How to create high vacuum cheaply - maybe?
How to create high vacuum cheaply - maybe?
(OP)
I'm wanting to generate a vacuum for purpose of making an LN2 dewar. I'd rather not buy a turbo molecular pump or diffusion pump if I dont have to. I've mentioned the idea to a couple of people who've thought it's dumb and wont work, but thought I'd try and get a vote here.
I have a 2 stage vane roughing pump that can go down a wee way (maybe 0.1mbar) - but I'd like to pump out as much as possible.
Firstly, the idea goes against all morals and virtue that a vacuum enthusiast has. Put a couple of grams of water into the flask! I can see them squirming on their seat now.
Then attach a vacuum pump to the tube attached to the flask and pump it down boiling out the water, this should purge the system of other gasses - ie nitrogen, oxy, argon etc. So by the time this is done you should have a flask that's at a pressure of maybe 0.1mBar of water vapor. Seal off the vacuum pump to the flask, leaving the glass tube a reasonable length.
Now dip the glass tube into LN2, this should drop the water vapor pressure quite low (around 10e-23mbar) leave for a couple of minutes for vapor molecules to collect in cold spot and seal the tube between the liquid nitrogen and flask.
Would love to hear feedback on the idea, I've not found anything like it after loads of searching on the net so I assume because it's so simple and not done it prolly wont work - but there is always that chance in a hundred that it will.
Thanks
Ben
I have a 2 stage vane roughing pump that can go down a wee way (maybe 0.1mbar) - but I'd like to pump out as much as possible.
Firstly, the idea goes against all morals and virtue that a vacuum enthusiast has. Put a couple of grams of water into the flask! I can see them squirming on their seat now.
Then attach a vacuum pump to the tube attached to the flask and pump it down boiling out the water, this should purge the system of other gasses - ie nitrogen, oxy, argon etc. So by the time this is done you should have a flask that's at a pressure of maybe 0.1mBar of water vapor. Seal off the vacuum pump to the flask, leaving the glass tube a reasonable length.
Now dip the glass tube into LN2, this should drop the water vapor pressure quite low (around 10e-23mbar) leave for a couple of minutes for vapor molecules to collect in cold spot and seal the tube between the liquid nitrogen and flask.
Would love to hear feedback on the idea, I've not found anything like it after loads of searching on the net so I assume because it's so simple and not done it prolly wont work - but there is always that chance in a hundred that it will.
Thanks
Ben





RE: How to create high vacuum cheaply - maybe?
RE: How to create high vacuum cheaply - maybe?
You would heat up the flask to as hot as you can get it, and supercool the pump transfer line to condense out the gas molecules leaving the flask. Seal off and let cool. If you want to get fancy, you can put angled vanes in the transfer line to trap molecules attempting to move upstream.
Additionally, you can add getters: http://en.wikipedia.org/wiki/Getter that you can fire after sealing to adsorb a few more molecules.
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RE: How to create high vacuum cheaply - maybe?
You will be better off if there is no water at all.
rmw
RE: How to create high vacuum cheaply - maybe?
I believe there was a scientific american article sometime in the 90s describing a method like yours (amateur scientist column).
http://www.belljar.net/amsci.htm
RE: How to create high vacuum cheaply - maybe?
rmw, am wondering how will the water limit ability to pull down into deeper vacuums? The water vapor is what's going to be 'producing' the vacuum?
RE: How to create high vacuum cheaply - maybe?
rmw
RE: How to create high vacuum cheaply - maybe?
Ben:
rmw is exactly correct. Don't think that you can pull an "end run" or "statue-of-liberty" play on mother Nature and - Physics in general. The basic truths always apply: a vacuum is the absence of matter. And while you have matter in the annulus, you will get a vapor pressure from it.
Just because you condense the vapor in the annulus (water, in this case) and it reverts to a liquid state doesn't mean that it doesn't exert a vapor pressure. Everything in Nature has a vapor pressure – even steel. Some substances may have negligible vapor pressure – or un-measureable vapor pressure – but basically molecular movement is trying to release molecules from their base phase at the temperature it finds it finds itself in. Water – or ice – will exert a vapor pressure and that will defeat the possibility of a vacuum ever being achieved in the annulus. Just refer to the steam tables (or a database) as rmw suggests.
Also, try it with CO2 (or Dry Ice) - the vapor pressure will be measurable. Even solids - like Dry Ice - sublime and exert a vapor pressure.
RE: How to create high vacuum cheaply - maybe?
I do understand that everything has a vapor pressure, but my thoughts were if the vapor pressure of the substance is less than the pressure my vacuum pump can pump, then it's gotta be better??
RE: How to create high vacuum cheaply - maybe?
RE: How to create high vacuum cheaply - maybe?
(*) see http://en.wikipedia.org/wiki/Cold_trap
RE: How to create high vacuum cheaply - maybe?
RE: How to create high vacuum cheaply - maybe?
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