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Vacuum chamber and the characteristics of glass?

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  • macona
    replied
    You need about 3/4 acrylic to withstand that pressure. That's more than the bell jar and plate I mentioned.

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  • EVguru
    replied
    Charity shop presssure cooker with acrylic lid; job done.

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  • The Artful Bodger
    replied
    I would be very dubious of a canning jar 'shattering' on cool down however I have seen jars crack on cooling, just a crack but no violent implosion which of course would be impossible when the jar is almost filled with liquid.

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  • macona
    replied
    Originally posted by Don Young View Post
    I was just commenting that the force is almost the same at moderate vacuums as it is at 'deep' vacuums. I agree that the force can be quite high but I don't think that you can break a canning jar even with a 'perfect' vacuum.
    I wouldn't be so sure, I have seen canning jars shatter on cool down. Not every one is going to be perfect. Why mess with on when you can get a real jar and base plate so cheap. Not worth the potential injury, especially around kids.

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  • darryl
    replied
    The canning process will get you pretty close to a vacuum, as Jpfalt suggests. Getting rid of the final 10% of air will not get you any closer to a catastrophe. If you were considering a canning jar in the first place, that suggests that a diameter of 4 inches would be enough for the project- 8 inch tube would just up the cost for you. Maybe compromise at 6 inch polycarb tubing at 1/4 inch wall. The base and the top can be grooved for the ends of the tubing, and that will prevent it from losing its round shape and possibly collapsing. For more safety you could also machine a round hole in a piece of flat sheet and place that over the tube at about the center point. Make that from the same stuff you use for the outer enclosure.

    Personally I'd have no problem capping a 6 inch diameter tube of polycarbonate and pumping it down. I'd be standing right beside it.

    As far as the feather and ball, I see the need to get below 1/2 atmosphere, but 1psi remaining air isn't going to screw up the demonstration.

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  • Don Young
    replied
    I was just commenting that the force is almost the same at moderate vacuums as it is at 'deep' vacuums. I agree that the force can be quite high but I don't think that you can break a canning jar even with a 'perfect' vacuum.
    Originally posted by macona View Post
    Atmospheric pressure may not seem like a lot but that is still a lot of energy.

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  • macona
    replied
    Originally posted by Don Young View Post
    Isn't the force on a vacuum chamber essentially 14.7 PSI at absolute vacuum and pretty close to that at most 'normal' vacuums? There is not much difference between 28" HG and 30" Hg. I never thought that force was a real concern in high vacuum work.
    Atmospheric pressure may not seem like a lot but that is still a lot of energy.

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  • darryl
    replied
    For an 8 inch poly tube, I think 3/8 wall thickness would be lots- a 1/4 inch wall would probably never show the strain either. One of the pieces in my vacuum system uses an 8 inch diameter plumbing fitting- I don't think that gets any thicker than about 1/4 inch. End caps are 3/8 thick polycarbonate, but I've also blown them into a dome shape. They take both the highest vacuum I can make, and up to 150 psi pressure. I can pump down to about 1 psi, which is about 95% of a full vacuum. In terms of the ability of something to withstand a vacuum, there's no difference between that and a high vacuum.

    It's equivalent to anything sealed and placed within another chamber which is pressurized to 15 psi. That might be a good test, actually, since it's way easier to achieve some relatively low positive pressure than a full vacuum. Of course the chamber you use would have to be rated to take the pressure.

    You could use water of course- tap water will be at least 30 psi, often up to 50 or so. If you really want to test a canning jar or similar for it's ability to withstand it, just rig up something you can seal and pressurize from your tap.

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  • Don Young
    replied
    Isn't the force on a vacuum chamber essentially 14.7 PSI at absolute vacuum and pretty close to that at most 'normal' vacuums? There is not much difference between 28" HG and 30" Hg. I never thought that force was a real concern in high vacuum work.

    Leave a comment:


  • The Artful Bodger
    replied
    The ball and feather demonstration might be appropriate for us too as we are trying to get young children (12-14 year olds) interested in things related to atmosphere, weather and of course aeroplanes! But I guess the ball and feather demo would require a much harder vacuum than my half atmosphere?

    How thick would the wall have to be and an 8" polycarbonate tube to be safe at all practically achievable vacuum levels?

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  • The Artful Bodger
    replied
    Originally posted by boslab View Post
    Stick a smoke detector in there and you can watch particle trails too!
    Am241 worked for me!
    Mark
    I need to know more please!

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  • KMoffett
    replied
    Originally posted by Edwin Dirnbeck View Post
    This sounds like a good idea to show my grandkids. Does the feather get slowed down by any kind of static electric charge that might be in an accrilic tube?Thanks,Edwin
    Never had a static problem with the demo. It never ceases to amaze me to watch the ball drop and the feather float down in ambient, and both " drop like rocks" in the vacuum.

    Ken

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  • boslab
    replied
    Stick a smoke detector in there and you can watch particle trails too!
    Am241 worked for me!
    Mark

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  • macona
    replied
    Or just buy this: http://www.ebay.com/itm/Bell-in-vacu...item3cd01cc5cb

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  • darryl
    replied
    I agree with all the safety issues, and would also suggest a polycarbonate shield box to house the apparatus. Beyond that though, I think most any glass jar would be able to sustain a full vacuum. But you might want to use a more appropriate thing so you can see the cloud formation, etc without optical distortions. I would tend to think that a length of polycarbonate tube would be the better thing to use for a chamber. Even at 8 inches in diameter, it is easy to make end caps capable of handling the pressure. Those don't have to be see-through, and one would become the base of it anyway. A piece of 1/2 thick aluminum plate will be more than strong enough to resist caving in as the top cover.

    Polycarbonate tubing isn't cheap, but the protective box would just use flat panels. You could use a larger tube as the protective cover for the actual 'vacuum' tube, but you'd probably have to take out a second mortgage on your house to be able to afford it

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