Re: Vacumm Pups Source

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Ed, All!

>1. Where do you get a decent glass aspirator?
>

Laboratory supply houses will stock them.  Lab glassware manufacturers
in particular.  Indexed as "aspirator pumps", "water pumps" (careful
with this one for obvious reasons!) or "venturi pumps".  You might
have to do some web searching to find one in your area (CA I think) I
don't know if you can access the "yellow pages" (commercial telephone
directory, just in case it doesn't translate from the English) for the
US on the web, but since you can here I'd be surprised if you can't.
There must be a handful in your locality, all the universities will
have these things so there must be a local supply house somewhere.
You might even find listings of approved suppliers on some university
web pages which should give you a start.  If the worst comes to the
worst, I have a chemist friend in one of the CA universities (whom I
haven't had contact with for ages!) and I'll ask him if you get stuck.

>2. If water temperature makes a difference, as I'm sure it does,
would a
>cold trap help in the process?  If so, that would sure be of value in
>the evacuation of capacitors prior to oil backfill.

Cold trap as in between the pump and vessel to be evacuated?  It
probably would.  Are you thinking liquid air, or dry ice/acetone?  I'd
think that'd work quite well, especially if you could put some
molecular sieve or silica gel inside the trap.  If you can bend and
fuse a glass tube and have some mercury lying around, you can even
make a simple J-tube mercury manometer to measure the vacuum attained.
However, filling one without air entrapment is an exercise in patience
and near-endless futility . . .

>3. Does the inlet water pressure change the ultimate pressure or just
>the pumping rate?


Both.  But obviously the ultimate pressure is the more important.  The
pressure drop in the venturi section is partially a function of the
velocity of the "working fluid".

Finally, a practical note.  Do disconnect the evacuated vessel prior
to turning off the water supply.  Otherwise water will suck back into
your nice dry capacitor.  A simple improvised vacuum tap and pressure
release between pump and trap (or trap and vessel) may be very useful
here, and if you can manage it, a large thick-walled "Buchner"
filtration flask between pump and trap will give a margin of safety if
the water should suck back for any reason e.g. fluctuation in water
pressure.  Don't trust pumps with in-line non-return valves, in fact I
wouldn't even buy one.  Go for the "plain Jane" pumps without any
fancy stuff in them.  They work with less fuss, often give better
vacuum, and are cheaper.  You may want to buy two.  They don't cost
much, and glass does break sometimes ;-)  Fix the pump to the rubber
tubing and the tubing to the water tap with hose clamps judiciously
tightened so as not to crack the glass.  It has been known for the
mains water pressure to force the pump out of the tubing, or the
tubing off the water tap, with broken glass everywhere.  The tubing
fits more easily onto the glass if you moisten the glass with water
first.  It's best to use thick-walled rubber pressure / vacuum tubing
for all the connections because mains water pressure has been known to
burst conventional thin-walled tubing (and the sight of a piece of
now-translucent rubber tubing a foot in diameter and filled with water
at mains pressure about to let go is not for the faint-of-heart . .
. )  If you need detail, shout off-list.  I can send a scanned diagram
on any of this if need be.

Dunckx
Geek#1113 (G-1)