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Vacuum measurement is an important part of vacuum technology. The instrument used to measure vacuum is called vacuum gauge. Vacuum gauges commonly used in vacuum heat treatment furnace and vacuum melting furnace are thermocouple vacuum gauge, ionization vacuum gauge, etc. Their working principles are as follows:

（1） Thermocouple vacuum gauge

Thermocouple vacuum gauge is composed of sensitive element, thermocouple gauge and measuring instrument. The thermocouple gauge tube is connected with the vacuum system to be measured. The shell is glass tube, and there are heating wire and thermocouple wire in the tube. When the temperature of the cold end and the hot end of the thermocouple wire is different, there is thermoelectric potential in the circuit due to the temperature difference effect. If the heating wire voltage remains constant, the potential of the thermocouple wire is limited to the temperature of the heating wire, which is related to the pressure of the gas to be measured. When the pressure is low, the thermal conductivity of the gas is small, and the heat carried away by the gas is less. When the temperature of the heating wire increases, the thermoelectric potential of the thermocouple wire increases; otherwise, the thermoelectric potential decreases. The thermoelectric potential in the loop is measured with a millivolt meter. The millivolt number in the meter reflects the vacuum degree. In order to ensure the voltage stability of the heating wire, a regulated power supply is connected. Therefore, the measuring instrument is composed of two parts: the millivolt meter for measuring the thermoelectric potential and the regulated power supply for the regulated heating wire.

（2） Ionization vacuum gauge

This kind of vacuum gauge is mainly used to measure high vacuum degree. In a low pressure and strong gas, the number of positive ions generated by ionization of gas molecules is directly proportional to the gas pressure. According to the different methods of ionization, the vacuum gauge which uses hot cathode to emit electrons to ionize the gas is called hot cathode ionization vacuum gauge; among them, the hot cathode ionization vacuum gauge consists of a hot cathode gauge and a measuring instrument. The measuring instrument is composed of regulatory power supply, emission current regulator and ion current measurement amplifier. The hot cathode ionization gauge is connected with the vacuum system under test. The hot cathode ionization gauge tube is a triode with cathode, grid and collector. The collector potential is relative to the negative potential of the cathode, and the grid is relative to the positive potential of the cathode. When the ionization gauge tube is electrified and heated, the cathode emits electrons. When the electrons reach the gate, they collide with gas molecules to produce ionization of positive ions and electrons. When the emission current is constant, the number of positive ions is directly proportional to the pressure of the measured gas. After the positive ions are collected by the collector and amplified by the measuring circuit, the vacuum degree to be measured can be read out by the approved ammeter.

（3） Composite vacuum gauge

Generally, the measurement of low vacuum and high vacuum can not be completed by one kind of vacuum gauge, but by composite vacuum gauge. The ionization and thermocouple type composite vacuum gauge is widely used. Its measurement range is 13.33-666.6 × 10-8 PA. The low vacuum of (10-1-10-3) × 133.32pa is measured by thermocouple vacuum gauge, and high vacuum of 133.32 × 10-3-666.6 × 10-8pa is measured by ionization vacuum gauge. The composite vacuum gauge is attached with a thermocouple gauge tube and an ionization gauge tube, which are respectively connected to the vacuum system. The two gauges can be heated respectively through the knob and can be used selectively.