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Removing air from water using vacuum

  1. Oct 13, 2011 #1
    Ok guys, I need help.

    I work for a Contract Manufacturer that is working to solve a problem for a customer of ours. The customer has designed a snowglobe (it's actually in a jar) with a cool little Christmas tree inside it. It's a great idea, but there are a ton of nooks and crannies that trap air inside them really really well when you insert the tree into the jar of solution (96.9% Distilled water, 3% glycerin, .1% IPA).

    What we're trying to do is find a way to dislodge the air from the Tree as the customer advertises them as being high quality and air free. Smacking the jar around dislodges some, but man those air bubbles can be stubborn!

    The customer has suggested using a vacuum system as their previous Chinese vendor had done (yes, we stole work back from China!) but the Chinese vendor is reluctant to disclose details of their methods.

    So we've tried various methods, but we have had no success. The customer has suggested placing the tree in the jar filled with the solution then placing that jar in a larger tank of solution and subjecting the whole thing to vacuum. The idea is that the vacuum will remove the air from the tree and the larger tank will keep air from re-entering while we remove the vacuum and complete the seal between the figure and the jar.

    I think they're crazy, but they insist that the Chinese vendor did it this way.

    Initial trials have achieved 0% success. The gauge on the pump maxes out at 30 mHg and I've left the container (a 1' tall, 8" diameter jar filled 75% with solution) under vacuum for as much as an hour (I've just been informed that the vacuum pump is now emitting smoke so I should probably hustle to check it out...).

    That being said, I'm appealing to those who might have a better understanding of what we're trying to achieve as the theory behind our goal is just beyond my understanding of the water/air/vacuum relationship. Does anyone have advice or can provide insight on how best to remove the air from the tree? This is an interesting issue and obviously there are financial implications for my company and our customer. We'd like to prove that we're as capable as our Chinese cometition and win one for the good guys!

    Please feel free to ask any questions that might clarify the situation. I cannot express how appreciating I am of anyone that would like to help!
  2. jcsd
  3. Oct 13, 2011 #2


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    You're placing a dry piece of plastic tree into a jar of water, and its dragging air along trapped in the feathery leaves? If you had agitated the tree in a large bath of water just before doing this, so the water displaced the trapped air, it would take water along with it, instead. Try a drop of detergent in the water to lower the surface tension, but not so much that you have froth forming. If you use water that has been boiled to fill the jar, then there will be almost no dissolved air to form bubbles in the jar.
  4. Oct 13, 2011 #3
    This is a common problem with investment casting. Does the tree start to boil as you reduce the pressure. You may need to only get to 1/2 atsm to usefully remove air bubbles.
    look at the machines in this link http://gemstone.smfforfree4.com/index.php?topic=4809.0
    There are also debubbling solutions that help a lot. In your case, if you want to keep your .1% IPA in your solution, you need to be careful how low of a vacuum you use.
  5. Oct 13, 2011 #4
    I suppose you mean 30 mm Hg... that's too much. And you'd better start with already degassed water in the first place, before you put the tree inside. Even a simple water Venturi pump may be better (and it won't start smoking!), although it's not a good idea for mass production. In fact, standard oil pumps don't like water vapour, there's a gas ballast valve to be used for that.
  6. Oct 13, 2011 #5
    Actually Harry, I was way wrong, the gauge reads inHg, sorry.

    I've shelved the vacuum until I can learn a bit more and decided to explore solving the problem at the source. If I can get the air out to begin with or try NO's suggestion of detergent (initial tests are promising btw, thanks!) to keep the air from sticking to the tree, we'll be in good shape.

    I'm not finished with the vacuum though as we still need to remove the air from the jar after the tree has been inserted. Also, it suits our production needs (mechanical, repeatable) and keeps the production within customer spec (solution ratio, fall rate of the snow), I just need to learn a bit more about it. You guys have been a big help so far, thanks again!

    @johnbbahm No, it does not start boiling, it does nothing.
  7. Oct 13, 2011 #6


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    As suggested, the addition of a surfactant to improve wetting appears to be the solution [no pun!] you require.

    If you are not going to improve wetting by altering the solution, then you need to cause the surfaces on the tree (and inside, wherever) more hydrophilic. At the moment, I would guess that the surfaces are quite rough and hydrophobic, hence an increased wetting angle compared with smoother surfaces that increases forces at the 3 phase wetting line to the point that they are, clearly, too great at the moment for simple mechanical or thermal (diffusion) processes to dislodge the gases.

