vacuum gauge classification and working principle
A vacuum gauge, also known as a gauge, is an instrument for measuring vacuum or air pressure. In general, the measurement of gas pressure is based on the change of some physical effect of gas under different pressure. It is widely used in scientific research and industrial production. According to the different physical mechanism of vacuum gauge measurement principle, the main vacuum meter can be divided into three categories, namely, the vacuum gauge using mechanical properties, using gas dynamic effect and using charged particle effect. The typical vacuum gauges using mechanical properties are Bolden gauge and film capacitance gauge; the typical vacuum gauges using gas dynamics effect are Pirani resistance gauge and thermocouple gauge; the typical vacuum gauges using charged particle effect are hot cathode ionization gauge and cold cathode ionization gauge.
Borden's fine copper tube will stretch under different gas pressure, which will drive the lever and gear to rotate, so that the pointer can indicate on different scales, and the corresponding pressure value can be read out. The measuring range of this gauge is generally between 100Pa and 1atm.
Under different pressures, the metal diaphragm of the film capacitance gauge will have different deformation scales, which makes the capacitance between the metal diaphragm and the electrode change. The measuring range of this gauge generally spans four orders of magnitude, such as 0.01pa to 100Pa, 0.1pA to 1000Pa, etc. the advantage of this gauge is that it has high sensitivity. The disadvantage is that it must be used at a constant temperature higher than the ambient temperature to eliminate the influence of different temperatures on the mechanical properties of the diaphragm. It generally needs to preheat for several hours before use.
Due to the different thermal conductivity of gas molecules under different air pressures, when a constant current is applied to the hot wire, due to the different air pressure, the heat carried by the gas is different, and the temperature maintained by the hot wire is different, which leads to the different resistance of the hot wire. The air pressure can be calculated by measuring the resistance of the hot wire The accuracy of measurement is improved effectively. Because different gases have different thermal conductivity at the same pressure, the gauge needs to be calibrated under different gases.
The basic principle of thermocouple gauge is consistent with Pirani resistance gauge, but it does not need to measure the change of hot wire resistance, but directly measures the temperature change of heat ratio by thermocouple. The measuring range is generally from 0.1pA to 1000Pa. Like Pirani resistance gauge, this gauge needs to be calibrated in different gases
The hot cathode ionization gauge emits electrons from the hot cathode, that is, the filament, ionizes the gas molecules in vacuum to generate ions. The ions generated are collected by the collector to form ion current. The density of gas molecules in vacuum can be calculated by measuring the ionic current, and then the gas pressure can be obtained. The measuring range of this gauge is generally. The lower limit of measurement can be reduced to 1.0e-09pa by the improved hot cathode ionization gauge. Because the ionization rate of different gases is different under the same pressure, the gauge needs to be calibrated in different gases.
Cold cathode ionization gauge is also used to measure atmospheric pressure by collecting ionic current by ionizing gas molecules. But different from hot cathode ionization gauge, it uses magnetic controlled discharge to ionize gas molecules to produce ions. The measuring range of this gauge is generally 1.0e-07pa to 0.1pA. For the same reason as the hot cathode ionization gauge, the gauge needs to be calibrated in different gases.