Poorly Performing Pump Advice

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Maxwell_Epstein
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Poorly Performing Pump Advice

Post by Maxwell_Epstein »

Hello All,

I am finally very close to replacing the cheap HVAC pump that I got at the start of my fusor project. This pump has unexpectedly lost vacuum, ejected oil into my chamber, leaks oil at a high rate, can only pump down to 50 microns, and has otherwise been a nuisance and a limiting factor in this project. I somewhat recently bought a Precision D-25 belt driven vacuum pump from a decent looking eBay listing to replace the old one and have had the new pump for about a month.

Over Christmas break, I flushed the pump thoroughly with LDS-FF-G flushing fluid until the oil drained completely clear, and refilled it with LDS-19U-G vacuum oil. After this, and despite the pump being theoretically capable of pumping below 1 micron (and other fusor.net members pumping as low as 8 microns with a similar model), my pump "bottoms out" at around 30 microns after 30 minutes of pumping (it takes less than one minute to pump below 100 microns, though). For this "bulkhead test," I attached a rubber hose to the barbed connector that the pump came with (I believe it to be Precision's standard barb connector) and a barbed kf-25 flange on the other end which I connected by TC gauge to.

The pump appears to be rather old (or at least poorly maintained), and there is some residue on the internals visible through the sight glass, but the belt wheel turns fine and it makes no gurgling (or other) noise when I run it.

What should my next steps be from here? Is there something I should try to improve performance, or is this lack thereof expected in older second-hand pumps? Assuming that it can only pump to 30 microns, what sort of secondary pumps should I consider to accommodate this in a fusion setup (I assume a turbo)? Would an oil trap improve my performance, and is that sort of thing worth it in my case? As you might be able to tell, I have very little experience with this sort of thing, so I would greatly appreciate any advice that you can provide

Thank you!
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Liam David
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Re: Poorly Performing Pump Advice

Post by Liam David »

I would first check the rubber hose and associated connections. Make sure there are no scratches on the barb, and that it is properly attached to the pump (I'd remove and re-teflon the threads if it has such a fitting). Ideally, you'd adapt the pump threads directly to KF25 and get rid of the hose entirely. There are a number of such cheap fittings on Ebay.

Assuming all is good there, do you know your gauge is accurate? For example, I have a (fully functional) convection gauge that reads ~13 microns high when the chamber pressure is actually ~3 microns but reads accurately at higher pressures.

Third, 30 microns is not unreasonable for an older pump, especially one that has not been properly maintained as you suggest. That pressure is plenty low enough for both a turbo and diff pump. It's about what my old refrigeration pump could hit, and it was in service until only recently backing two turbos. It will not prevent you from doing fusion. Sure, pumping to sub-10 microns would be nice, but it is far from necessary. But first, make sure both my previous points aren't interfering.
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Richard Hull
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Re: Poorly Performing Pump Advice

Post by Richard Hull »

I assume you put suitable tight clamps on those barb hose connections and that you did not rely on the barbs to seal the two connections. Unclamped barbs are fine for class room demos, but not for letting the pump go as deep as it can.

The pump is doing OK, but one would like to see one go to 20 microns or better at the head. What is the brand and model? Photo of your hookup to the gauge? Picture is worth a thousand words.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Maxwell_Epstein
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Maxwell_Epstein

Post by Maxwell_Epstein »

Thank you very much for your advice. I have attached pictures of the pump and connections.

I bought my Varian 531 TC gauge and controller from a post on the items for sale forum of this site and the seller told me that he had calibrated the controller against another gauge (I forget the type and the post has been removed). The gauge reads within the atmospheric range when it is in air and my plasma results in my chamber look similar to the plasma pictorial in the vacuum faq, but beyond that I'm not sure if the gauge is perfectly accurate. Is there a way that I could test my gauge's accuracy without simply buying a known gauge and comparing the two?