    (PS, if that answer helps you earn the cash from your contract, feel free to PM me with a sub!)
  8. Oct 13, 2011 #7


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    Incidentally, there is one other way I am totally confident will fix the problem, but I don't think there is a point mentioning it because it'd not be useful in production volumes. You have to come up with a solution fit for a production line, and this is a case of fixing material/supply issues. I reckon you'll find the tree is too rough and hydrophobic.
  9. Oct 13, 2011 #8
    "@johnbbahm No, it does not start boiling, it does nothing."
    I am thinking something is wrong with your vacuum system. At .5" Hg water will boil itself into ICE. (fun to watch). In vacuuming the air out of slurry mixtures, you have to ease down the vacuum, so the rapid release of gas does not splash the slurry all over the place.
    You have to watch it, I suspect that is one of the reasons the bell jars are clear.
  10. Oct 13, 2011 #9
    And I may have exaggerated a bit. I often use a little water venturi pump to degas water, and while it can in principle reach lower pressures than 30 mm Hg (amazingly it's even possible to get the water to boil at room temperature), I guess that it usually doesn't go that low - but still good enough for my purpose. It depends on the application.

    However 30 inHg is about 1 bar (assuming it means inches of mercury). That corresponds to you saying "maxes out": I guess that it starts out at 0 and reaches 30 inHg. If so, it measures the pressure difference between the environment and the inside. Thus it doesn't directly measure the vacuum pressure which is low (near to zero), but you don't know exactly how low!

    It may be that your vacuum pumping system will work if you combine it with one of the other tricks.
  11. Oct 13, 2011 #10
    I really don't like admitting this, but I've got some egg on my face. I found the pump vent valve partially opened and after I closed it air started to surface.

    My newest issue is that it has not completely purged from the inside of the jar. If you remember, I'm placing the tree inside a small jar, partially sealed, then placing that into a larger jar (my "chamber") filled with water.
    The outer tank was under vacuum for 1.5 hrs and there were bubbles still forming on the inside of the interior jar. As long as I can prove that the technology works we'll invest in a better system. (Right now I've got a piece of aluminum that I cut out of a sheet pan with some rubber gasket material glued to the bottom placed over a glass flower pot. Yup, that's my chamber!)

    I had seen the ice form in other tests I had done, wicked cool, but no idea why it's happeneing.
  12. Oct 13, 2011 #11


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    Put the jar in the microwave oven and make the water boil. When the water is still at 100 degrees celcius, seal the jar and cool it down. Now there will be pretty much vacuum inside as the steam turns into water. Everything that once was air is replaced by water or vacuum. Maybe you will have a small bubble inside, but that makes it easier to shake the snow off the ground. But no more small bubbles on the tree.

  13. Oct 13, 2011 #12
    Be extremely careful of any containers not designed for vacuum. They can violently implode and cause injury.
  14. Oct 13, 2011 #13
    Have you considered ultrasound, such as used in cleaning jewelry and fine mechanisms?
  15. Oct 13, 2011 #14


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    That was the impractical idea I didn't bother to mention.

    You guys are nuts! Add some wetting agent/detergent!!!!
  16. Oct 14, 2011 #15
    One thing that might be worth trying is to dip the tree into Alcohol, The alcohol may keep the bubbles away from the tight areas of the tree, and then boil away.
  17. Oct 14, 2011 #16
    Put the tree in the jar, pull vacuum on the jar with tree already in it, and then let the vacuum pull water into the jar. In otherwords, remove the air from the tree before adding the water.
  18. Oct 14, 2011 #17


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    Curses. I read this thread from the beginning and someone else seems to have made all the useful comments that I could think of!! Some good, inventive minds at work here.
    I agree that thorough cleaning of the tree, with alcohol could help.
    Just a thought - but I can't think of a specific solution. When they go for the final low vacuum stage in the manufacture of radio valves, they use a 'getter' which mops up the residual few atoms in the envelope (after it's been sealed) by blasting them against the side of the tube with a small 'exploding' piece of metal - heated with RF, i believe. There could be an equivalent to this if you flushed all air out of the system, first, with CO2, or some other gas. Then, after doing the best job that you can, to remove gas bubbles, some absorbing pellet could mop up any dissolved flushing gas after the bulb has been sealed.

    Btw, has anyone suggested using already-boiled, distilled water, which should be much more free of dissolved gas, in the first place?
  19. Oct 14, 2011 #18


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    I don't think anyone in the thread is actually looking for a simple, practicable idea like that.
  20. Oct 14, 2011 #19
    I made approximately the same suggestion in post #4, although I'd do everything by vacuum pumping for simplicity. And I did not think of the difference between distilled water and tap water - are you sure that distilled water holds less gas than tap water?
  21. Oct 14, 2011 #20


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    If it's been boiled recently, it's bound to have less dissolved gas.
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