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What material I should use for the "vacuum"?

  1. Nov 12, 2014 #1
    I put helium in a balloon (with a maximum pressure of 1.5-1.7 atm), but I do not want changes in the volume, so I have to use a rigid material (non-elastic). To make sure that there is only helium in the balloon, I have to create a "strong vacuum."
    My fear 'that, in the material can break or deform.
    Can you recommend a rigid material (not heavy) that resist also vacuum ??

    I have think at PVC, but i'm not sure that is a good idea...


    Please


    Thanks!
     
  2. jcsd
  3. Nov 12, 2014 #2

    mfb

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    Many materials will work if the wall can be thick enough. That won't look like a balloon, however.
    Do you really need vacuum? If you have a balloon, you can simply push all air out of it first. Alternatively and if your container really has to be rigid, you can flush with helium long enough to get rid of any reasonable remains of air (a bit more expensive, but probably still cheaper than a solution involving vacuum).

    What is the application of this?
     
  4. Nov 12, 2014 #3
    great idea!
    It is an aerostat, the height is varied in two ways:
    1) valve for placing helium (high-pressure), to increase the density of the helium and bring down the aerostat
    2) the outflow valve to decrease the density of helium and pick up the aerostat

    You're advising me to do so:
    1) both valves open, pass the helium in the aerostat for a few hours and close the valves? in this way in the aerostat there will be only helium ??
    I just want to helium (not air), otherwise my calculations will be incorrect.


    sorry for my bad English but I am Italian
     
  5. Nov 12, 2014 #4

    e.bar.goum

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    I'm with mfb. Without knowing what you're doing in detail, the easiest thing to do would probably be to flush with helium. I'd be tempted to flush with dry nitrogen first, but I'm not sure if that would actually help matters.

    Also: Check the purity of the helium gas you're using. If you're using 99.9 % pure He (grade 3), working really hard on your vacuum will pointless. If you're paying for 99.99999% He, then (a) I really want to know what you're doing, and (b) working on your vacuum might be worth it.

    But in terms of light, rigid, good for vacuum materials, aluminium is very commonly used in vacuum chamber design. If you're that worried about impurities, remember to bake out the chamber.

    ETA: An aerostat. Ok, you probably don't want to make it out of aluminium. How important is that it be totally pure He?
     
  6. Nov 12, 2014 #5

    e.bar.goum

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    Yeah. But make sure the valves are far apart (other sides of the chamber would be best). If they're right next to each other, you won't get sufficient flow. This is a common problem in gas detector design, so there's lots of literature on the matter.
     
  7. Nov 12, 2014 #6

    Doug Huffman

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    To prevent dilution of the He, put an impervious boundary between the air and the He.
     
  8. Nov 12, 2014 #7

    mfb

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    The aerostat just has to be prepared once? Then just flush it with some multiple of its volume (like 10 times) as helium volume. That should remove more than 99.9% of the air if you do it slowly.
     
  9. Nov 12, 2014 #8
    helium must be truly pure, I need to get the density (measuring pressure and temperature sensors), using the formula of van der waals (with the constants a and b).
    I can not afford that helium is not pure or that there is air in the aerostat.

    So if I understand correctly, the vacuum is not needed, just pump the helium in the aerostat (and have the two valves far)?
    so that material? any material that does not expand to 1.5-1.7 atm?


    @Doug Hoffman
    I did not understand.



    @mfb
    so if the balloon has a volume of 2 m ^ 3, I have to pump 20 m ^ 3 of helium? 'Why slowly? then at that pressure?
     
  10. Nov 12, 2014 #9

    e.bar.goum

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    It's an experimental reality that you won't have 100% pure helium. It's just not going to happen. You can try really hard - by baking out whatever materials you're making the aerostat out of, and spending a whole heap of money on a lot of grade 7 helium (it's very expensive, and still not 100% pure), will get you a lot of the way, but you will never get 100% pure helium.

    Without infinite money, and without infinite time, what you have to do is decide how many impurities you can live with to get the experimental results you want to the precision you desire. So do your research - if you have 99% helium, what is the error on your measurement? How sensitive are your pressure and temperature sensors in the first place? How sensitive are your derived quantities to impurities?

    If you assume you've gotten 100% pure helium in your experiment, you will be wrong, and you will get the wrong answer. So quantify your error! There's a reason experimentally derived quantities have error bars!
     
  11. Nov 12, 2014 #10
    you are right,
    it's good enough Helium 5.0
    Now, however, 'I doubt:
    I just pumped helium for a few hours (wash)
    or
    I need to create a vacuum?
    and what material to use?
     
  12. Nov 13, 2014 #11
    Maybe you have more serious problems than the purity of the gas.
    If you have a rigid balloon, pumping helium in it will not change the buoyant force. And the weight of the aerostat will not change either, unless the new helium comes from somewhere outside the aerostat. Otherwise you just transfer some helium from a tank to the baloon.
    So what kind of calculations did you make? And how come that they are so critical to the purity of helium?

    Getting rid of some helium will indeed reduce the weight and make it rise, if it's rigid balloon. If that helium is released in the air and not pumped back in a tank.
     
  13. Nov 13, 2014 #12
    Are you sure?

    If i have an rigid aerostat with a big Volume, you say that there is not a bayount force? So, the aerostat will be not up ?
     
  14. Nov 13, 2014 #13
    No, I did not say that there is no buoyant force. There is a buoyant force no matter how large is the volume. If it goes or not up depends on how the buoyant force compare with the total weight. Here is where your calculations should give you the answer.

    My comment is about your intended method for making the device to go down by pumping more helium in the rigid balloon.
     
  15. Nov 13, 2014 #14

    ah ok. I understand, excuse me but i don't speak and write english very well. So, for me, is very difficult.

    in the latest posts, you said that to have only Helium in the aerostat, i must pump it (inside aerostat) a large volume (like 10 times) very slowly...

    can you tell me, why very slowly? and at what pressure?
     
  16. Nov 13, 2014 #15
    No, it was not me. That was the discussion about your requirement about helium purity. Which I believe is irrelevant anyway. There is no reason to spend so much effort on this.

    You may have real problems which you overlooked in your calculations.
     
  17. Nov 13, 2014 #16

    How slowly?
     
  18. Nov 13, 2014 #17

    mfb

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    It should not matter in most setups, you just want to avoid a stream of gas that enters the balloon at one side and directly hits the exit at the other side.
     
  19. Nov 13, 2014 #18
  20. Nov 13, 2014 #19
    see the imagine.... is ok ?
     

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  21. Nov 14, 2014 #20
    the problem is that i don't want to mix the two gases (air and helium), so i would to know how slowly, i must flush the aerostat with Helium.
    To have a good result, i think that this operation i need to do it, with the aerostat in vertical position e not horizontal position.
    The weight's air is more heavily than helium, so if the valve to immission helium is on top, the helium will push the air on the bottom, and so the air will go out by valve.
    If i will do this operation with aerostat in horizontal position will be more difficult
    Please, i would to know what do you all think ?
    thanks
     
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