To wet, or not to wet, that is the question.

How to get to vacuum, what the classes are, and what is needed for what job.

To wet, or not to wet, that is the question.

Postby chrismb » Sun Jan 16, 2011 5:56 pm

I've been doing various adjustments and trials with my new chamber setup, which I'll pin up once I've settled on the final arrangement, and have configured circuitous venting in front of and behind my flow restriction. This has added many bits of additional plumbing to an already 'well-holed' set-up.

In total I now have 22 rubber seals [directly exposed to the chamber vacuum], it must be some 150-200 cm worth of rubber seal length! (Mostly buna, with a few viton)

The point of this post is to explain that I did not wet any of the seals. 'Common sense' tends to suggest one might want to add some sort of substance to aid the rubber-surface sealing, usually vac oil, and many suggest to do just that. However, as I was just experimenting and have not finalised a set-up, I did not bother to do so.

But it turns out that the chamber seems far better sealed than I might have expected. The pressure comes up so slowly from e-5's torr that I am getting bored waiting, when I want to run the next flow-rate test starting at a higher pressure! Maybe 1 micron in 5 mins, poss longer! I did not expect such good sealing from so many seals, and the difference to past installations is that all the seals have gone in bone dry, straight from an IPA wipe-over.

Does anyone else have any 'wet-versus-dry seals' anecdotes? I guess most go for CF-copper gaskets so may not have so much experience at the 'softer end' of kit build that I am at, but I would like to get a feel for whether mounting wet or dry seals have any other pros or cons people have identified that I've not thought of... Basically, once finalised, if I have to go back and refit them all, for a reason I've not yet thought of, there are so many that it might take hours!
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Re: To wet, or not to wet, that is the question.

Postby Joe Jarski » Sun Jan 16, 2011 6:10 pm

My sputtering chamber has over 200cm of viton seals in it also, all installed dry and it seems to seal up fine. Through my reading, I came to the conclusion that there are people on both sides of the fence and they're equally adamant about how it should be done, so it's mostly personal preference I think. If I'm trying to seal up a scarred surface or something, then a light coating of grease may be required, but other than that I prefer to have them clean.
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Re: To wet, or not to wet, that is the question.

Postby Jerry » Sun Jan 16, 2011 7:52 pm

I use krytox LVP. Good from 0 F to 570F and is totally inert.
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Re: To wet, or not to wet, that is the question.

Postby Doug Coulter » Sun Jan 16, 2011 10:22 pm

When you can, I think it's better to just fix the scarred thing -- buff it. But when you can't, I use a mix of about 80% beeswax and 20% forepump oil, which is pretty thick stuff (barely spreadable at room temp). Probably not as good as Jerry's suggestion, but it gets the job done. You wipe off as much as you can after applying it. Only takes a tiny amount to get the job done. More just makes other problems.

What I've found is that viton will seal over wavy stuff fine -- it's the "shiny or not" that really matters, and whether any micro-scratches are across the seal, vs along it. Buffing cures almost anything, even if you leave a little "wavy" on it doing that. Get it shiny, you're done, no need for anything else. But there are cases where you can't get in there to buff as well as you might want to.

I don't believe that any of these wetting agents help the problem that water diffuses through viton....I'd love to be corrected on that one. So far, water is still the enemy.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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