How to create high vacuum cheaply - maybe? 2

[NZBen]

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

 

[Compositepro]

Actually your concept is not bad at all. You could also use calcium oxide to absorb residual water vapor. Calcium metal on an electric filament is use as a getter to remove residual gasses in vacuum tubes. That's why they have a mirrored appearance.

 

[IRstuff]

You are sort of describing a rudimentary diffusion pump:

http://en.wikipedia.org/wiki/Diffusion_pump

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.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[rmw]

Any residual water will limit your abililty to pull down into deeper vacuums.

You will be better off if there is no water at all.

rmw

 

[ivymike]

I've been told that the same thing has been done with CO2 in the pressure vessel, and liquid N2 outside. Purge with CO2, then freeze it out.

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

 

[NZBen]

Thanks for your input so far guys, I'll give it a go and see what happens.

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?

 

[rmw]

Your pressure is limited by the vapor pressure of any moisture present. As long as there is any water available to "boil off" the pressure will determined by the vapor pressure of water at that temp. A quick check of the steam tables will reveal that the vapor pressure of water even at low pressures is higher than you can attain with vacuum equipment when there is no water present.

rmw

 

[Montemayor]

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.

 

[NZBen]

The vapor pressure graph was my inspiration, if you look at the vapor pressure chart for water at liquid nitrogen temperatures, the pressure is far lower than any standard vacuum pump could achieve. Ie may vacuum pump can go to I think 1.3e10-2Pa where as water vapor pressure is somewhere around 1e10-19Pa - thats a difference of 17 zeros! or 130 quadrillion times greater vacuum - or am I missing something here in my logic???

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??

 

[NZBen]

I meant to say water vapor pressure AT LIQUID NITROGEN TEMPERATURE is around 1e10-19Pa

 

[btrueblood]

The OP is describing a cold trap(*), although attempting to use it as a getter for water, and assuming that the water-soaked flask he is attempting to evacuate will evacuate further when a cold trap is attached. It might work in the manner described, provided (A) the line attached from the cold trap to the vacuum pump is valved closed, to prevent air from backflowing thru the vacuum pump to the device, and (B) the flask is held at high enough temperature to actually allow the water in it to boil off, and (C) the line from the flask to the cold trap doesn't develop an ice plug somewhere along its length. The effectiveness of the cold trap at removing all the water vapor from the flask will be limited by molecular flow of the water vapor (i.e. how many random bounces of the molecules will it take before they find the opening of the pipe leading to the trap). That gives a sort of decaying exponential rate of pressure decrease, and it may be many days for even low vacuum (1 to 10 torr) to be reached, dunno, it will depend on actual line sizes relative to the volume and surface area of the flask. Probably worth a try, though I'd put a good vacuum gage upstream of my final seal, as a test method for the process.

(*) see

http://en.wikipedia.org/wiki/Cold_trap

 

[btrueblood]

Oh, but - I do also agree with rmw and monte in general - adding water to a system before you try to pump it to high vacuum is usually a really dumb idea, as the water tends to freeze as you are trying to pump it out, and cause all sorts of trouble. Whenever the spacecraft guys tried to do thermal/vacuum tests of spacecraft, they would spend days, weeks even, trying to get the pressure below the vapor point of water. The stuff adsorbs/absorbs into every nook and cranny of your hardware.

 

[IRstuff]

A cold trap coupled with a high-end roughing pump can serve as a high vacuum pump all by itself, provided that the cold trap is cooled by sufficiently cold fluids like nitrogens, argon, or even helium. This is the basis of the cryopump:

http://en.wikipedia.org/wiki/Cryopump

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize