Vacuum Knowledge Primer - Explanations of Some Common Vacuum Terms!

　1. Definition of Vacuum

A vacuum system refers to a state of rarefied gas that is below the atmospheric pressure of the area.

　　2. Vacuum Degree

The degree of rarity of gas in a vacuum state, usually expressed as "high vacuum" and "low vacuum". High vacuum indicates a "good" vacuum degree, while low vacuum indicates a "poor" vacuum degree.

　　3. Units of Vacuum Degree

Typically measured in Torr (Torr), in recent years, the international standard has adopted Pascal (Pa) as the unit.

1 Torr = 1/760 atmospheric pressure = 1 mm of mercury

　4. Conversion between Torr and Pascal

1 Torr = 133.322 Pascal or 1 Pascal = 7.5×10^-3Torr

　　5. Mean Free Path

The average distance traveled by gas particles undergoing random thermal motion between two successive collisions, represented by the symbol "λ".

　6. Flow Rate

The amount of gas passing through any cross-section per unit time, represented by the symbol "Q", with units of Pa·L/s or Torr·L/s.

　7. Conductance

Indicates the ability of a vacuum pipeline to conduct gas. The unit is L/s, and under steady state, the conductance of the pipeline equals the flow rate divided by the pressure difference at both ends of the pipeline. Represented by the symbol "U".

U=Q/(P2- P1)

　　8. Pressure or Stress

The force exerted by gas molecules on a unit area of the container wall, represented by "P".

　9. Standard Atmospheric Pressure

Pressure of 101325 dyne per square centimeter, symbol: (Atm).

　　10. Ultimate Vacuum

The vacuum degree that a vacuum container stabilizes at after sufficient pumping, referred to as ultimate vacuum. Typically, the vacuum container must undergo 12 hours of gas refining, followed by 12 hours of vacuum pumping, and in the last hour, measurements are taken every 10 minutes, with the average of 10 measurements taken as the ultimate vacuum value.

　　11. Pumping Speed

At a certain pressure and temperature, the amount of gas removed from the pump inlet per unit time is called the pumping speed, abbreviated as Sp=Q/(P-P0).

　12. Thermocouple Vacuum Gauge

A vacuum gauge that measures vacuum degree based on the principle that the electromotive force of a thermocouple is related to the temperature of the heating element, which in turn is related to the thermal conductivity of the gas.

　13. Ionization Vacuum Gauge (also known as Hot Cathode Ionization Gauge)

Composed of a cylindrical collecting electrode, a grid, and a filament located at the center of the grid, with the cylindrical collecting electrode outside the grid. The hot cathode emits electrons that ionize gas molecules, and the ions are collected by the collecting electrode, measuring gas pressure based on the size of the collected ion current.

　　14. Composite Vacuum Gauge

Composed of a thermocouple vacuum gauge and a hot cathode ionization vacuum gauge, measuring range from atmospheric pressure to 10^-5Pa.

 

 

　15. Cold Cathode Ionization Gauge

A vacuum gauge that has a pair of cathode plates at both ends of the anode tube, and under the influence of an external magnetic field, ionization occurs in the anode tube due to Penning discharge, measuring gas pressure based on the size of the ion current collected by the cathode plates.

　　16. Resistance Vacuum Gauge

A vacuum gauge that measures vacuum degree based on the principle that the resistance of a heating element is related to temperature, which in turn is related to gas conduction, using a bridge circuit.

 

 

　　17. McLeod Vacuum Gauge (Compression Vacuum Gauge)

A type of absolute vacuum gauge that compresses the gas to be measured with mercury (or oil) to a very small volume, then compares the liquid column difference between an open tube and a closed tube, using Boyle's law to directly calculate gas pressure.

　18. B-A Gauge

This is a type of hot cathode ionization gauge with inverted cathode and collecting electrode. The collecting electrode is a thin wire placed at the center of the grid, while the filament is outside the grid, thus reducing the influence of soft X-rays and extending the lower measurement limit, capable of measuring ultra-high vacuum.

　　19. Water Ring Vacuum Pump

A mechanical vacuum pump that generates a water ring due to the rotation of the pump's impeller rotor. The eccentric rotation of the rotor causes periodic changes in the volume between the water ring and the blades, facilitating gas removal.

　　20. Reciprocating Vacuum Pump

A mechanical vacuum pump that removes gas through the reciprocating motion of a piston.

　21. Oil-Sealed Mechanical Vacuum Pump

A mechanical vacuum pump that uses oil to maintain sealing, which can be divided into fixed blade type, rotary blade type, sliding valve type, and other types.

　22. Roots Vacuum Pump

A mechanical vacuum pump with a pair of synchronously rotating shoe-shaped rotors. This pump cannot operate alone and requires a pre-stage vacuum pump such as an oil-sealed or water ring pump that can directly exhaust to the atmosphere.

　23. Turbo Molecular Vacuum Pump

A vacuum pump with a high-speed rotating impeller. When gas molecules collide with the high-speed rotating turbine blades, they are driven towards the exhaust port and then removed by the pre-stage pump.

　24. Oil Diffusion Vacuum Pump

The diffusion pump ejects a high-speed steam flow from its nozzle. Under molecular flow conditions, gas molecules continuously diffuse into the steam flow and are carried towards the pump outlet, where they are compressed step by step before being expelled by the forepump.

　25. Low Temperature Vacuum Pump

A vacuum pump that utilizes low-temperature surfaces below 20K to condense and adsorb gases.

　26. Cold Trap (Water-Cooled Baffle)

A device placed between the vacuum container and the pump, used to adsorb gases or capture oil vapors.

27. Gas Bypass Valve

A small hole is opened on the compression chamber of an oil-sealed mechanical vacuum pump, equipped with a regulating valve. When the valve is opened and the gas flow is adjusted, air is introduced through this hole into the compression chamber to reduce the compression ratio, preventing most of the steam from condensing and allowing it to be expelled along with the introduced gas. This valve is called a gas bypass valve.

　28. Vacuum Freeze Drying

Vacuum freeze drying, also known as sublimation drying. The principle is to freeze the material, turning its moisture into ice, and then sublimate the ice under vacuum to achieve drying.

　29. Vacuum Deposition

In a vacuum environment, heating the material and depositing it onto a substrate is called vacuum deposition, or vacuum coating.

　30. Vacuum Drying

A method of drying items by utilizing the characteristic of low boiling points in a vacuum environment.

　　31. Common Names for Vacuum Systems

(1) Main Pump: In a vacuum system, a vacuum pump used to achieve the required vacuum level to meet specific process requirements, such as the oil diffusion pump in a vacuum coating machine.

(2) Forepump: A vacuum pump used to maintain the pressure before a certain vacuum pump below its critical forepressure. For example, the rotary vane or sliding valve pump configured before a Roots pump is a forepump.

(3) Rough Pump: A vacuum pump that starts pumping from atmospheric pressure and reduces the system pressure to the level where another vacuum pump can operate. For example, the sliding valve pump in a vacuum coating machine is a rough oil pump.

(4) Maintenance Pump: In a vacuum system, when the gas volume is very small, the forepump cannot be effectively utilized. Therefore, a smaller auxiliary pump is configured to maintain the operation of the main pump, called a maintenance pump. For example, a small rotary vane pump configured at the outlet of a diffusion pump is a maintenance pump.