[DMCForum] Re: PCV Routes Manifold Vacuum Through The Crankcase

["checksix3" <jetjock11@xxxxxxxx>]

> We manufacture superconducting magnets and the common method of 
> operating them is in a bath of liquid helium.<

Yes, I'm familiar with that. The Holy Grail of superconductors is the 
temp problem.
 
> The cryopumping you mention can cause us problems with helium 
> diffusion through the welds in the cryogenic vessel. This degrades 
> degrades the vacuum and the vessel will no longer keep the helium 
in > a liquid state. These "cold leaks" are difficult to avoid and it 
> takes a very specialized welder to construct these vessels.<

Where are you getting He from? The helium is contained within the 
cryopump and is not exposed to the chamber. Also, helium can't be 
cryopump...like H2 it's a non-condesable and must be absorded.
I guess I don't follow your equipment setup. I've welded dozens
of 316 SS chambers and a few Ti ones and never had a problem but He 
is a tiny bugger a will permeate like crazy thru a weld occulsion, as 
it will thru many other materials. It's why it's difficult to use 
elastomers for sealing below 10E8 Torr or so.

> Dilution refrigerators using Helium-3 can get down to 100 milli-
> Kelvin or less. This is in a very small sample space and the time 
at the lowest temperatures is somewhat short - measured in minutes.

100 milli-kelvin? Thats impressive...darn close to absolute.

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