The vacuum measurement in the vacuum coating machine cavity refers to the measurement of the vacuum level in a specific space with specific instruments and devices. This instrument or device is called a vacuum gauge (instrument, gauge). There are many types of vacuum gauges, which are usually divided into absolute vacuum gauges and relative vacuum gauges according to the measurement principle. All vacuum gauges that directly obtain gas pressure by measuring physical parameters are absolute vacuum gauges, such as U-shaped pressure gauges and compression vacuum gauges. When the gas pressure is very low, it is extremely difficult to measure directly; and the vacuum gauge that obtains the pressure value by measuring the physical quantity related to the pressure and comparing it with the absolute vacuum gauge is called a relative vacuum gauge, such as a discharge vacuum gauge, Thermal conductivity vacuum gauges, ionization vacuum gauges, etc., are characterized by slightly poor measurement accuracy and are related to the type of gas. In actual production, except for vacuum calibration, most of the relative vacuum gauges are used. This section mainly introduces the working principle and measurement range of resistance vacuum gauge, thermocouple vacuum gauge and ionization vacuum gauge.

1. Resistance vacuum gauge
      The resistance vacuum gauge is a kind of thermal conduction vacuum gauge, which indirectly obtains the degree of vacuum by measuring the temperature of the hot wire in the vacuum. The principle is that the heat conduction of the gas under low pressure is related to the pressure, so how to measure the temperature parameters and establish the relationship between the resistance and the pressure is the problem to be solved by the resistance vacuum gauge.
      The structure of the resistance vacuum gauge, the heating filament in the gauge tube is a tungsten wire or platinum wire with a large resistance temperature coefficient, and the heating wire resistance is connected to a Wheatstone bridge and serves as an arm of the bridge. When heated under low pressure, the heat Q generated by the filament can be expressed as:
Q = Q1 + Q2
       In the formula, Q1 is the heat radiated by the filament, which is related to the temperature of the filament; Q2 is the heat taken away by the gas molecules colliding with the filament, and the size is related to the pressure of the gas. When the temperature of the hot wire is constant, Q1 is a constant, that is, the heat radiated by the hot wire does not change. Under a constant heating filament current condition, when the pressure of the vacuum system decreases, that is, the number of molecules of the gas in the space decreases, Q2 will decrease accordingly, and the heat generated by the filament will increase relatively, then the temperature of the filament will decrease. When it rises, the resistance of the filament will increase. There is a relationship between the pressure of the vacuum chamber and the resistance of the filament, such as P↓→R↑, so the pressure can be indirectly determined by measuring the resistance of the filament.
      The resistance vacuum gauge measures vacuum in the range of 105 ~ 10-2Pa. Since it is a relative vacuum gauge, the measured pressure is highly dependent on the type of gas, and its calibration curves are all for dry nitrogen or air, so if the measured gas composition changes greatly, the measurement results should be corrected to some extent. In addition, after the resistance vacuum gauge is used for a long time, the zero point drift of the hot wire will occur due to oxidation. Therefore, it is necessary to avoid prolonged exposure to the atmosphere or work under high pressure during use, and it is often necessary to adjust the current to calibrate the zero point position.
2. Thermocouple vacuum gauge
       Schematic diagram of the structure of the thermocouple vacuum gauge. The regulation of the thermocouple vacuum gauge is mainly composed of heating filaments C and D (platinum wire) and thermocouples A and B (platinum-rhodium or constantan-nickel-chromium) used to measure the temperature of the hot filament. The hot end of the thermocouple is connected to the hot wire, and the cold end is connected to the millivoltmeter in the instrument, and the thermocouple electromotive force can be measured from the millivoltmeter. During measurement, the thermocouple gauge is connected to the vacuum system under test, and the hot wire is supplied with a constant current. Unlike the resistance vacuum gauge, a part of the heat Q generated by the filament will be between the filament and the thermocouple wire. Conduction dissipates. When the pressure of the gas decreases, the temperature at the thermocouple junction will increase with the increase of the temperature of the hot wire. Similarly, the thermoelectric potential of the cold end of the thermocouple will also increase, and there is such a relationship between the gas pressure and the electromotive force of the thermocouple. Relationship: P↓→ε↑.
      The measurement results of thermocouple vacuum gauges for different gases are different. This is because the thermal conductivity of various gas molecules is different. Therefore, certain corrections need to be made when measuring different gases. Table 1-3 gives correction factors for some gases or vapors. The measurement range of the thermocouple vacuum gauge is roughly 102 ~ 10-1Pa, and the measurement pressure is not allowed to be too low. This is because when the pressure is lower, the heat escaped by the heat conduction of the gas molecules is very small, and the hot wire and the thermocouple wire are used. The heat loss caused by heat conduction and heat radiation is the main reason, so the change of thermocouple electromotive force will not be caused by the change of pressure.
