Chamber contamination by vacuum grease

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Yang Minju
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Chamber contamination by vacuum grease

Post by Yang Minju » Thu Aug 23, 2018 1:37 pm

Hello.
I started my first chamber yesterday. But the vacuum didn't drop below 15,000 microns, and I used IPA and find the leak at the diff pump. But I tried using a lot of Dow corning vacuum grease to stop the leak because leak looked small. The pressure was reduced to 600 microns as soon as, but soon the pressure was increased to 4000 microns. Then I use the IPA back in the same place and used a vacuum grease again, but the vacuum kept going up and stopped at 6,000 microns. I think this is because the vacuum grease is backflowing, can I recover the vacuum by blocking the leak point with a hysol 1c vacuum epoxy, cleaning the chamber with mineral spirits and then baking it in the oven? Also, will vacuum grease be removed if we immerse the chamber in mineral spirits? I don't know what to do.... Thank you.
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Roberto Ferrari
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Re: Chamber contamination by vacuum grease

Post by Roberto Ferrari » Thu Aug 23, 2018 1:55 pm

Hi Yang
Vacuum problems must be solved, not disguised.
Vacuum grease must be used in small quantity, just to lubricate a surface, not as a plug.
HiSol is a very good sealant, apt for hi vac use, but be sure where do you plan to apply it.
Many leaks can be solved cleaning, adjusting, polishing scratched sealing surfaces or replacing a damaged part.
That o-ring looks badly dented. Replace it.
Vacuum grease may be cleaned with acetone. Do not bake a greased part or system.
Good luck!
Roberto

John Futter
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Re: Chamber contamination by vacuum grease

Post by John Futter » Thu Aug 23, 2018 6:59 pm

That O-ring is your problem
O-rings must be smooth and round in crossection any deviation from this will cause a leak.
Only apply vacuum grease to O-ring and then only enough to make the O-ring look wet

Yang Minju
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Re: Chamber contamination by vacuum grease

Post by Yang Minju » Fri Aug 24, 2018 6:27 am

Thank you for your advices!
Unfortunately, I don't have an extra Viton O-ring, but I don't think I'll handle this part again, so I could seal it with a hysol 1c. Also, Dow Corning Vacuum Greece have low vapor pressure, so i think it does not seem to affect the chamber if contaminated.
First of all, I will buy the NW40 blank and connect directly to the rotary pump to test the level of contamination and gas emissions in the chamber. From now on, I am afraid of using vacuum grease....

Roberto Ferrari
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Re: Chamber contamination by vacuum grease

Post by Roberto Ferrari » Fri Aug 24, 2018 1:16 pm

Yang
Doesn’t seem a good strategy to impair the original design of the pump.
Give me the o-ring size and a postal address and I will send you some new ones.
Please send a drawing of your setup.
In order to deal with high vacuum issues you need to be prepared.
Minimum o-ring stock, spare flanges and blank ones, gauges, valves, spare oils and yes, of course, HySol…
You will learn that after real leaks then come virtual leaks. To control your vacuum system is similar to tame a horse. Time and effort.
Roberto

Yang Minju
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Re: Chamber contamination by vacuum grease

Post by Yang Minju » Fri Aug 24, 2018 2:21 pm

Sadly, I have already sealed a leak point using hysol...I'm sorry. I heartily Thank you for your consideration.
The next day, when the hysol is fully cured, it will connect to the chamber again and test the vacuum.
This time, I hope the chamber will go below 50 microns without any leaks.
By the way, I used a vacuum grease to seal the scratched ISO connection on diffusion pump before, is it fine? I am worried about this...Thanks.
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Roberto Ferrari
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Re: Chamber contamination by vacuum grease

Post by Roberto Ferrari » Fri Aug 24, 2018 2:57 pm

I see. You are welcome.
Send a sketch of your setup.
Best way to deal with scratches is to polish the surface, recovering flatness.
Depending how bad is the scratch, grease and o-ring may solve the leak.
What flange is the one scratched? The large one or the small one?

Yang Minju
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Re: Chamber contamination by vacuum grease

Post by Yang Minju » Fri Aug 24, 2018 4:30 pm

The scratch is large enough to be seen and touched. O-ring and vacuum grease seem to be keeping the vacuum well, considering that no problem when IPA was sprayed. The scratch is located above the ISO63 flange.
I will be able to reply after 20 hours. Thank you very much for your help!
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Roberto Ferrari
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Re: Chamber contamination by vacuum grease

Post by Roberto Ferrari » Sat Aug 25, 2018 12:30 pm

Always is useful to have a valve between diff pump and chamber.
Helpful for diagnostics and to keep a finer flow control.

The scratch is in the sensitive side. You will need to monitor it.

The whole setup is good. A gauge between diff pump and mechanical pump is very important to diagnostic purposes.

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Dennis P Brown
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Re: Chamber contamination by vacuum grease

Post by Dennis P Brown » Sat Aug 25, 2018 8:45 pm

A good design overall; however, I would suggest an anti-oil vapor back-streaming device on the roughing pump between it and the diffusion pump. Not too difficult to make one using fine Stainless Steel mesh packed in a small housing. This prevents the roughing pump oil from contaminating the DP or the chamber (especially, if you have no valve.) Also, it is important that the deuterium gas is very dry - so that section of your design needs to be large enough to handle the water vapor and also, be accessible for replacement as needed. That, ultra dry deuterium, as I understand, is an important consideration for successful fusion. Successful fusion operation really does require a throttling valve. Yes, leaking D2 in appears to work but it has two major issues: first, one tends to use far more gas than required; and second, controlling fusor vacuum pressure is far easier, in my experience.

I should add, once the DP oil is hot, letting air into the system isn't possible. As such, the roughing pump must then run until the oil cools before you can break vacuum to prevent back flow from the roughing pump. Also, one cannot then keep the fusor under vacuum. So, a valve between the roughing pump and DP and/or a vapor trap is a useful addition for that reason.

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