Common Vacuum Measurement Methods of Vacuum Furnace

1. Compression Vacuum Gauge

The compression vacuum gauge is connected with a capillary closed at the top, the lower end is connected with a port of the “Y” tube, and the lower part of the “Y” tube is connected with a mercury reservoir. When in use, connect the vacuum system with the “Y” tube orifice, rotate the vacuum gauge to find the mercury compressed gas, and then turn to the vertical position to read the liquid level difference between the “measuring capillary” and the “specific capillary”, that is, the measured vacuum degree. Compression vacuum gauge has the advantage of high measurement accuracy and can be used as a standard for vacuum measurement. His disadvantage is that it is not convenient to use, the response is slow and can not be measured continuously. Because the compression vacuum gauge is made according to the gas law, it can only measure the permanent gas partial pressure, not the vapor pressure. In the vacuum system with large steam partial pressure, the reading of the compression vacuum gauge can not mark its vacuum degree.

2. Thermal Conductivity Vacuum Gauge

Based on the principle that the number of molecules in the vacuum system is related to the conduction heat, two kinds of vacuum meters are made: thermal conductivity vacuum meter, resistance vacuum meter and thermocouple vacuum meter. When passing the current, the temperature rises, the periodic gas is heated, and the temperature of the regulatory tube wall is low. The heat is transmitted by the surrounding gas molecules, and the temperature of the resistance wire changes, that is, it reflects the number of gas molecules. It is inconvenient to directly measure the temperature of the resistance wire. Because the resistance value of the resistance wire is related to the temperature, the resistance value can be measured by an electric bridge. The advantages of thermal conductivity vacuum gauge are: 1. It can measure the total pressure; 2. Continuous measurement; 3. It is easy to use and can be measured remotely with wires; 4. Simple structure and easy manufacture; Its disadvantages are: 1. It has thermal inertia. When the pressure changes quickly, the reaction lags behind; 2. The external temperature has a certain influence on the measurement results; 3. The change of the surface condition of the resistance wire can cause zero drift and affect the accurate reading.

3.Ionization Vacuum Gauge

The vacuum degree is measured based on the ionization phenomenon of gas. It can be divided into hot cathode ionization vacuum gauge and magnetron discharge vacuum gauge. When a certain power supply is applied to each pole, the filament is incandescent and emits hot electrons. These electrons collide with gas molecules in the tube to ionize the gas and produce whole ions. The certified ions are collected by ions to form a current.

The current also changes with the change of pressure. The vacuum can be measured by measuring the current. The advantages of hot cathode ionization vacuum gauge are: 1. It can measure the total pressure; 2. Continuous measurement; 3. Wide measurement range and high sensitivity; 4. The ionization gauge has a small volume and can be directly connected to the position to be measured; 5. Insensitive to mechanical vibration and small inertia; 6. The calibration curve is linear; Its disadvantages are: 1. The reading is related to the type of gas, and different curves should be made for different gases; 2. The filament is incandescent, and the filament will be damaged if the vacuum system leaks suddenly or the pressure rises suddenly; 3. The gas is adsorbed on the cathode and collection surface, which is easy to produce measurement error; 4. When the low voltage is strong, the discharge phenomenon between the regulatory shell and the electrode affects the reading;Magnetron discharge vacuum gauge also uses the principle of gas discharge, but there is no hot cathode in the regulation. There are two parallel cathodes in the gauge and a frame electrode in the middle of the two parallel plates. The electrode part of the whole gauge is in a magnetic field. Due to the action of external magnetic field and the frame shape of anode, it is easy for electrons to pass through, causing electrons to make multiple spiral movements back and forth in space. The sensitivity of the vacuum gauge increases with the increase of ionization effect. The advantages are: 1. There is no hot cathode and the filament will not be damaged due to air leakage; 2. Measure the total pressure, which can be measured continuously; 3. Large discharge current, simple circuit and instrument; 4. The structure is firm and insensitive to the outside world; Its disadvantage is that under pressure, its sensitivity is not as high as that of hot cathode ionization vacuum gauge, and different calibration curves are also required for different gases.