Foreline pump base pressure degraded

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
ian_krase
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Foreline pump base pressure degraded

Post by ian_krase » Sun May 20, 2018 1:35 am

Hello, everyone.

I use a Pfeiffer Duo (the newer version with the white case) as a foreline pump. When I first got this (used) pump, it performed pretty well with about 8 microns of base pressure. Now, however, I can't get lower than about 60 microns. I didn't really notice since lately I've been using it exclusively as a foreline pump.

The oil is fairly new though maybe I should replace it. The pump doesn't sound any different or anything like that.

Do you think I need to do an outright rebuild?

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Re: Foreline pump base pressure degraded

Post by Bruce Meagher » Mon May 21, 2018 9:09 pm

Ian,

After helping Tyler through his pump rebuild it’s not evident the parts in the minor rebuild will have much impact on performance. There are a couple o-rings inside that could be leaking, but I'd think this would be unlikely. Of course cleaning during the rebuild process could make a huge impact if contamination is an issue. The first thing I always assume when I have a vacuum issue is that I have a leak somewhere. Have you taken the pump off the foreline and just tested directly at the intake port? Next would be contamination in the oil. How long ago did you measure the 8 microns (weeks, months, years,…). How much usage has the pump seen in that time, and what type of processes / contamination have been put through it? I probably put less than 100 hours a year on my vacuum pumps. Maybe it’s just water vapor and a few hour run with the ballast open would clear everything? Also, could it be a faulty or leaky gauge? I like to check / cross calibrate my gauge with another system to make sure they are in good working order. A couple pictures of your test setup might be helpful.

Bruce

ian_krase
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Re: Foreline pump base pressure degraded

Post by ian_krase » Tue May 22, 2018 11:43 pm

I tested with nothing but a valve between the pump and the gauge, should go with nothing but the gauge.

8 microns was measured over a year ago, with the pump on a system reaching sub-30-microns routinely for months later. However, I haven't measured the pump's ultimate vacuum in some months since then until now.

By rebuild I meant a more complete rebuild including replacing the vanes. Have found a source, but don't want to do it if not needed -- 70 dollars plus hours of disassembly on grease-spattered table.

Really pretty much just air that has gone through it though it's been exposed to trace quantities of solvent vapors and plasma-burned dust-off occasionally. Total of a few hours run a week.

The gauge is unlikely to be faulty or leaky -- it's a new-to-me convectron that I tested the calibration of at both zero and atmosphere, though it always could have a cracked feedthrough or something.

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Richard Hull
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Re: Foreline pump base pressure degraded

Post by Richard Hull » Wed May 23, 2018 4:13 am

The obvious assumption is that you have fresh oil in the pump of very recent input. If not and you want to try to keep the oil. Follow the suggestion of gas ballasting for two to three hours with the inlet blocked off. Smart money would just replace the oil. with new oil, still gas ballast for a few minutes before attaching the TC gauge tube. Glow cleaning and blasting off dust over a year will foul oil.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

ian_krase
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Re: Foreline pump base pressure degraded

Post by ian_krase » Sun Jun 03, 2018 5:33 am

I tried blankoff and gas ballast for a while but the only result was a bit of foaminess in the oil. Going to try changing the oil when I have a chance.

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Richard Hull
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Re: Foreline pump base pressure degraded

Post by Richard Hull » Sun Jun 03, 2018 10:21 am

Foam in the oil is a definite sign of air and moisture in the oil. OOPs ....something is not sealed well.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Mark Kimball
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Re: Foreline pump base pressure degraded

Post by Mark Kimball » Mon Jun 04, 2018 12:49 am

At work we had a nice gold sputtering system for preparing samples for SEM examination. It used a small 2-stage mechanical pump to evacuate the chamber. It didn't take long for the pump oil to absorb a _lot_ of water (this was in Oregon, infamous for its damp weather). Symptoms included extended pump-down time and foamy pump oil observed in the pump's sight glass. You might be able to enable the ballast on your pump and -- slowly -- pull the water out of the pump oil. A CDA or dry nitrogen purge would be best, if you have access to either one.

We also had an RIE (reactive ion etcher) that used a turbo pump + mechanical pump to evacuate the etch chamber. Most who used it would leave the thing at atmospheric pressure after they were done. Very few understood (or perhaps cared) that the next user would have to wait much longer to get the chamber down to operating pressure as the water (slowly) desorbed from the chamber walls.

Mark

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Re: Foreline pump base pressure degraded

Post by ian_krase » Mon Jun 04, 2018 8:10 am

Oh boy. I've been letting my whole system up to atmosphere this whole time.

New oil it is.

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Richard Hull
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Re: Foreline pump base pressure degraded

Post by Richard Hull » Mon Jun 04, 2018 9:14 pm

Your whole system??? I thought this was just a blank off test. You can let a pump, by itself, up to air on a regular basis. When you do this, it is wise to cap off the inlet so constant exposure to atmosphere is obviated.

Entire systems, plumbing, diff pumps, fusor chamber, etc., are best left under a valved off vacuum and never let up to full atmosphere unless repairs of major components are needed.

Letting a mechanical fore pump up to air is normal provided its inlet is then sealed off from contact with outside air over periods of time.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Tom McCarthy
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Re: Foreline pump base pressure degraded

Post by Tom McCarthy » Tue Jun 05, 2018 9:54 am

The first part of your message gave me a shock Richard, relieved to see the caveat for forepumps.

My standard procedure when finished pumping and using the system is to first turn off the turbo pump. I leave the mechanical forepump running. After an hour or so I look for a noticeable rise in pressure as a proxy for turbo slowdown. Then I close the valve between rotary pump inlet and turbo outlet and turn off the turbo pump. Finally I undo the KF connection on the forepump inlet and bring the inlet up to air, then re-clamping the connection.

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