Re: Need info on Edwards Vacuum pump

["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 Mike, All !

Date: 04 February 2001 19:44
Subject: Need info on Edwards Vacuum pump


>Original poster: "Mike Harrison by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <mike-at-whitewing.co.uk>
>
>I just picked up an Edwards E2M5 2-stage vacuum pump at a
hamfest
>(UKP25 - I think that was a bargain!)

So do I - they're over a thousand quid new :-)

>
>Does anyone have any info on this pump

2,5 cubic feet per minute pumping rate, ultimate vacuum
better than 0,01mm Hg (depending on how the previous user
has treated it).

>, and any general 'care and
>feeding' type tips for someone like me whose knowledge of
vacuum
>systems is comparable to the amount of air in them?!

Always release the vacuum on the input side of the pump
before switching off.  This means you may need to improvise
a valve or two between the pump and the evacuated vessel if
you wish the vessel to remain evacuated.  These pumps have
automatic valves which are _supposed_ to shut off the input
if there is a vacuum present at power down, the idea being
that pump oil is not then sucked back into the evacuated
vessel when the pump stops working.  Unfortunately they
don't always make the trip in time.  I have seen many a
vacuum line filled with pump oil and many a scientist
seething and cussing as they faced the reality of having to
strip everything down and clean it all out ;-)

The Edwards pumps have a gas ballast valve which can be
opened (it's a black plastic screw cap on top of the pump)
to admit air to the compression cycle before the gas is
ejected from the pump, which is essential when pumping
condensable gases and vapours.  It works thus:

1) The expansion cycle allows gas from the vessel being
evacuated to expand into the pump chamber.

2) The compression cycle then increases the pressure of this
gas (maybe from say 0,5mm to say a bit over atmospheric)
before the exit valve opens.

This is fine if you are pumping dry gases and highly
volatile gases, but for gases which can condense or gas
containing e.g. water or ether vapour there is the danger
that these will condense in the pump oil on the compression
cycle, which of course will spoil the ultimate vacuum
achievable by the pump when it is subsequently working on
non-condensable gases and vapours, because their presence in
the oil will raise the internal vapour pressure.  The gas
ballast valve allows extra air from the atmosphere (rather
than the vessel you are evacuating) into the compression
cycle and by doing so prevents (or at least greatly reduces)
the condensation of vapours in the pump.  It will reduce the
ultimate vacuum of the pump a little but protect it a lot.
Simply loosening the gas ballast cap a turn or two (no need
to undo it completely) will allow the pump to suck some air
in and you will hear the pump note change - it sounds
"hollow", as opposed to the "tight" sound you get when
everything has been pumped down.  I always used to gas
ballast the pump for at least an hour after each oil change
to clear everything out.  A well-cared-for E2M5 will get
down to 10^-3 mmHg on a good day.

There is a downside to this.  Oil mist of micron particle
size is greatly increased under gas ballast and this is very
hazardous to health (lung cancer).  Even if you are not
using the gas ballast valve, the initial stages of
evacuating a vessel will have large volumes of gas bubbling
through the pump oil and giving large clouds of mist.  You
_need_ an oil mist filter on the pump outlet - I feel it is
close to criminal that these things are not shipped with a
filter as standard.  If all else fails, I can contact a
friend who I know has the details of a UK stockist and from
memory these filters are only a few pounds.  Likewise for
Edwards rotary pump oil, there are from memory at least two
grades, and I'm sure I can get details if you need them.  I
may even be able to turn up a pump manual.  Shout off list
if need be.  I've used Edwards pumps, oil, mist filters etc
for years.

Wear gloves when doing that first oil change and throw them
away afterwards.  Don't get the oil on your hands or
clothes.  You have no idea what it was used to pump before
you got it.  Dispose of the old oil and the first flushings
with the new _responsibly_ e.g. via the local authority.  It
will have to go to a toxic waste facility.

>Is the vacuum created by this type of pump likely to be
good for
>plasma-sphere/crookes tube type effects and messing with
low-pressure
>argon?

No kidding!  0,01mm Hg will give you a half-decent soft
X-ray tube if you fancy irradiating yourself ;-)  Isolate
the evacuated vessel from the pump and use a "getter" in the
vessel (e.g. a hot titanium wire) and you can make your own
valves, vacuum down to 10^-6 mmHg !!!  If you pump systems
containing plastics, your ultimate vacuum will be limited
because of the presence of plasticisers and residual
monomers which have a significant vapour pressure and you
may find it impossible to do better than say 0,5mmHg in this
case.  I would always gas ballast when pumping on plastics
unless I had a liquid air trap before the pump.  Ideally,
all vac pumps ought to be protected by a liquid air trap,
but I realise this is unlikely to be practical outside of
science labs!

This is one beautiful piece of vacuum technology !


>Anyone know of any  'fun things' (HV or otherwise) to do
with a vacuum
>pump?


You can make a crude Pirani gauge to indicate the degree of
vacuum with a piece of resistance wire, a battery and a
milliammeter !  The convection cooling of a hot wire is
obviously reduced according to the degree of vacuum, so
monitoring the current in the wire will tell you how the
vacuum is changing - higher vacuum equals poorer cooling
equals hotter wire equals higher resistance.  The useful
range will be from say 1 mmHg down to 10^3 mmHg. At a pinch,
you can carefully break a torch bulb envelope without
damaging the filament and fix it in your vacuum system.  Hey
presto - instant vacuum gauge!  Use the same volts the bulb
needs to get it to very dull red heat. When the vacuum in
the system is as good as you think it's going to get, heat
the walls of the vessel up with a hot air gun (if the vessel
will stand it) and you'll see the gauge indicate that more
gas is desorbing from the walls !

Getting a really good vacuum is hard work - and one man's
vacuum is another man's waste tip.  10^-6 mmHg is really
good for a polymer chemist like me and 10^-1 mmHg good
enough for most things I've done - but a spectroscopist
might not be satisfied with worse than 10^-9 mmHg.  Yet
again, 10-20 mmHg is the sort of thing you need for most gas
discharge applications.  You are always welcome to email me
off list if you need more details.

See also: http://www.boc-dot-com/evt/pdf/catalog/info.pdf
This at least will give you contact info, spares parts
numbers etc.

Dunckx