Fusor Forums

Thanks for the reply, Dave, I should have explained myself better. When the vacuum performance changed overnight, I changed the oil. No change, still 200 microns.

When I open the ballast valve, it has no effect, still 200 microns. Does this mean that the second stage is the problem? My understanding of pumps (which is limited) is that the ballast valve vents the interstage vacuum, correct?

My power supply is designed around a transformer that has one side grounded. That means the DC output from a full bridge is bipolar. (my chamber is a bell jar). The vacuum pump case potential is halfway between the cathode and the anode, and sometimes I've seen glow going down the vacuum line towards the pump. Is this a problem?

This problem started the day after I added a 0.1 ufd filter capacitor to the power supply, but I don't see why that would have caused it.

Curious, don't see my original response......


Ummm... Gary , that ionization you see in the vacuum inlet, might be related to the pressure problems. Not sure.... but if you do have a glow discharge happening in a vacuum line that's non-metallic... you could be getting serious outgassing from localized heat/beating-up of the inner hose surface.

From your description, the electrical circuitry sounds correctly done. the center tapped supplies are built with the bipolar use in mind. Can't really see what effect a capacitor might have, unless it is somehow shifting the ground point, and confusing the pressure gage. But if your low vacuum gage is a Convectron, TC or Capacitance manometer, it probably doesn't really care. I have seen Ion Gages, confused by stray potentials, but these operate with very much smaller currents.

I'm thinking your pump is not the problem. The ballast meters outside air into the high pressure stage... so that condensation is minimized. This lowers the internal last stage pressure (eventually) as the volatiles are pumped away.

Dave Cooper

My supply isn't center tapped it's end tapped - I have a hipotronics 30kv 600VA transformer I took out of a hi-pot tester. So relative to ground (the pump) I'm half wave bipolar.

I'm pretty sure it's the pump because I'm running it blanked off now, with out the chamber and voltage. It still has about 6 inches of 1" copper pipe and soldered fittings but these were leak free a few days ago.

I just ran it for awhile and observed the following: With the ballast valve closed it barely breaks 200 micron. With the ballast valve open, it creeps up to 225 micron - not much difference, making me think the problem is in the high pressure stage. Another thing is that it used to hold vacuum well when I shut it off. Now the vacuum quickly goes up to several thousand microns.

The pressure gage (Robinaire thermistor) is not confused, I tested it on my other pump, a small refrigeration type, and it went right down to 5 micron as expected.

I'm just wondering now if I abused the Edwards unit somehow or if it was just one of those things. The night before the problem when I added the filter capacitor, I noticed a lot of sparkler activity on my anode (it's aluminum). I've seen this before on new aluminum, burning off the oxide. I don't know why the capacitor would greatly increase this activity and I don't see how it's connected, but I'm suspicious of coincidences too.

The rapid leak up does seem to point to the pump itself. Might be a leaking exhaust valve.. they're often reed valves...

The Welch Pump general overhaul kit includes a replacement reed, so it is a maintenance item, along with seals. Not sure what's inside an Edwards pump.

Check out an Edwards service center.??

Dave Cooper

I think I might have spotted the problem. I looked closely at the oil I drained to see why it's a little dark. I see thousands of tiny, tiny black pinpoint particles. I'll bet it's burned oil, maybe from little electrical events going on inside the pump. When I added the filter capacitor, I changed the voltage relationship of the pump case from half wave pulses to filtered DC. (I can't ground either the fusor anode or cathode, due to the transformer ground)

Cleaning this out will be a chore. Just changing the oil didn't do any good.

I assume you have taken all these readings at the pump head, itself, and not in your chamber. That will tell the tale. At the pump head use a near zero length stub of hose.

Richard Hull

Richard
These readings were at the pump head, blanked off with no chamber. Have you seen these little black specks before?

I have never seen black specks, I am worried to internal flapper valve (usually a piece of spring steel or spring loaded ball is corroded or deteriorating. Sound bad.

Richard Hull