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I'm trying to calculate a conductance for free molecular flow of a tube which is closed by a disc. The distance between the tube and the disc will be approx 10 micro meters while inner and outer radius of the tube is around r1~10cm ; r2~20cm.

Here are some pictures: http://tinypic.com/r/5mjsbd/4, http://tinypic.com/r/2zi4eus/4

The conductance of the tube is given in most textbooks on this subject, but I don't know how to find a conductance of a volume enclosed by the disc and the wall thickness (r2-r1) of the tube.

My first idea was to approximate the enclosed volume by rectangular tubes and then integrate the relation for rectangular tube conductance from 0 to 2*Pi*r2, that is:

[itex]\int[/itex][itex]^{2*Pi*r_{2}}_{0}[/itex](31.1[itex]\frac{b^{2}*c^{2}}{(b+c)L}[/itex])db

where c is the rec. tube height, b ist the tube thickness and L=r2-r1 is the length of the tube.

But I'm not sure whether this is good.

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# Vacuum conductance calculation, tube closed by a disc.

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