Vacuum pumps are used in a variety of vacuum systems, along with chambers and operating methods. In some cases, more than one pump (in series or parallel) will be employed in a single application. A positive displacement pump that carries a gas load from an intake port to an outlet (exhaust) port can be used to generate a partial vacuum, often known as a rough vacuum. Such pumps can only reach a low vacuum due to their mechanical constraints. Other procedures, generally in sequence, must subsequently be utilized to produce a greater vacuum (usually after a negative displacement pump is used to produce a fast vacuum). An oil-sealed rotary vane pump (the most typical positive displacement pump) might be used to support a diffusion pump, or a dry scroll pump could be used to support a turbomolecular pump. Other combinations exist, depending on the strength of vacuum required.
A high vacuum is difficult to achieve because the outgassing and vapor pressure characteristics of all materials subjected to the vacuum must be carefully examined. Oils, greases, and plastic or rubber gaskets used as vacuum chamber seals, for example, must not boil off when contacted with the vacuum; otherwise, the gasses produced will prevent the appropriate degree of vacuum from being achieved. All surfaces exposed to the vacuum must often be roasted at a high temperature to drive off absorbed gasses.
Desiccation prior to vacuum pumping can also help to prevent outgassing. Metal chambers with metal gasket seals, such as Klein flanges or ISO flanges, are more frequent in high vacuum chamber seals than rubber gaskets, which are more common in low vacuum chamber closures. The system must be clean and devoid of organic debris to reduce outgassing. All materials, solid or liquid, have a low vapor pressure, and when the vacuum pressure goes below this vapor pressure, outgassing becomes critical. As a result, many materials that function well at low vacuums, like epoxy, may be outgassing at higher vacuums. With these safeguards in place, vacuums of 1 mPa may be obtained with a variety of molecular pumps. It is feasible to achieve 1 µPa with proper design and operation.
Pumps of various sorts can be used in series or in parallel. A positive displacement pump would be used to eliminate the majority of the gas from a chamber in a standard pump down sequence, starting at the atmosphere (760 Torr, 101 kPa) and working down to 25 Torr (3 kPa). The pressure would then be reduced to 104 Torr using a sorption pump (10 mPa). A cryopump or turbomolecular pump would be employed to reduce the pressure to 108 Torr (1 Pa). Below 106 Torr, an extra ion pump can be initiated to remove gasses that a cryopump or turbo pump cannot handle, such as helium or hydrogen.
Custom-built equipment, stringent operational procedures, and a fair amount of trial-and-error are often required for ultra high vacuum. Stainless steel vacuum systems with metal-gasketed vacuum flanges are the most common. To temporarily boost the vapor pressure of all outgassing elements in the system and boil them out, the system is normally baked, ideally under a vacuum. This outgassing of the system can also be done at room temperature if required, although it will take significantly longer. The system can be chilled to lower vapor pressures to reduce residual outgassing during real operation once the bulk of the outgassing materials have been boiled off and evacuated. Liquid nitrogen is used to chill some systems deep below room temperature in order to stop residual outgassing while also cryopumping the system.
Some unusual leakage pathways and outgassing sources must be addressed in ultra-high vacuum systems. Aluminum and palladium's water absorption becomes an intolerable cause of outgassing, and even hard metals like stainless steel and titanium's absorptivity must be addressed. In severe vacuums, some oils and greases will boil out. It may be necessary to address the porosity of the metallic vacuum chamber walls and the grain direction of metallic flanges.
The influence of molecule size must be taken into account. Smaller molecules are easier to leak in and absorb by specific materials; thus, molecular pumps are less efficient at pumping gases with smaller molecular weights. Although a system may be able to remove nitrogen (the major component of air) to the appropriate vacuum, leftover ambient hydrogen and helium may still be present in the chamber. Outgassing issues arise in vessels lined with a highly gas-permeable material such as palladium (a high-capacity hydrogen sponge).
