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Originally Posted by UNDERBYTE
shrader valve - interesting
hard plastics may be OK, Bic lighters do pretty well with butane.
is that a shrader fill valve on the bottom of a bic?
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If those are Schrader Valves, they're not the kind I'm thinking of:
http://www.rparts.com/Catalog/Major_...s/schrader.jpg
I got a 5 pack for a project for about $4 locally.
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I don't think a schrader will hold vacuum - at least not to any greater (negative) pressure than the spring holding the valve closed - when holding positive pressure the internal pressure also holds the valve closed, so seal clamping force goes up with the pressure it's holding.
If you were thinking of something that required positive pressure to move state-change temp (like butane, maybe) then they should work fine.
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Interesting point. I've not tested the negative pressure limit, however I believe its fairly low. At least I've never seen or heard of a problem on systems running at below ambient pressure. Most likely you would not need to go that low for common coolants.
The worst case is probably water, where you'd be around 40 mmHg. Typically refrigeration systems are vacuumed at much lower pressure then this, often for hours and I've not heard of valves leaking. But I suppose its possible they leak slow enough that the vacuum pump overcomes the leak. At any rate, if I were to design something like this I'd probably use a coolant that worked at a slightly positive pressure anyway. R134a might be a good choice because its so cheap, nonflammable and comes in pure form. Pressure would be a bit high though, so you'd have to braze or at least use flare fittings on everything.