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Vacuum Pump Issues

Posted: Sun May 20, 2018 6:23 am
by Samuel Berman
So, my Pfeiffer Duo 2.5 is now creating a vacuum low enough that I can measure it with my thermocouple gauge (thanks Ian). When I turn on the pump cold, it quickly sucks down to 40-50 microns. However, after 30 seconds or so, the pressure starts going up over 200 microns. I'm reasonably sure the pressure is reliable since I have measured it with 2 different gauges. What I think is causing this pressure increase is tiny bubbles forming in the oil. I don't think this is normal but let me know if it is. They come from the bottom and the pressure starts going up about when they reach the surface. I tried running the pump with the ballast open for an hour and also changing the oil. The bubbles form more with the ballast open. I'm wondering whether there could be air leaking in through the outside of the pump, since I did remove the case to look inside earlier. Would there be any way to check for air leaking in from the outside? Maybe using dust-off?

Thanks in advance,


Bubbles after running for ~1 minute

Re: Vacuum Pump Issues

Posted: Sun May 20, 2018 1:59 pm
by Rich Feldman
Good picture. Moisture in the oil?

I bet the experts will advise you to start with an oil change. Not worth the trouble to dry or degas the used stuff. In one pump users' manual, refrigeration techs are reminded that they could change the oil more than once during evacuation of a single system, if it's large and dirty.

My gallon of rotary pump oil was bought over the counter at Duniway in Mountain View.
If you are out of spare oil, I could give you some. Am in San Jose most weekdays. PM if you want to get in touch.

Re: Vacuum Pump Issues

Posted: Sun May 20, 2018 4:12 pm
by Samuel Berman
Well, I did change the oil and run the pump with the ballast open for an hour. I suppose the oil itself could possibly be a problem since I'm using very cheap stuff.

Re: Vacuum Pump Issues

Posted: Mon May 21, 2018 12:28 am
by Tyler Meagher
Your pump is similar to the pump I just rebuilt. I do not think the problem is because of the seal on the case. The seal on the case keeps the oil from leaking out. It does not have anything to do with the vacuum. The inside of the case is at atmosphere pressure. The pump inside is sealed to the face of a solid aluminum piece with oil. This connects directly to the intake port. You can see a picture in the manual. What vacuum oil did you use when you changed it?

Re: Vacuum Pump Issues

Posted: Mon May 21, 2018 1:38 am
by Samuel Berman
Okay, yeah, that makes sense. I used pretty much the cheapest oil I could get my hands on. I guess the problem is likely the oil then.

The oil I used (it was on sale): ... m+pump+oil

Re: Vacuum Pump Issues

Posted: Mon May 21, 2018 8:24 pm
by Bruce Meagher
Using cheap vacuum oil in your deluxe scientific vacuum pump was a mistake. See ... pumps.html and ... p-oil.html for a good overview of vacuum fluids.


P.S. - I'm intrigued by your thermonuclear gauge. I sure hope it always reads zero!

Re: Vacuum Pump Issues

Posted: Mon May 21, 2018 11:05 pm
by Samuel Berman
Haha, I think thermocouple was corrected to thermonuclear by spell check! Thanks for the info, I will try flushing with better oil. Would this oil be good?

Re: Vacuum Pump Issues

Posted: Tue May 22, 2018 2:04 am
by Bruce Meagher
I believe the recommended oil for your pump is Pfeiffer P3 oil. If you’re set on a Duniway brand oil I’d call and ask for their equivalent. I’ve personally had good luck with VacOil, and their cross reference chart says VacOil 195 is equivalent to Pfeiffer P3. I’m curious to know how much a simple oil change will reduce the ultimate pressure given the high vapor pressure of your current oil. Maybe this could be a good test for Tyler to do in parallel...

Re: Vacuum Pump Issues

Posted: Tue May 22, 2018 3:46 am
by prestonbarrows
A bit hard to tell from your pic but the oil through viewport should look clear and translucent(like maple syrup) not milky or opaque (like caramel).

Opaque means air bubble or water emulsion. If it gets bad, only solution is to flush the oil. You should do initial tests with the pump deadheaded to the gauge with minimal pumping volume. Any bypass valves should only be used if you are chugging through a large chamber full of wet air, rtfm for details.

Using the correct pump oil is also important. Again rtfm. The exact mixture is not critical but at least rotary vane vs direct drive.

Use of tracer gas to find leaks requires a sufficient level of vacuum and a molecularly dependant vacuum gauge. Usually a rough pump on its own does not get low enough for this technique.

If you have a power meter and something mobile with a low vapor pressure like an alcohol, gross leaks can be seen. But not in the case it sound like you are having where the leak is into the oil/pump body itself.