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Create a vacuum by expansion

  1. Feb 12, 2014 #1
    I want to create a mechanical vacuum (semi not necessarily complete), I've heard the easiest way to create one is by expansion (basically expanded a pump). But is this also the easiest way physically, as in takes the least amount of energy? Or is there any other 'mechanical' way possible for this (excluding the use of external electricity or chemicals). Any pointers would be appreciated.
  2. jcsd
  3. Feb 12, 2014 #2


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    If you have a building with four floors, or a hill, you could make a barometer with water.
    Set up a tube, with a 10 metre height difference between the ends. Place a valve at each end.
    Close the bottom valve, then fill the tube with water from above.
    Close the upper valve, and place the bottom of the tube in a bucket, (water trap).
    Open the bottom valve and allow the water level in the tube to fall.
    You now have a partial vacuum in the tube.
    The tube will contain only a small amount of water vapour.
  4. Feb 12, 2014 #3
    Use a glass bottle with a cork or other air tight seal.
    Open the bottle and heat it up.
    The cork the bottle and let it cool down.
  5. Feb 12, 2014 #4
    Thanks both. But I need this object/device to be constantly changing from an atmospheric pressure to vacuum to atmospheric pressure and so on back and forth, without any external aide except movements such as pull/push, turn. Basically a pump device would work like as syringe or something, but when using it takes up a lot of energy creating this pull action. Could anything aide to make this movement take up less physical energy, some sort of design?
  6. Feb 12, 2014 #5


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    What scale or volume are you considering?
    What partial pressure do you need?
    You must apply some external energy to overcome the thermal losses in the system.
    Where does that energy come from?
  7. Feb 12, 2014 #6
    Cheers. I'm not an engineer or anything so have tried to answer to the best of my abilities:

    What scale or volume are you considering? Very small, not like a hill or anything lol. I have a 100ml syringe so was hoping to work around it

    What partial pressure do you need? Medium vacuum should be sufficient about 25 to 1×10−3

    You must apply some external energy to overcome the thermal losses in the system.
    Where does that energy come from? I'm not 100% sure what you mean here?
  8. Feb 12, 2014 #7
    How quickly do you need to go back and forth from vacuum to normal pressure?

    I doubt that the syringe is going to draw 0.025 Atmospheres.
  9. Feb 13, 2014 #8
    After every 2 seconds.
  10. Feb 13, 2014 #9


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    There are some limitations here that decide the geometry of the system.
    When at atmospheric pressure, what is the volume, a, you need.
    When there is a partial vacuum, what proportion, p, of one atmosphere do you require. 0.025?

    If a syringe started with one atmosphere at the a = 1 ml mark and you wanted p = 0.1 atmospheres, then you would need to draw the syringe out to a/p = 10 ml mark.

    It is clear that the length of the syringe is determined by the partial vacuum you require and the atmospheric volume. Any additional experimental volume that is external, but connected to the syringe, is part of the volume a.

    So, what volumes and pressures do you really need?
  11. Feb 13, 2014 #10
    oh ok. Thanks, so according to this I would need want the 100 ml syringe to start at 70ml and then draw suction. So 30ml draw would be expansion of the 70ml atmospheric pressure already there.
  12. Feb 13, 2014 #11


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    Absent a complicated energy recovery scheme, yes, a reciprocating pump comes close to the theoretical maximum efficiency possible.
  13. Feb 13, 2014 #12


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    Would a hand vacuum pump like this work for you? I've use these in the past to pull a modest vacuum...

    Last edited by a moderator: May 6, 2017
  14. Feb 13, 2014 #13
    Why does it have to be efficient? Is it because it has to be portable and battery operated. Or is it because you are going to be powering it yourself. Are we dealing with a weight limitation? Are we dealing with a size limitation?

    If you are operating it near running water, have you considered a venturi pump?
  15. Feb 14, 2014 #14
    Thanks all, I think Im going to try the hand pump Berkeman suggested, hopefully itl work.
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