Vacuum pump, compression turbomolecular

Mechanical pump (vacuum technology) A compression-type vacuum pump with moving parts. The term is generally applied to pumps used for roughing or backing (Example oil-sealed mechanical pump, piston pump, diaphragm pump, etc.) and not high vacuum pumps (Example turbomolecular pumps). See also Vacuum pump. 

A mechanical pump providing even lower vacuum levels is the turbomolecular pump, in which one or more balanced rotors (turbine blades) spin at 20,000 to 50,000 rpm. At these rotation rates, the periphery moves at a speed that exceeds the mean molecular speeds of most molecules, and gas-rotor collisions impart a momentum component to the gas in the direction of the exhaust. Compression ratios up to 10 can be achieved as long as the outlet pressure is kept below about 0.1 Torr by a forepump. 

Mechanical pumps are used directly to produce a low and medium vacuum, as well as extensively to back Roots vacuum, turbomolecular and diffusion pumps. These pumps are also called oil-sealed rotary vane pumps as they rely on the use of vanes or blades to compress gases. 

The ultimate total pressure that can be attained by means of turbomolecular pumps mainly depends upon the partial pressures of the various gases on the pre-vacuum side of the turbo-molecular pump. The partial pressure p0 of a gas on the high-vacuum side is calculated from the partial pressure ppatt on the fore-vacuum side, divided by the compression for this gas acording to (41). 

For the turbomolecular pumps mostly used at present the given rating is defined for pumping nitrogen atmosphere. The important performance data of turbo-molecular pumps is the compression ratio. The compression ratio describes the ratio of the outlet pressure (forepump) of a particular gas to the inlet pressure (MS). The turbomolecular pump gives a completely background-free high vacuum and exhibits excellent start-up properties, which is important for benchtop instruments or those for mobile use. The use of helium lowers the performance of the... 

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