On Nov 1, 8:53 am, Mark Hickey wrote:
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?

No, but it is much easier to build a small one than a large one.

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?

Buckling.

The skin of a pressure vessel is in almost pure tension, so they can
be thin and not buckle. Any bending moment on a flat section bows
it outward reducing the bending moment (essentially converting it to
tension) The sides of vacuum vessel see compression and bending,
and any flat sections will buckle inward which will increase the
bending
moment.

The bathyscape and similar vessels are cylindrical with hemi-spherical
ends so that their skin is in almost pure compression with very little
bending moment.

A 55-gallon drum can be cut down to make the sides of the vacuum
chamber but I'm not clear on how to make the end hemispherical.
An option is to use nested vessels, with partial vacuum between them.

There is a reason why bell jars have a hemispherical top, and it is
not esthetics.

--

FF