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Common Terminology for Vacuum
A vacuum system refers to a state of rarefied gas with a pressure lower than the atmospheric pressure of the region.
The degree of gas rarefaction under vacuum conditions is typically expressed as "high vacuum" and "low vacuum." High vacuum indicates a "good" level of vacuum, while low vacuum indicates a "poor" level of vacuum.
Usually measured in torr, in recent years the international standard has adopted the pascal (Pa) as the unit.
1 torr = 1/760 atmosphere = 1 millimeter of mercury
4. Switching between To and Pa
1 torr = 133.322 pascals, or 1 pascal = 7.5 × 10⁻³ torr
The average distance traveled by gas particles undergoing random thermal motion between two successive collisions is denoted by the symbol “λ”.
The amount of gas flowing through any cross-section per unit time is denoted by the symbol “Q,” with units of pascal-liters per second (Pa·L/s) or torr-liters per second (Torr·L/s).
Indicates the ability of a vacuum pipe to transmit gas. The unit is liters per second (L/s). Under steady-state conditions, the pipe's flow conductance equals the pipe's flow rate divided by the pressure difference between the two ends of the pipe. The symbol used is “U.” U = Q / (P2 - P1)
The force exerted by gas molecules on a unit area of the container wall is denoted by “P.”
9. Standard atmospheric pressure
Atmospheric pressure, with a value of 101,325 dynes per square centimeter; symbol: (Atm).
After the vacuum container has been thoroughly evacuated, it stabilizes at a certain degree of vacuum. This degree of vacuum is referred to as the ultimate vacuum. Typically, the vacuum container must undergo a 12-hour outgassing period followed by another 12-hour evacuation period. During the final hour, measurements are taken every 10 minutes, and the average of these 10 readings is taken as the ultimate vacuum value.
Under a given pressure and temperature, the amount of gas pumped away from the pump's inlet per unit time is called the pumping speed, or simply the evacuation rate. Mathematically, it is expressed as Sp = Q / (P - P0).
A vacuum gauge that measures vacuum degree by utilizing the principle that the thermoelectric potential of a thermocouple is related to the temperature of the heating element, and the temperature of the element, in turn, is related to the thermal conductivity of the gas.
13. Ionization vacuum gauge (also known as hot-cathode ionization gauge)
It consists of a cylindrical collector, a grid, and a filament located at the center of the grid. The cylindrical collector is positioned outside the grid. The thermionic cathode emits electrons that ionize gas molecules; the ions are then collected by the collector. This vacuum gauge measures gas pressure based on the magnitude of the ion current collected.
It consists of a thermocouple vacuum gauge and a hot-cathode ionization vacuum gauge, with a measurement range from atmospheric pressure down to 10^-5 Pa.
15. Cold Cathode Ionization Gauge
At both ends of the anode cylinder, there is a pair of cathode plates. Under the influence of an externally applied magnetic field, a Penning discharge is generated within the anode cylinder, producing ions. The vacuum gauge measures the gas pressure based on the magnitude of the ion current collected by the cathode plates.
A vacuum gauge that measures vacuum degree by utilizing the principle that the resistance of a heating element is temperature-dependent, and the element's temperature, in turn, is related to gas conduction, through a Wheatstone bridge circuit.
17. McLeod vacuum gauge (compression-type vacuum gauge)
An absolute vacuum gauge that compresses the gas to be measured into an extremely small volume using mercury (or oil), then compares the liquid-column difference between the open and closed tubes, and directly calculates the gas pressure using Boyle's law.
This is a hot-cathode ionization gauge with the cathode and collector electrodes reversed. The collector electrode is a fine wire positioned at the center of the grid, while the filament is located outside the grid. This arrangement reduces the influence of soft X-rays, extends the lower measurement limit, and enables the measurement of ultra-high vacuum conditions.
A mechanical vacuum pump that generates a water ring by rotating the impeller rotor of the pump. The water ring and the spaces between the blades undergo periodic volume changes due to the eccentric rotation of the rotor, thereby achieving gas extraction.
A mechanical vacuum pump that uses the reciprocating motion of a piston to evacuate air.
21. Oil-sealed mechanical vacuum pump
Mechanical vacuum pumps that use oil to maintain a seal can be classified into types such as fixed-vane, rotary-vane, sliding-valve, and hypocycloidal.
A mechanical vacuum pump equipped with a pair of synchronously high-speed rotating shoe-shaped rotors—this pump cannot operate independently for gas extraction and must be paired with a preliminary-stage vacuum pump, such as an oil-sealed or liquid-ring pump, that can directly vent to the atmosphere.
23. Turbine molecular vacuum pump
There is a high-speed rotating impeller. When gas molecules collide with the rapidly spinning turbine blades, they are propelled toward the outlet and then removed by the preceding-stage pump.
A high-speed steam jet is ejected from the nozzle of a diffusion pump. Under molecular flow conditions, gas molecules continuously diffuse into the vapor stream and are carried by the steam toward the pump outlet, where they are progressively compressed stage by stage before being exhausted by the preceding-stage pump.
25. Low-temperature vacuum pump
A vacuum pump that uses low-temperature surfaces below 20 K to condense and adsorb gases.
26. Cold trap (water-cooled baffle)
A device placed between a vacuum container and a pump, used for adsorbing gases or capturing oil vapors.
A small hole is opened in the compression chamber of the oil-sealed mechanical vacuum pump, and a regulating valve is installed there. When the valve is opened and the intake volume is adjusted, the rotor rotates to a certain position, allowing air to enter the compression chamber through this hole, thereby reducing the compression ratio. As a result, most of the vapor does not condense but is expelled from the pump along with the injected gas. The valve that performs this function is called an air-blowing valve.
Vacuum freeze-drying, also known as sublimation drying, works by freezing the material to turn its contained moisture into ice crystals, and then sublimating the ice under vacuum conditions to achieve the drying effect.
In a vacuum environment, heating and depositing a material onto a substrate is called vacuum evaporation or vacuum coating.
A method of drying items by taking advantage of the low boiling point under vacuum conditions.
31. Common names for vacuum systems
(1) Main pump: In a vacuum system, the 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—is referred to as the main pump.
(2) Forepump: A vacuum pump used to maintain the forepressure of a particular vacuum pump below its critical forepressure. For example, a rotary vane or slide valve pump configured upstream of a Roots pump serves as a forepump.
(3) Roughing pump: A pump that starts evacuating from atmospheric pressure and reduces the system pressure to a level at which another vacuum pump can begin operating. For example, the rotary vane pump used in vacuum coating machines is a roughing oil pump.
(4) Backup Pump: In a vacuum system, when the gas flow rate is very low, the primary pump cannot operate efficiently. To address this, a smaller auxiliary pump is configured to maintain the operation of the main pump; this pump is called the backup pump. For example, installing a small rotary vane pump at the outlet of a diffusion pump serves as a backup pump.
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