Vacuum measurement of vacuum furnace

Vacuum measurement is an important part of vacuum technology. The instrument used to measure the degree of vacuum is called a vacuum gauge. The vacuum gauges commonly used in vacuum heat treatment furnaces and vacuum melting furnaces are: thermocouple vacuum gauge ionization vacuum gauges, and the like. They work as follows:

Thermocouple vacuum gauges consist of sensitive components, thermocouple gauges and measuring instruments. The thermocouple tube is connected to the vacuum system to be tested, and the outer casing is a glass tube with a heating wire and a thermocouple wire.

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When the cold end and the hot end temperature of the thermocouple wire are different, a thermoelectric potential is generated in the loop due to the temperature difference effect. If the heating wire voltage is kept constant, the potential of the thermocouple wire is limited by the temperature of the heating wire, and the temperature of the heating wire is related to the pressure of the gas to be measured. The pressure is low, the thermal conductivity of the gas is small, the heat taken away by the gas is small, the temperature of the heating wire is increased, and the thermoelectric potential of the thermocouple wire is increased; otherwise, the thermoelectric potential is decreased. The thermoelectric potential in the loop is measured in millivoltmeters, and the millivolts in the table reflect the degree of vacuum. In order to ensure the voltage of the heating wire is stable, a regulated power supply is connected. Therefore, the measuring instrument is composed of two parts: a millivoltmeter for measuring the thermoelectric potential and a regulated power supply for the heating wire.

This vacuum gauge is mainly used to measure high vacuum. In a low-pressure strong gas, the number of positive ions generated by ionization of gas molecules is proportional to the pressure of the gas. According to the method of generating, the vacuum gauge that uses the hot cathode to emit electrons to ionize the gas is called a hot cathode ionization vacuum gauge; wherein, the hot cathode ionization The vacuum gauge consists of a hot cathode gauge and a measuring instrument. The measuring instrument is composed of a regulating working power source, a transmitting current regulator, an ion current measuring amplifier and the like.

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The hot cathode ionization gauge is in communication with the vacuum system under test. The hot cathode ionization gauge is a triode with a cathode, a grid, and a collector. The collector potential is relative to the cathode electrical negative potential; the gate is electrically positive with respect to the cathode. When the ionization gauge is energized and heated, the cathode emits electrons, and when the electrons reach the grid, they collide with the gas molecules to generate ionization of positive ions and electrons. When the emission current is constant, the positive ion number is proportional to the measured gas pressure. After the positive ions are collected by the collector, they are amplified by the measuring circuit, and the degree of vacuum to be measured can be read by the meter.

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Generally, the measurement of low vacuum and high vacuum cannot be done with a vacuum gauge, but a composite vacuum gauge should be used. The most widely used is an ionization and thermocouple composite vacuum gauge. Its measurement range is 13.33--666.6×10-8Pa. A thermocouple vacuum gauge measures a low vacuum of (10-1--10-3) x 133.32 Pa; an ionization vacuum gauge measures a high vacuum of 133.32 x 10-3-666.6 x 10-8 Pa. The composite vacuum gauge is equipped with a thermocouple gauge tube and an ionization gauge tube, which are respectively connected to the vacuum system. The two gauge tubes can be respectively heated by the knob and selected for use.