rubber/vinyl gaskets - ?

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ReeceVance
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rubber/vinyl gaskets - ?

Post by ReeceVance »

I've run into a snag and I'm hoping it's something I can remedy with reasonable effort. I've been using rubber gaskets to seal my main chamber but my vacuum won't drop low enough to kick the plasma to the center of my inner grid. I'm wondering if anyone has had trouble maintaining vacuum with rubber gaskets or if the issue is just with my pump.
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Brian McDermott
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Re: rubber/vinyl gaskets - ?

Post by Brian McDermott »

Can the plasma "see" the gasket? If so, it may be causing it to outgas and ruin your vacuum. If it's something like a KF/QF or ISO flange, which use metal centering rings that tend to shield the rubber, this generally isn't an issue, but if you're using a rubber gasket on CF, ASA or custom-made flanges, the plasma may be bombarding and vaporizing the rubber.
Tidbit77
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Re: rubber/vinyl gaskets - ?

Post by Tidbit77 »

Viton and buna-n work fine for me.
For my demo fusor's bell jar gasket, I just took a sheet of viton, cut it with an x-acto knife, and applied a small amount of vacuum grease to either side, and put it between a VERY flat aluminum plate and my bell jar. This didn't leak a bit.

In order to pinpoint your problem, please provide a more accurate description of your setup.
What are the gaskets made of? What are they sealing to? What type of chamber/pump? What pressures are you achieving? What else is a candidate for leaking/major outgassing?

-Will Jack
Richard Hester
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Re: rubber/vinyl gaskets - ?

Post by Richard Hester »

Vinyl will outgas like crazy - it needs a lot of additives to be flexible. Vacuum gasket meterial is made using buna, viton, and silicone, all of which are much better suited to vacuum - the last two are preferred.
Dan Tibbets
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Re: rubber/vinyl gaskets - ?

Post by Dan Tibbets »

Other than out gassing, shape may also be important. Depending on the precision of the mating surfaces, a flat gasket may not seal well unless one of the metal surfaces has a knife edge protrusion. I've found that round gaskets work better (O-rings) in most of my cobbled assemblages. The exception is glass jars. Of course they have a dull knife edge due to the lip of the glass. Having said that, the thick flat gasket in my pressure cooker works fairly well. There is plenty of pressure to push down the lid (60 square inches, or ~ 900 lbs of force. In a smaller chamber, the bolts(if used) need to apply adequate compression without distorting the shape. This is why thick collars with many screws are often used. A rounded o-ring or knife edge will concentrate the pressure to a small area. A simple 1.5 inch wide o-ring held in place by atmospheric pressure on the glass window seems to work well, at least for demo fusor work. A 1 inch o-ring doesn't seem to work as well (~ 30 lbs of compression vs ~ 15 lbs of compression).

Dan Tibbets
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