Index Home About Blog
From: (Jonathan M. Elson)
Newsgroups: rec.crafts.metalworking
Subject: Re: Care and feeding of vacuum pumps.
Date: 12 Jul 1997 04:57:22 GMT

Ian Stirling ( wrote:

: I've not really got a good handle on what vapour pressures are of common
: compounds, say things like lead, epoxy, PVC, .....
: Presume no websites exist with info like this, going to have to make a
: trip to the nearest uni library I guess.

Vapor pressure of LEAD?  Don't even think you can get within a couple
orders of magnitude of that!  We do nuclear instruments, and have all
SORTS of junk materials in our instruments, plastic covered wire,
plexiglas, epoxy, molded plastic connectors, x-y positioner with
greased stepper motors and leadscrews, etc. etc. and we still get down
to 10 ^ -5 torr in a few minutes.  You can do fair vacuum coating at
that pressure, although 10 ^ -7 would be a lot better.
Huntington Vacuum products makes a vacuum epoxy which is
indistinguishable from the 'brand-name' Torr-Seal (from Varian vacuum
products) except for the price being 10% of varian's.
I've made a lot of devices, including electrical feedthroughs in
vacuum flanges with this stuff.  Its a tad expensive to use for ordinary
epoxy work, but it is such a fine epoxy, I just might start.

PVC, like water pipe from the hardware store might be an outgassing
problem, I don't know.  That is one material we haven't used much
in our work, although we have used something that looks a bit like
grey PVC for vacuum flanges.  it comes from a huge supply of scraps
in our shop, so I don't know for sure what it is.

Anyway, a single fingerprint will louse up your vacuum far worse than
a whole bunch of plastic parts in the chamber.  Of course, the finger
print will evaporate in 6 hrs or so, while a gassy piece of plastic
will just keep pumping it out.

Hmmm, a pretty cheap vacuum pump is a cryo-pump.  If there is helium
present, of course, you need to get down to 15K or so to pick it up.
But, if your problem is organic stuff, a big grid cooled with liquid
nitrogen at atmospheric pressure will do wonders for a dirty vacuum
system.  A LN trap is a good thing to put on a DP, to keep oil from
backstreaming, too.

OK, don't flame me about cryo-pumps being cheap!  I was just thinking
that a poor-man's cryopump (really just a LN trap in the chamber)
could do a good job.

Good luck,

From: (Jonathan M. Elson)
Newsgroups: rec.crafts.metalworking
Subject: Re: Care and feeding of vacuum pumps.
Date: 11 Jul 1997 18:41:55 GMT

Ian Stirling ( wrote:
: I've just found a nice vacuum pump in the shed, which I'd somehow forgotten
: about.
: This was just after I'd been looking at a catalog, listing them at silly
: prices, as I was wondering how expensive it would be to buy new the
: equipment to play with vacuum deposition.
: The pump currently has some oil in it, obviously I need vacuum oil, but
: looking at the catalogs, it comes in several viscosities.
: As it's been sitting for 10 years at least, I'd like to replace the oil,
: anyone know of a simple way of measuring viscosity?

I think the different viscosities are for belt-drive vs direct-drive
vacuum pumps.  Tell the supplier of the oil the make and model of your
pump, and they should be able to recommend the correct oil.  There are
also some exotic oils for use with corrosive gas products, when plasma
etching or doing CVD processes.  You shouldn't need these more expensive
While you're at it, you might get a rebuild kit for the pump.  They are
not too expensive, and contain a bunch of seals, etc. to bring the pump
back up to original performance.

: (specs are that it'll pump down to about 5um of mercury (on the side, wish
: I could get a manual, but doubt it) which should be enough for initial
: experiments.
: Then I guess I need to make a diffusion pump.

This is doable, but it is not easy to make a GOOD DP.  You might keep
your eye out for one on the jumk market.  A mechanical forepump is
a lot more salable at the junkyard than a DP.

: Or has anyone got any comments on using a multi-stage blower (as in
: vacuum
: cleaners) housed in the vacuum chamber as a poor mans turbomolecular pump?

NOT very easy.  I work with these.  The small ones run at 50,000 RPM.
Getting ball bearings to run at 50,000 RPM in vacuum is tough.
The rotor blades are stamped out of titanium sheet and then stress
relieved.  That sounds pretty tricky.  You should know that while a
TMP LOOKS like an aerodynamic axial flow fan, it IS NOT.  The blades need
to move faster than the mean molecular velocity of the gas they are
pumping, which means PRETTY FAST!  Those multi-stage blowers (best
ones come from old computer tape drives) are centrifugal blowers.
Centrifugal won't work at all at vacuum pressures, because the
molecules are not interacting much with each other.


Index Home About Blog