Thank you again,
Max E.
Attachments
The threaded connection coming out of the pump that the barb screws on to. I have not tried using teflon tape on these threads, but will add it to see if it has an effect
The threaded connection coming out of the pump that the barb screws on to. I have not tried using teflon tape on these threads, but will add it to see if it has an effect
I bought my TC gauge embedded in a KF-25 flange that I attached to the kf-25 to barbed adapter that connects to the hose.
I bought my TC gauge embedded in a KF-25 flange that I attached to the kf-25 to barbed adapter that connects to the hose.
The pump came with a rubber hose from Fisher Scientific that I attached to the barb. I could not push it down all the way over the barb, but I don't think that had any effect as i did not experience better performance with the slightly wider diameter hose that I tried as well (see below)
The pump came with a rubber hose from Fisher Scientific that I attached to the barb. I could not push it down all the way over the barb, but I don't think that had any effect as i did not experience better performance with the slightly wider diameter hose that I tried as well (see below)
It is pretty difficult to see, but looking through the sight glass there is some corrosion or residual oil residue on the metal inside the pump
It is pretty difficult to see, but looking through the sight glass there is some corrosion or residual oil residue on the metal inside the pump
I have also used a 2 ft length of gum rubber hose from idealvac with a slightly larger diameter<br />https://www.idealvac.com/Pure-Gum-Rubber-Vacuum-HoseTubing-58-Inch-ID-with-38-inch-Wall-Thickness-(Sold-by-the-Foot)/pp/P101427
I have also used a 2 ft length of gum rubber hose from idealvac with a slightly larger diameter
https://www.idealvac.com/Pure-Gum-Rubber-Vacuum-HoseTubing-58-Inch-ID-with-38-inch-Wall-Thickness-(Sold-by-the-Foot)/pp/P101427
rubber hose 1.jpg
The Precision D-25 pump I am using
The Precision D-25 pump I am using
pump motor.jpg
pump label.jpg
The inside of the screw-on Precision barb
The inside of the screw-on Precision barb
It looks like the previous owner of the pump added the red wax-like substance around the bottom of the inlet, presumably to plug a leak. Should I try re-sealing the bottom with epoxy?
It looks like the previous owner of the pump added the red wax-like substance around the bottom of the inlet, presumably to plug a leak. Should I try re-sealing the bottom with epoxy?
precision barb 1.jpg
The label on the bottom of the pump with all of the model info
The label on the bottom of the pump with all of the model info
other pump label 1.jpg
The pump connection when I used the larger diameter hose
The pump connection when I used the larger diameter hose
The ballast connection. When I first was testing the pump, I could only pump down to ~200 microns and the oil made a gurgling sound. I tightened the ballast bolt with a wrench and the gurgling stopped and I could pump down to 30 microns.
The ballast connection. When I first was testing the pump, I could only pump down to ~200 microns and the oil made a gurgling sound. I tightened the ballast bolt with a wrench and the gurgling stopped and I could pump down to 30 microns.
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Richard Hull
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Re: Poorly Performing Pump Advice

Post by Richard Hull »

That screw-on barb demands a rubber ring or gasket!! If it has a rubber ring, it may be flattened due to age. Replace it and put a tiny amount of vacuum rated grease on the new ring. Give the pump a chance.



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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Maxwell_Epstein
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Re: Poorly Performing Pump Advice

Post by Maxwell_Epstein »

I think that there is a rubber gasket seated within the threaded inlet coming directly out of the pump (the black ring in the first picture). Do I need to replace it with a special high vacuum rated new gasket? If so, what source do you recommend for one? If not, can I use a standard hardware store gasket as a replacement?

