jgourlay
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Vacuum Bell????
Gents:
I need a vacuum bell, but all the ones I find online are REALLY expensive. Anyone here have good ideas on how to either homejob one, or repurpose
some fairly common household item?
I need to degass investment for wax casting....
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12AX7
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Well, if you don't need to hold a vacuum, just lower the pressure, that's fairly easy (though the mechanical load is about the same).
I've heard of using a hunk of PVC tubing (8" schedule 40, let's say) with a thick slab of acrylic sitting on top, rubber gasket inbetween. Bottom can
be whatever (same, or something bolted, or cemented, etc.).
Tim
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jgourlay
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I'm stoopid: I have all that at home. Thanks for bringing my attention to the obvious! 
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Mr. Wizard
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How big of a volume do you have to enclose? How high a vacuum do you have to maintain? Large containers have to be fairly strong, as normal
construction that will hold internal pressures of 15 psi will collapse like junk under a vacuum. A fairly sturdy vacuum chamber could be made out of
a normal 20 pound propane tank. Using an air chisel to cut off an end will leave you with a straighter line than a torch. Use eye and ear
protection. You then slide the rough cut on a flat concrete surface kept wet with a slow stream of water. Rotate the tank as you slide it back and
forth. You will be surprised how quickly it flattens out the opening, and how loud it is. You might speed things up with a grinder or belt sander, but
it's easy to cut too much and make a gap. Be careful not to cut a groove or notch, and waste all your work.
The bottom of the chamber could be a 18"x18" x 1/2" steel plate available from a steel company scrap pile. The usual price is about 50 cents a
pound. The might even just give you the metal if you tell them about the project.
The end of the tank you cut off will depend on whether you want to take advantage of the convenient pipe threads on the tank or not.
The tank can have a window by cutting a small hole and putting a thick glass or plastic window on the outside. The sealant will depend on your needs.
Putting the window n the outside uses the atmosphere to hold the window in place. Using small windows means you don't have to have exotic thick
windows. Thin material will work if the hole is small.
Just an idea to start with.
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JohnWW
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The design of pressure vessels (usually cylindrical or spherical, including hemispherical ends and manholes) is something which, as a chemical
engineer, I am very familiar with. However, the design of a vessel for holding a vacuum - an external pressure - is more difficult, being analogous to
the mechanics of loading an unsupported column with a vertical load in compression, and finding the maximum safe load without its buckling. A full
mathematical analysis of the problem requires use of elliptic integrals.
Naturally, the accuracy of fabrication of such an item is important, because any buckling would occur initially at an imperfection, at below the
calculated design stress. Stiffening rings are thus often welded onto large vacuum vessels. The design and fabrication of vessels for both internal
and external pressure is dealt with in ch. 24 of Perrys Chemical Engineers' Handbook, in the References section, including particularly of stiffening
rings and flanges for vacuum vessels. The ASME Standards and British Standards also deal with such design.
Many pressure vessels designed for internal pressures will also withstand a significant external pressure. This estimated maximum allowable external
pressure is usually specified by manufacturers of pressure vessels designed for specified internal pressures. On a small scale, it may be possible to
adapt an autoclave for use as a vacuum vessel, by giving it an outlet and valve for connection to a vacuum line powered by a vacuum pump.
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tumadre
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Try using the glass cover from an electric skillet. They tend to be tempered and fairly strong.
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Magpie
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It would be interesting to see how much vacuum a small pressure cooker could withstand.
The single most important condition for a successful synthesis is good mixing - Nicodem
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zed
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Geez, my buddies in the jewelry trade, use a metal plate as a base, then they flip over the bottom half of a vacuum desiccator , and use that as their
bell.
You can often buy such units at auction, for 20 bucks or so, used. New, figure $50.00 plus.
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12AX7
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I know a guy who uses one (with the seal inverted, so it seals vacuum instead of pressure) for impregnating transformers with varnish.
Tim
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