      The thermocouple vacuum gauge has thermal inertia. When the pressure changes, the change of the hot wire temperature often lags for a period of time, so the reading of the data should also lag for some time; in addition, like the resistance vacuum gauge, the heating filament of the thermocouple gauge is also Tungsten wire or platinum wire, if used for a long time, the heating wire will have zero drift due to oxidation, so when using it, the heating current should be adjusted frequently, and the heating current value should be recalibrated.
3. Ionization vacuum gauge
      The ionization vacuum gauge is a widely used vacuum gauge, which uses the principle of ionization of gas molecules to measure the degree of vacuum. According to the different gas ionization sources, it is divided into hot cathode ionization vacuum gauge and cold cathode ionization vacuum gauge. The former is divided into ordinary hot cathode ionization gauge, ultra-high vacuum hot cathode ionization gauge and low vacuum hot cathode ionization gauge. Figure 1-7 shows the structure of an ordinary ionometer gauge, which mainly has three electrodes: the filament that emits electrons as the emitter A, the spiral-type grid that accelerates and collects electrons (also known as the accelerating electrode) B and the cylindrical type The ion collector C is composed of three parts, in which the emitter is connected to zero potential, the accelerating electrode is connected to positive potential (hundreds of volts), the collector is connected to negative potential (tens of volts), and there is a repelling field between B and C. The working principle of the ionization meter is that the hot cathode A emits electrons, which are accelerated by the acceleration pole, and most of the electrons fly to the collector. Flying to the B pole again, when the electron flies to the BC space, it is also affected by the repelling field. When the speed is reduced to zero, the electron turns back and flies to the C pole, and the repeated movement of the electron in the BC space will interact with the gas molecules. Continue to collide, so that the gas molecules gain energy and generate ionization. The electrons are finally collected by the accelerating electrode, and the positive ions generated by ionization are accepted by the collecting electrode and form an ion current I+. For a certain regulation, when the potential of each electrode is constant, I+ has the following linear relationship with the emitted electron current Ie and the pressure of the gas
I+ = kIeP
      In the formula, k is the proportionality constant, and its meaning is the current value of the ion obtained under the unit electron current and the unit pressure, and the unit is 1/Pa, which can be determined by experiments. For different gases, the magnitude of k is different, and it exists in the range of 4-40. When the emission current is constant, the ion current is only proportional to the gas pressure, so the gas pressure value in the vacuum chamber can be determined according to the size of the ion current.
       The measuring range of the ordinary hot cathode vacuum gauge is 1.33×10-1—1.33×10-5Pa. Whether it is higher or lower than this measurement limit, the linear relationship between the ion current I+ and the gas pressure will be lost. When the pressure is high, the probability of multiple collisions between electrons and molecules is greatly increased. Since the acceleration potential is much higher than the ionization potential of the gas (tens of volts), the electrons generated by ionization are enough to cause gas ionization, which will make the ionization regulator At the same time, due to the high density of the gas, the free path of electrons is very short, and most collisions are low-energy collisions that cannot cause ionization. Many factors lead to the fact that the ion current and pressure at higher pressures no longer maintain a linear relationship ; When the pressure is low (below 1.33×10-1Pa), the high-speed moving electrons will generate soft X-rays when they reach the acceleration pole, and the soft X-rays will then be directed to the ion collector C, which will cause the collector to produce photoemission, The electron flow is emitted, so that the pressure-independent current is superimposed in the original ion flow measurement circuit, and the linear relationship between the ion flow I+ and the gas pressure is lost, and the ionization vacuum gauge cannot measure the pressure in the vacuum chamber. .
       The vacuum coating machine uses an ionization vacuum gauge to quickly and continuously measure the total pressure of the gas to be tested, and the gauge is small and easy to connect. However, the emitter in the gauge is made of tungsten wire. When the pressure is higher than 10 When the temperature is -1Pa, the life of the gauge will be greatly reduced, or even burnt. Avoid working under high pressure; when the vacuum system is exposed to the atmosphere, the inner surface of the glass bulb and each electrode of the ionometer gauge will absorb gas, which will affect the vacuum. Therefore, when the vacuum system is exposed to the atmosphere for a long time or has been used for a period of time, regular outgassing treatment should be carried out.