Vac pump advice

[guest]

Hi Guys, I'm having a few problems with my dual stage Edwards
vac pump getting a good vacuum. I've fitted a seal kit thanks to
Robin from Recalvac and thoroughly cleaned the pump housing
out as it had a grey sludge in the sump. The wear on the internal
surfaces of the pump were minor: I lapped a few faces with 1200
wet and dry on glass to remove some wear marks but even then
this did not remove the original machining marks from the unit.

Currently I can pull about 80 millitorr vacuum measured with my
"Belljar" thermocouple gauge when the pump is cold but this
goes up to 100 millitorr when warm. I think my pump oil is the
main culprit as the pressure goes up when I close the ballast
and as things warm up.

Is there any advice as to correct oils and viscosity I should use
on a belt drive unit for best performance?.

Mark Harriss

[fizz]

You have not said what oil you are using, it maybe to thin, and
when the pump warms up and the clearances expand you
loose pressure.
Try to find a thicker oil.

Robin

[Richard Hull]

Your second stage in the pump may be worn or its seal bad. The pressure MUST always go down when a gas ballast is closed!! An open ballast reduces most pumps to an effective single stage pump. Closing a ballast causes the second stage to function to its max., thus, the pressure always plunges after closing the ballast. The ballast open at the moment of pump turn on will allow the output of the first statge to vent to air rather than be further compressed and volatiles condensed in the second stage.

Richard Hull

[guest]

The oil is intended for direct drive fridge service pumps, I think
it's around 30 SAE viscosity and is known as R31.

Mark Harriss

[guest]

Thanks for that advice, I'll concentrate on that area on the
pump. This pump has a ball bearing and spring ballast valve
arrangement inside a hollow tube that doubles as the knob
shaft. When I first pulled it apart the ball was on the bottom and
the unit blew air through the ballast, this was the opposite to the
drawings with the ball on top and the unit sucking air through
the ballast. I tried both ways last night ball on top, or ball on
bottom (sounds like a playboy forum letter doesn't it?) and it
gets an extra 10 millitorr with it blowing air and the ball on bottom
, contrary to the assembly drawings. The pressure still goes up
though once the valve is closed. It looks like there's a simple
reassembly problem at work here, I'll have to examine the
drawings a bit closer.

Mark Harriss

[AllenWallace]

We bought our vac pump oil at the local farmers coop. It seems that
milking machines use vacuum pumps. I don't really know the
viscosity... As I recall, It didn't seem that the price was excessive.

[guest]

I'm now down to 25 millitorr on my homemade thermocouple
gauge after accidentally reversing the filament polarity while
swapping the octal socket for soldered joints. I wasn't sure if the
DST-531 was polarity sensitive for filaments but my unit is using
a separate transformer to power the tube via a current regulator
(LM317) set to the recommended current and so is running off
DC. At least now I'm in the ballpark.

Mark Harriss