Thank you,
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Richard Hull
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Re: Poorly Performing Pump Advice

Post by Richard Hull »

A hardware store gasket would be a great start. Harbor freight has three or four large boxed collections of round rubber rings. One box they sell are all viton. If you have a Harbor Freight store, go there. Viton is the best. Northern Tool might be a good stop as well. My city has both, but I prefer HF

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Poorly Performing Pump Advice

Post by Dennis P Brown »

One thing, that pump would never go below 1 micron - you mention that in your first post. Getting an old pump that has seen years of service even to 10 microns would be an achievement. The inside is likely rather dirty (still) and parts are worn in any case. Certainly keep at this to try and see if you can improve its performance - a worthwhile endeavor - but don't assume something is basically wrong (major leak) if it struggles to get below 30 microns. If the pumping speed (volume) is good most turbo's and diffusion pumps are fine with 30 microns. Just saying.
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Maxwell_Epstein
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Re: Poorly Performing Pump Advice

Post by Maxwell_Epstein »

Thank you for pointing that out, Mr. Brown. Although the name plate claims .1 microns as I noted earlier, I was only expecting around 10-20 microns at best. I should have made more clear.

I replaced the gasket on the pump inlet with one I got at a local Lowes hardware (I have very few options locally and no harbor freight). I used some vacuum grease and Teflon tape on the threads, but was unfortunately not able to pump any lower.

I am going to look for a threaded to kf adapter just to make sure that the barbed connections aren't hurting my performance. Other than that, are there other things that I should try to improve performance?

It also seems worth noting that after about one to two minutes (around when I reach 150-100 microns) my pump rate slows dramatically. I assume this will be exacerbated when I attach my entire chamber which has significantly more internal volume than the length of hose I'm using right now. Is this also something to worry about (especially before I add a secondary diff or turbo pump)?

Thank you!
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Dennis P Brown
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Re: Poorly Performing Pump Advice

Post by Dennis P Brown »

What I did with an old pump that was performing badly was open it up, and clean the insides of all the gunk (it still had a lot after a flush) and as much rust as reasonable; I then diss-assembled the main rotors and cleaned them as well (they were the real problem and partly clogged). I was super careful not to damage gaskets and keep the order of parts straight for reassembly. This did improve its performance from - junk that was worthless (really didn't produce much vacuum) to able to achieve a few tens of microns. However, this might be beyond your interest but this is how I learned to repair two stage modern pumps. Since this was a through away, failure on my part wasn't important - that is not the case with a pump that is working that you want to keep. Also, if one does decide to do this one should verify that they can obtain a rebuild kit (and the cost is reasonable) before attempting this - so if worse case of gasket/o-ring destruction the pump can still be repaired.
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Maxwell_Epstein
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Re: Poorly Performing Pump Advice

Post by Maxwell_Epstein »

As this is my only usable pump right now, I am going to refrain from disassembling it. The cost of a rebuild kit and risk that I might damage something irreplaceable is too high for me to risk it, especially without a backup, for at most 20 microns of benefit. Once I receive some more KF clamps and gaskets, I am going to attach it to my chamber (valves, tees, and other stuff have been sitting on a shelf for weeks) and see if I can pump down to a decent vacuum.

Thank you all for your guidance and advice,
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
John Futter
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Re: Poorly Performing Pump Advice

Post by John Futter »

dramatic slow down of pumping speed at around 100 microns usually suggests water in the pump oil
you get rid of this by either leaving the pump running dead headed for an extended time (days) or gas ballast the pump for 2 - 6 hours
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Maxwell_Epstein
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Re: Poorly Performing Pump Advice

Post by Maxwell_Epstein »

Mr. Futter,

Thank you very much for your advice. I ran the pump with the ballast open for five and a half hours and it made a huge difference! I was able to pump down to 30 microns in under one minute and got all the way down to 15 after four minutes of pumping. I guess I was sabotaging my pump and results with water in the pump oil and now that I have remedied that problem, it's doing much better.

Clamps to put the whole system together are arriving on Thursday and I'm very excited for that, especially since I can now pump down way into the pressure range where fusion actually occurs (albiet I can only do so in air, not deuterium).

Thank you again to everyone who gave me advice,
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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