Vapour diffusion pumps

Occasionally a distillation will require a higher vacuum than that produced by a rotary pump. Higher vacuums are normally obtained by employing a mercury or oil vapour diffusion pump, in conjunction with a rotary pump. Oil vapour pumps are most commonly used today and various models are now commercially available which will produce a vacuum of about lO mmHg. These pumps are only used occasionally but it is very useful to have one shared between a large group or section. 

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To attain pressures lower than that produced by the oil immersion pump, a vapour diffusion pump is employed which gives pressures down to 5 x... 

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