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How do you make a near-perfect vacuum?

  1. Dec 17, 2008 #1
    I'm just curious on how a near-perfect vacuum is created.
     
  2. jcsd
  3. Dec 17, 2008 #2

    Vanadium 50

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    Pumps. Lots of pumps, of various kinds. Sorry to be so generic, but it's hard to be specific with a question that's so general. Maybe if you would tell us what kind of answer you are looking for we could craft an answer that suits you.
     
  4. Dec 17, 2008 #3
    How about.. How much (-) pressure would be required to completely evaccuate a volume of one liter?
     
  5. Dec 17, 2008 #4

    mgb_phys

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    nuby - it doesn't really work like that.
    The air will only flow from higher pressure to low (a bit like heat going hot->cool)
    The size of the pump depends on the size of the vessel to be emptied and how quickly.
    The type of pump sets the final pressure reached.
     
    Last edited: Dec 17, 2008
  6. Dec 17, 2008 #5

    rcgldr

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    Powerful pumps, and for extreme vacuum, metal surfaces nearly free of any gases, and very low temperatures to reduce the vaporization of any gases left in the walls of a chamber. From what I've read, the extreme chambers get down to around 1 molecule / cm^3.
     
  7. Dec 18, 2008 #6
    After lots of pumps, you can use appropriate absorbents to absorb all (nearly) remaining gas atoms.
     
  8. Dec 18, 2008 #7

    Ranger Mike

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    once yo uhave the air tight vessel constructed and before evacuating its content, would it make sense to pump in Nitrogen first?
    This will push out any air, thus moisture.
     
  9. Dec 18, 2008 #8
    One professor I talked to about his experiment which also uses a vacuum said they cool the apparatus (or whatever they need to be a vacuum) down until all the air liquifies and then they just pour it out. Of course that is pretty crude, but it seems fairly decent.
     
  10. Dec 18, 2008 #9
    Do you know where I can buy a pump? And if not, how to make one?
     
  11. Dec 18, 2008 #10

    ZapperZ

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    A turbo pump costs around... what... $10,000? An ion pump, without the controller, might be a bit cheaper. A cryopump costs around... $30,000 with the compressor. If you have that much money, can I be your friend?

    And we haven't even gone into the type of vacuum chamber, valves, flanges, gauges, etc.

    Zz.
     
  12. Dec 18, 2008 #11

    Ranger Mike

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    assuming you want to experiment with vacuums and the like, a compressor from a refrigerator will generate limited vacuum..cheap. we use one to suck air out of the tires before replacing with nitrogen
     
  13. Dec 18, 2008 #12

    fluidistic

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    Wow... why are they so expensive? Is it because they are big devices?
     
  14. Dec 18, 2008 #13

    mgb_phys

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    A turbo pump is only about the size of a coke can but has about a dozen sets of compressor fans (like a mini jet engine) but it has to spin at 50-100,000RPM without destroying itself - tricky engineering.

    It's not generally worth flushing with Nitrogen, the water in the air will pump out very easily. The problem is water absorbed onto surfaces which the N2 isn't going to help.
    Usually you fit electrical heater pads to the walls of the dewar and heat it to 50-100degC while pumping to out gas any water or contamination.

    To get high or ultra high vac on a large chamber, the easiest way is often a cryopump.
    You take a very large surface area such as a long roll of al foil or a big box of activated charcoal/pourous ceramic and cool it to liquid nitrogen. The air molecules lose energy and stick to thec cold surface. You get an enourmous pumping rate but it takes a while to heat it to boil them all off again and pump them out.
     
    Last edited: Dec 18, 2008
  15. Dec 18, 2008 #14
    thank you very much... all of you. :)
     
  16. Dec 18, 2008 #15
    For what kind of pumps you should use, you first have to identify how high vacuum you need (E-5, -6 or -9 etc..), and does it have the intake (how much). How big the manifold is also very important.
     
  17. Dec 19, 2008 #16
    Well... I want a vacuum for a cathode ray experiment, and it's about a foot long and 3 inches in radius.
     
  18. Dec 20, 2008 #17
    Dont forget that even the solid material from which the vacuum chamber is made from evaporates and more so as the pressure inside falls.
     
  19. Dec 20, 2008 #18

    ZapperZ

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    You should have mentioned this before.

    You don't need "near-perfect vacuum" for something like that. The CRT tubes that we all use barely get to 10^-5 Torr. Any good mechanical or scroll pumps will do, but this also costs money. You will need to consider what kinds of connectors/valves/seals you can use.

    Zz.
     
  20. Dec 20, 2008 #19

    OmCheeto

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    I can think of three simple ways to evacuate the tube:

    1. Build a Toepler pump. Works because a column of mercury will draw a vacuum at around 30"
    2. Build a steam generator/condenser. A mason jar with some extra piping and valves might work. Fill the jar about 1/4 full of water. Just like when canning vegetables, boil the water until most all of the air is evacuated, then let the sealed jar cool down. It should draw a nice vacuum. ~29". You may have to perform this several times to remove all of the air from your cathode ray tube.
    3. Call one of your shuttle rider friends and have them take your tube for a space walk. Open and then close a valve connected to the tube. Only 2.5 H2 atoms per cm3.

    Warning. Performed improperly, these methods may result in poisoning, death, dismemberment, or putting one of your eyes out.
     
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