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Registered Member #3304
Joined: Mon Oct 11 2010, 04:18AM
Location:
Posts: 9
This is a proposal for a real cheap way to get a vacuum of 1 micron or better with a simple method. It would be interesting to see if a water aspirator pump could be used inline with a dry ice cold trap on a small chamber. If the chamber were about the size of a canning jar it would pump down to about 30 mm in a reasonable time. Most, if not all, of this base pressure would be H2O vapor. Now, if the pump is valved off and the dry ice trap is opened, the pressure should drop to the vapor pressure of H2O @ dry ice temperature which is below 1 micron.
The system would have to be tight because any air leak would not be pumped on. The trap could have some molecular sieve in it for greatly added pumping capacity. It might work even better if a very small piece of dry ice was placed into the chamber before aspirating. I can think of variations of this setup where a diaphragm pump might replace the aspirator.
This is only a thought experiment right now, but maybe someone will beat me to the testing part.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
It's an interesting concept and could probably work to an extent. One possible problem though is that the water will have all sorts of gases dissolved in it which will outgas from the water aspirator pump limiting the ultimate amount of gases you can remove from the chamber. The dry ice cold trap can condense water, but most of the gas in your chamber are nitrogen and oxygen, neither of which will be condensed by dry ice, so your vacuum would not get all the way to 1 micron, but it would be closer than the aspirator pump by itself.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
It doesn't sound too unreasonable.
I recall seeing somewhere once, a "pump" that comprised of nothing more than a bottle filled with Molecular Sieve. The "Pump" was attached to a small chamber, and the bottle immersed in liquid Nitrogen.
Although nothing more than a glorified trap, it would apparently pull an x-ray vacuum in a small chamber.
Registered Member #3304
Joined: Mon Oct 11 2010, 04:18AM
Location:
Posts: 9
That was probably the article in a 1995 issue of "The Bell Jar". Dry ice was not as available then as it is now in almost every super market. Back then LN2 was available at most of the welding supply outlets, so that was the choice. I've been musing lately about what might be done at higher temperatures that border on cryogenic.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
How about flushing the apparatus with a suitable gas first, then cryo-pumping that out. Gets rid of all that pesky hard-to-condense O2 and N2.
The question of a suitable gas remains though. Each has pros and cons. Many of the ones that come to mind are corrosive
One obvious one would be water vapour. This needs a fairly high temperature to acheive a pressure approaching atmospheric, but could be used with a roughing pump at modest temperature, and cryo-pumped with CO2.
Lower boiling point alternatives are alcohol, ammonia, butane, and various CFCs. Perhaps butane is a good one, what residual vapour pressure does that have at dry ice termperatures?
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Dry Ice / Water Vapor Vacuum Pump
