Vacuum Pump: Techniques

Vacuum pumps interact with chambers and operative procedures into a deep range of vacuum systems.
Sometimes in an individual application are used more than one pump (in series or in parallel). A rough vacuum, called also partial vacuum, can be created utilizing a positive displacement pump that carries a gas load from an inlet port to an outlet port; of course they have some limitatios so they can only reach a low vacuum.

To reach a high vacuum you can use different techniques, such as the use of an oil sealed rotary vane pump backing a diffusion pump, or a dry scroll pump backing a turbomolecular pump.
It is a difficult process, as all of the materials exhibited to the vacuum must be cautiously valued for their outgassing and vapor pressure specifications.
Frequently, the surfaces exhibited to the vacuum must be baked at really high temperature to dispel adsorbed gases.
High vacuum systems usually necessitate metal chambers with metal gasket seals.

The system have to be uninfected and with no organic matter to reduce outgassing. Some materials that can work good in low vacuums, can be the cause of outgassing at higher vacuums.

As previously said, you can utilize different kinds of pumps in sequence or in parallel. In a regular pumpdown sequence, a positive displacement pump is used to get rid of most of the gas contained in a chamber, starting from atmosphere (760 Torr, 101 kPa) to 25 Torr (3 kPa). Then the use of a sorption pump can bring the pressure down to 10−4 Torr (10 mPa), and the use of a cryopump or turbomolecular pump can bring the pressure further down to 10−8 Torr (1 µPa). Is also possible to start an extra ion pump below 10−6 Torr to get rid of gases which are not sufficiently handled by a cryopump or turbo pump.

There is also the ultra high vacuum that need a custom-built equipment, rigorous operational procedures, and a fair amount of trial-and-error.
Ultra-high vacuum systems are often made of stainless steel with metal-gasketed conflat flanges and are often baked under vacuum to increase temporarily the vapour pressure of all outgassing materials and boil them off. After that the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures to reduce residual outgassing during actual operation.

The impact of molecular size is also a factor to consider. Smaller molecules can leak in more easily and are more well assimilated by determinate materials, and molecular pumps are less efficacious at pumping gases with lower molecular weights.

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