Principles of pumping

     Since vacuum technology extends on so many ranges of pressure, no single pump has yet been developed, which is able to pump down a vessel from atmospheric pressure to the high vacuum or ultra-high vacuum range. Although all the vacuum pumps are concerned with lowering the number of molecules present in the gas phase, several different principles are involved in the various pumps which are used to attain low pressures.
    Vacuum pumping is based on one or more of the following principles:
    Compression-expansion of the gas, in piston pumps, liquid column or liquid ring pumps, rotary pumps, Root's pumps:
    Drag by viscosity effects, in vapour ejector pumps; Drag by diffusion effects, in vapour diffusion pumps; Molecular drag, in molecular pumps: lonization effects, in ion pumps; Physical or chemical sorpiion in sorption pumps, cryopumps and gettering processes.

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