On Nov 1, 12:21 pm, "Maxwell" wrote:
"Mark Hickey" wrote in message

...



Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.


Ok so at 18" diameter that clear plastic dish had a surface area
of about 254 square inches, so it saw a force of about 3700 lbs,
less if you were significantly above sea level.

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FF