density of the gas, while the pressure P = NKT. At a certain temperature, N and P are in direct proportion. If we can measure the size of ionization attack ion current, we can measure the vacuum degree.
The ionization vacuum gauge is made according to this principle. Assuming that high-speed electrons are emitted from a heated cathode, it is called a hot cathode ionization vacuum gauge.
The measuring circuit of hot cathode ionization meter consists of the following three parts.
1. Ion current amplifier.
2. Transmitting current stabilizer.
3. Voltage stabilizer to adhere to the stability of regulatory parameters.
The characteristic of hot cathode ionization meter is that the pressure of measurable gas and vapor can be measured in a long distance. The higher the vacuum degree is, the higher the activity is. But the reading is related to the type of gas, and the cathode is easy to burn when the measuring pressure is lower than 10-3pa.
In order to use conveniently, there is a compound vacuum gauge, which combines the measuring parts of the thermocouple and the ionization meter together, but they are still independent when they are used.
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