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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Has anyone made one out of thin aluminum or stainless steel tubing? I have a vacuum pump and I was thinking...
I could take two slightly different sized aluminum tubes with both ends sealed. I could put on a valve and pull a vacuum. This would require TIG welding and some lathe/mill work.
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
Go for it. It certainly doesn't seem impossible. But I suppose it depends on the quality of dewar you want/need in terms of holding time. Personally, my two two main concerns are:
1) The top connection from the inner chamber to outer shell. This needs to be designed in such a way that insulates but can also withstand the extreme temperature gradient and thermal shock if you are going to pour LN2 or liquid air from it. I've only worked closely with one particular dewar, and it was a dry vapor shipper type. It had some type of composite (i.e. fiberglass/phenolic) tube, which I assume was a very good insulator, between the inner and outer shells. See pic below for cutaway of almost the exact model. Note the very long neck. Welded stainless would be my choice for a homemade one.
2) The quality of vacuum required. You need a surprisingly high vacuum to make an efficient dewar. According to this , 1 micron (10^-3 torr) or lower is ideal. Not sure what your vacuum setup is, but this will be pretty difficult without a diffusion or turbo pump. Maintaining it for a long period of time in a sealed container will be even more difficult. I wonder if commercial dewars use some type of getter. Of course you could always just vacuum it out every so often if you feel that its losing its insulating abilities.
*in this type, LN2 is held in an absorbent material in order to keep some biological specimen cool while remaining spill-proof.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Commercial dewars "cheat" on the vacuum. They have some charcoal held in contact with the inner vessel so that when it has liquid nitrogen in the charcoal is kept very cold. It then acts as a getter.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Bored Chemist wrote ...
Commercial dewars "cheat" on the vacuum. They have some charcoal held in contact with the inner vessel so that when it has liquid nitrogen in the charcoal is kept very cold. It then acts as a getter.
Interesting trick. How much does this bring the vacuum down the unit is in use? Does it bring it down from what a roughing pump can do (5-20 mm-hg) to a much harder vacuum? or does it go from something already pretty good, to even better?
edit: did some reading, looks like single digit torr-militorr down to a harder vacuum
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Bored Chemist wrote ...
Commercial dewars "cheat" on the vacuum. They have some charcoal held in contact with the inner vessel so that when it has liquid nitrogen in the charcoal is kept very cold. It then acts as a getter.
This is interesting. Do you mean that the cold charcoal absorbs the residual gas in the insulating chamber? Would you elaborate on your comment if I am wrong.
edit: I read the link. So, how many dewars use this method vs a pure vacuum?
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making a vacuum dewar
