differential pumping


Differential pumping is a technique where you have different stages of a vacuum system between which you have to maintain a large difference in pressure. This could be between a detector which is operating in UHV (like an electron spectrometer or a residual gas analyzer) and the rest of the chamber which is not at UHV and containing, e.g. liquids you want to analyze. The pressure difference is maintained by a small aperture between the two parts and extra pumping on the UHV part. This strategy can work because of the very long mean free path of molecules in UHV.

Differential pumping is also used for some rotary feedthroughs where the sealing is achieved by viton o-rings. This it is not sufficiently good to maintain UHV, especially while rotating, but different stages of sealing are build in the feedthrough, such that the pressure difference is never actually between UHV and ambient pressure. Such a feedthrough is shown below. The upper and lower platforms can be rotated against each other, keeping UHV in the centre tube. There are three o-rings and two stages of differential pumping. The first stage maintains medium vacuum between the ambient pressure and the second stage. The second stage maintains high vacuum between the first stage and the UHV. So for this to work, one needs two extra pumps.


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