If you want a very good vacuum, use stainless steel construction,
flanges with knife edges and oxygen free copper rings for sealing.
Avoid screw holes and such which can trap dirt or moisture on the
inside. Flat flanges and copper wire also works for sealing. Brass
construction is also ok, but beware of the zinc vapour pressure.
Epoxy may be OK, but unless it is cured by heat, I would be vary of
outgassing. Apiezon has a full range of sealing compounds for UHV.

Stainless steel is actually better than glass, as glass will allow
hydrogen through in time. Bead blast or pickle to get rid of any
oxides after welding. Clean everything in ultrasonic cleaner or
similar equipment, rinse with distilled water, clean again with
degreasing agent such as acteone or hexane. A vapour degreaser is
excellent. Bake everything after assembly while you are pumping. This
helps the outgassing. Heat can be applied with a heat gun or heating
tape.


"Z" == Zak writes:

Z Allan Adler wrote:
It seems to me that if one is interested in making one's own tubes, it
must be a lot easier to machine a metal tube, seal it with flanges and
O-rings, evacuate it and have electrodes passing through the metal than
it is to learn to make reliable glass-to-metal seals. The discussion also
focuses on the large number of parameters to worry about, each of which
presumably requires a new tube. But this all metal construction presumably
makes it possible to open it up and change things in it. So this approach
seems to be more suited to experimenting with tubes if one is so inclined.

Z Experimenting yes, if you can leave your pump running.

Z But for manufacturing it is not so good. The glass-to-metal seal is
Z easy enough: make sure your glass and pin material belong together.


Z I agree that blowing a nice multi pin glass base with the pins in
Z correctly is not easy at all, but it lends itself well to an
Z industrial process.


Z I'd think you can buy these for custom tube work, in a specified glass.

Z Now, if you leave your tube connected to the pump you could even use
Z vacuum epoxy to seal everything.


Z O-rings are tricky. If you have a square channel with a round ring in,
Z gas will be trapped where round meets square; this air will eventually
Z escape. The same applies to flanges coming too close together, and
Z really anything that you cannot heat very well.



Z Thomas


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