Back-Streaming Diffusion Oil

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.
Jeremy Adams
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Back-Streaming Diffusion Oil

Post by Jeremy Adams » Fri Jun 29, 2018 3:09 am

Hello,

Today I was running my pumping system - a 2 stage roughing pump connected to an oil diffusion pump when I found that my roughing pump was actually pulling oil out of the diffusion pump through a bellows connecting them. The bellows was dripping in diffusion pump oil on its connecting end to the roughing pump - not good! How can I prevent this? And is this a normal problem with diffusion pumps connected to foreline? The bellows connecting them is about ~1" diameter and 16" long, so perhaps a longer bellows could trap some oil and less will reach the roughing pump. Also my roughing pressure was not lower than 10^-3 torr, it was around 7x10^-2 at its lowest so I don't think this was a vapor back streaming issue.

After this I did another test connecting two bellows and raised the roughing pump so it would take more work to get the diffusion pump oil up by my roughing pump. There was less oil that reached the roughing pump compared to the earlier test but some oil still made it up there. Attached is a picture of the bellows from the second test (with a paper towel jammed in it to keep oil from dripping after being disconnected from the roughing pump).
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file-2.jpeg
Diffusion Pump and Roughing 2nd Test

John Futter
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Re: Back-Streaming Diffusion Oil

Post by John Futter » Fri Jun 29, 2018 8:07 am

yes the oil will move to the roughing pump if the diff pump is not part of a system as in your photo.
you must have the diff pump sealed off at its top opening not aluminium foil.
the back ing pump needs to be pumping in the molecular flow regime not bulk flow.

Bellows tube is not my favorite for backing connections I use milking rubber as used on dairy farms
very cheap, vacuum tight, and gives better vibration isolation and far higher pumping speed due to lower surface area.
do not try what you have done until your system is all together.
PS it seems your are not using DC704 or DC705 as these are extremely viscous at room temp and would not move as you say it has

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Richard Hull
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Re: Back-Streaming Diffusion Oil

Post by Richard Hull » Fri Jun 29, 2018 8:15 am

I can't get but so good a look at every thing, but how much Diff pump oil do you have in you pump? I hope it wasn't about the diff pump exhaust hole. If all was good there, what was the pressure at the throat of the diff pump or your fusor chamber pressure? I worry that you have a leak somewhere in the diff pump system and the oil is boiling off and not condensing on its way to the boiler allowing the oil to be pumped by the mechanical pump.

I attach some image taken back in 2003 as I assembled fusor IV's station. I checked my mechanical pump, then I checked my sealed of diff pump before mounting the chamber. I am still assembling this post

Richard Hull
Attachments
IV const 04 (4).JPG
mechanical pump tested with rubber line 9 microns (ok)
IV const 04 (17).JPG
System with diff pump in place with discharge gauge connected
IV const 04 (20).JPG
meter on discharge gauge shows diff pump is working (ok)
IV dark 2011.JPG
Finished system doing fusion
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.

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Dennis P Brown
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Re: Back-Streaming Diffusion Oil

Post by Dennis P Brown » Fri Jun 29, 2018 10:47 am

Besides what others have suggested, was the DP cooling water flowing and flowing fast enough?

Jeremy Adams
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Re: Back-Streaming Diffusion Oil

Post by Jeremy Adams » Fri Jun 29, 2018 4:44 pm

The roughing pump is not connected to the diffusion pump in the picture above because it was taken after I disconnected the bellows and noticed oil flowing backwards to the roughing pump. Also aluminum at the top of the diffusion pump is around a blank flange, just there to keep it clean. The diffusion pump was not on when I did this test, so perhaps heating up the oil some before roughing will help because the oil can be low viscosity at room temperature.

Jeremy Adams
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Re: Back-Streaming Diffusion Oil

Post by Jeremy Adams » Fri Jun 29, 2018 4:59 pm

Oil was being sucked out from the diff pump exhaust hole (the diffusion pump outlet) which was connected to the roughing pump. Perhaps we need a forline exhaust baffle? The pressure of the roughing pump was about 7x10^-2 torr at its lowest.

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Dennis P Brown
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Re: Back-Streaming Diffusion Oil

Post by Dennis P Brown » Sat Jun 30, 2018 6:15 pm

You found oil in the roughing pump line and the diffusion pump was never even heated up? Either the oil was far too high in the DP (as someone suggested) or else you have some water or other high vapor pressure substance that might have splattered oil up into the fore-line when you pulled vacuum on the DP. While my DP has a small baffle in its fore-line, even without that device, no significant oil would enter the fore-line just by operating the fore pump even if the oil is cold or hot.

Also, 70 microns isn't a very good vacuum for a fore pump; again, maybe something is in the DP oil. Have you tested the fore pump alone? It should get under 10 microns or so.

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Re: Back-Streaming Diffusion Oil

Post by John Futter » Sat Jun 30, 2018 7:23 pm

What I do not understand is
low viscosity
cold diff pump oil is like treacle
you did not say which oil you have DC704 or DC 705 I doubt you have Santovac due to cost.
Diff pump oil charge is critical if it says 75mls then 75 mls it is, it does not work better with 100mls in fact it will tend to burp in operation,
I would remove all the oil from the diff pump by turning the heater on in air for a couple of minutes to get it to flow and pour it into a measureing container. Then clean the diff pump (search this forum there is plenty on how to) and put the correct amount of good silicon diff pump fluid back.
you need to heat the oil to get it to flow back into the pump.
hopefully you are not using octoil or similar in the pump as diff stacks are fussy about the working fluid, diff pumps are a little more forgiving

Jeremy Adams
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Re: Back-Streaming Diffusion Oil

Post by Jeremy Adams » Sat Jun 30, 2018 7:32 pm

I am using ECO-704 Silicone Diffusion pump oil (which is an improved formula of DC-704). This fluid is pretty viscous but I should probably have a baffle on my diffusion pump exhaust just in case there is any back flow.

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Richard Hull
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Re: Back-Streaming Diffusion Oil

Post by Richard Hull » Sun Jul 01, 2018 4:43 am

There is something very wrong here. Diff pump oil never backstreams in a properly assembled and run system. This includes how to properly start and stop a system. It is more likely in mismanaged systems to have mechanical pump oil find its way into the diff pump

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.

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