Common terms for vacuum

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　　1. Definition of vacuum: A vacuum system refers to a state in which the gas density is lower than that of the atmospheric pressure in that area.
2. Vacuum level: It denotes the degree of thinness of the gas in a vacuum state, and is usually expressed as “high vacuum level” and “low vacuum level”. A high vacuum level indicates a \"good\" vacuum condition, while a low vacuum level indicates a \"poor\" vacuum condition. 　　3. Vacuum unit 　　Torr is commonly used as the unit, but in recent years Pa has been adopted internationally as the unit. 　　1 torr = 1/760 atmosphere = 1 millimeter of mercury. 4. Conversion between torrs and pascals: 1 torr = 133.322 pascals, or 1 Pascal = 7.5×10-3 torr. 5. Mean free path: The average distance that a gas particle in random thermal motion travels between two successive collisions, denoted by the symbol “λ”. 　　6. Flow rate 　The amount of gas that passes through any cross-section per unit of time; denoted by the symbol “Q”, with units of Pa·L/s or Torr·L/s. 　　7. Conductance: Represents the ability of a vacuum tube to allow gas to pass through it. The unit is liters per second (L/s); in a steady state, the flow conductance of a pipe is equal to the pipe flow rate divided by the pressure difference across the pipe ends. The symbol is denoted as “U”. 　　U = Q/(P2 – P1) 8. Pressure The force exerted by gas molecules per unit area on the walls of a container, denoted by “P”. 　　9. Standard atmospheric pressure: A pressure of 101,325 dynes per square centimeter, symbol: (Atm). 　　10. Ultimate vacuum: After the vacuum chamber has been thoroughly evacuated, it stabilizes at a certain level of vacuum, and this level is referred to as the ultimate vacuum. Typically, a vacuum vessel requires 12 hours for gas purification, followed by another 12 hours of vacuum pumping. During the final hour, measurements are taken every 10 minutes, and the average value of these 10 measurements is taken as the ultimate vacuum value. 　　11. Pumping rate: At a certain pressure and temperature, the amount of gas removed from the pump’s inlet per unit of time is referred to as the pumping rate, or simply the pump speed. That is, Sp = Q/(P – P0). 12. Thermocouple vacuum gauge: A vacuum gauge that measures the degree of vacuum by utilizing the principle that the potential of a thermocouple is related to the temperature of the heating element, and that the temperature of the element in turn is related to the heat conduction of the gas. 　　13. Ionization vacuum gauge (also known as heat-cathode ionization gauge) consists of a cylindrical collector, a grid, and a filament located at the center of the grid, with the cylindrical collector being outside the grid. A vacuum gauge in which a hot cathode emits electrons to ionize gas molecules; the ions are collected by a collector electrode, and the gas pressure is measured based on the amount of ion current collected. 　　14. Compound vacuum gauge: Composed of a thermocouple vacuum gauge and a hot-cathode ionization vacuum gauge; its measurement range is from atmospheric pressure to 10-5 Pa. 　　15. Cold cathode ion gauge: A pair of cathode plates are located at both ends of the anode tube; under the influence of an external magnetic field, a Penning discharge is generated within the anode tube to produce ions. This vacuum gauge measures the gas pressure based on the amount of ion flow collected by the cathode plates. 　　16. Resistance vacuum gauge: A vacuum gauge that measures the degree of vacuum by using the principle that the resistance of a heating element is related to temperature, and that the temperature of the element is related to gas conductivity, through a bridge circuit. 　　17. McLeod vacuum gauge (compression vacuum gauge): An absolute vacuum gauge that compresses the gas to be measured using mercury (or oil) to a very small volume, then compares the difference in liquid column heights between the open and closed tubes, and uses Boyle’s law to directly calculate the gas pressure. 　　18. B-A type: This is a thermionic ionization gauge with the cathode and collector reversed. The collector is a thin filament placed at the center of the grid, while the filament is located outside the grid; this reduces the influence of soft X-rays, extends the lower limit of measurement, and enables the detection of ultra-high vacuum conditions. 　　19. Water ring vacuum pump: The impeller rotor of the pump rotates to create a water ring. A mechanical vacuum pump that achieves pumping by the periodic change in volume between the water ring and the blades, caused by the eccentric rotation of the rotor. 　　20. Reciprocating vacuum pump: A mechanical vacuum pump that uses the reciprocating motion of a piston to evacuate air. 　　21. Oil-sealed mechanical vacuum pumps: Mechanical vacuum pumps that use oil to maintain sealing, which can be classified into fixed-vane type, rotary-vane type, slide-valve type, epicyclic type, etc. 　　22. Roots vacuum pump: A mechanical vacuum pump that features a pair of shoe-shaped rotors rotating at high speeds in synchronization. This pump cannot evacuate air on its own; it requires a vacuum pump equipped with oil seals, water rings, or similar components that allow direct discharge to the atmosphere as a preceding stage. 　　23. Turbomolecular vacuum pump: It has a high-speed rotating impeller; when gas molecules collide with these rapidly rotating turbine blades, they are directed toward the exhaust port and then removed by the pump in the previous stage. 　　24. Oil Diffusion Vacuum Pump A high-speed stream of vapor is ejected from the nozzle of the diffusion pump. Under molecular flow conditions, gas molecules continuously diffuse into the vapor stream and are carried by the steam to the pump outlet, where they are compressed stage by stage before being expelled by the preceding pump. 　　25. Cryogenic vacuum pump: A vacuum pump that uses low-temperature surfaces below 20K to adsorb gases. 　　26. Cold trap (water-cooled baffle) – A device placed between the vacuum vessel and the pump, used to absorb gases or capture oil vapors. 　　27. Air purge valve: A small hole is made in the compression chamber of an oil-sealed mechanical vacuum pump, and a control valve is installed there. When the valve is opened and the amount of air introduced is adjusted, the rotor moves to a certain position, allowing air to enter the compression chamber through this hole. This reduces the compression ratio, preventing most of the vapor from condensing; it then exits the pump along with the introduced air. The valve that serves this purpose is called an air purge valve. 　　28. Vacuum freeze-drying Vacuum freeze-drying is also known as sublimation drying. The principle involves freezing the material to turn the water contained within it into ice, and then using vacuum to cause the ice to sublimate, thereby achieving drying. 　　29. Vacuum evaporation: Heating a material in a vacuum environment and coating it onto a substrate is known as vacuum evaporation, or vacuum coating. 　　30. Vacuum drying A method of drying items by taking advantage of the low boiling point in a vacuum environment. 　　31. Common names for vacuum systems 　　(1) Main pump: In a vacuum system, it is the vacuum pump used to achieve the desired level of vacuum in order to meet specific processing requirements; for example, the oil diffusion pump in vacuum coating machines is the main pump. 　　(2) Pre-pump: A vacuum pump used to maintain the pressure upstream of a certain vacuum pump below its critical upstream pressure. Pumps such as vane pumps or slide valve pumps installed in front of a Roots pump serve as pre-pumps. 　　(3) Roughing pump: A vacuum pump that starts pumping air from atmospheric pressure and reduces the system pressure to the level at which another vacuum pump can begin to operate. For example, the slide valve pump in vacuum coating machines is a crude oil pump. 　　(4) Maintenance pump: In a vacuum system, when the gas volume is very low, the pre-pump cannot be utilized effectively. To this end, a auxiliary pump with a smaller capacity is configured to keep the main pump operating; this pump is called a maintenance pump. If a small rotary vane pump is installed at the outlet of the diffusion pump, it serves as a backup pump.