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Vacuum - An argument to settle

  1. Jan 30, 2007 #1
    In the world of woodworking wood verneer is often glued onto a board by putting the piece into a vacuum bag which applies an even preasure to the verneer giving good claming results.

    If the vacuum bag was put into a steel container with 14.6 psi or 1 atmosphere in the bag and in the steel container and a vacuum was pulled on the bag from the outside and a vacuum pulled on the container from the outside would the bag still have clamping preasure?

    Thanks, I believe that it would.
     
  2. jcsd
  3. Jan 30, 2007 #2

    russ_watters

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    No, it wouldn't. You can never have a negative absolute vacuum (since a vacuum is just an absence of pressure).
     
  4. Jan 30, 2007 #3

    Fredrik

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    I have read the question over and over, but I can't understand it.
     
  5. Jan 30, 2007 #4
    Russ,
    Isn't the vacuum bag inside the container which has a vacuum pulled on it a separate system from the vacuum that has been pulled on the inside of the steel container? It seems to me that being a separate system that a vacuum would be pulled on the bag.
     
  6. Jan 31, 2007 #5
    I don't really understand what you're asking either.

    Are you asking whether the bag - itself in a vacuum - experiences the effects low pressure?
     
  7. Jan 31, 2007 #6

    russ_watters

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    A vacuum would be pulled on the bag - but it wouldn't be a greater vacuum than the one in the vacuum chamber.
     
  8. Jan 31, 2007 #7
    Took me a bit of time to work this one out. If it is what I think it is the answer is no.

    You are trying to get a uniform pressure on the veneer. By putting a vacuum in the bag which contains the surface being glued you are using the pressure of the outside air acting uniformly on the bag to apply pressure to the surface.
    The pressure applied is equal to the pressure outside the bag (1 atmos) minus the pressure in the bag (lets say 0.2 atmos). So you are uniformly applying 0.8 atmos to the surface, which is quite a lot.
    (If you want to know how much pressure somebody will know the psi of atmos at sea level in their head, I would have to look it up)

    In your box idea, the same ratio is applied, the pressure outside the bag minus the pressure in the bag. So the pressure outside the bag, in the box is reduced to lets say 0.2 atmos and the pressure in the bag is also reduced to 0.2 atmos so you have no pressure difference and hence no force applied.
     
  9. Jan 31, 2007 #8
    If this vacuum bag was sitting on a tabletop with wood and veneer in it and a vacuum was pulled on the bag to glue the veneer the question is, what effect does the 1 atmosphere (ambient) that the system is in have any effect on the clamping preasure that is being applied by the bag. It seems to me that when a vacuum is applied to the bag that all of the clamping preasure is being applied internally by the bag trying to attain the smallest shape possible. The vacuum bag being a closed system shouldn't be effected by the 1 atmos. ambient.

    In trying to figure this out I was theoreticaly putting the vacuum bag in another steel container where a vacuum could be pulled to negate any effect that the ambient 1 atmos. may have.

    I hope this helps to clairify the question. Thanks all.
     
  10. Jan 31, 2007 #9

    FredGarvin

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    Like has already been mentioned, the ambient atmospheric pressure has everything to do with the clamping force. The force is not directly created by the bag. The force is created by the pressure of the ambient pressure collapsing the bag. Like Russ already mentioned, this is the pressure difference across the bag. Once you evacuate the bag, you have 14.7 psi acting over the surface of your part (assuming a perfect vacuum inside the bag). For a part that is 1 ft^2, that's a little over 2000 Lbf of clamping force.
     
  11. Jan 31, 2007 #10
    OK the problem is that you are considering the vacuum as being physical where as it is not. I will try and simplify this, so note to physists there will be a lot of undeclared assumptions going on here.
    The general equation I will use is Pressure, Volume and Temperature (PVT) is constant. This is simplified by saying temperature is static so
    Pressure * Volume = Constant

    Air is very light but it does not weigh nothing. If you think of a column of air stretching from the top of your head all the way up to the edge of the atmosphere all the air weighs 14.7lb for every square inch of your head, so the pressure on your head is 14.7lb per sq". You don't notice this as there is an equal amount of pressure coming from every other direction counteracting it.

    So you have this air tight box 12" sq and in it is air at 14.7lb per sq" and outside is air at 14.7lb per sq", so there is no force on the surface of the box.
    You then take half the air out of the box. The outside of the box is being pushed in by 14.7lb per sq" the inside of the box is being pushed out by 7.3lb per sq", so the net force is 7.3lb per sq" pushing on the outside of the box, which is the same as putting a 1000lb weight on the lid.

    With a bag the same thing effectively happens:
    To start Air in the bag equals the air outside the bag so no pressure is applied.
    You take half the air out of the bag and because the bag is not rigid the bag collapses. So the volume has reduced by half and the pressure of the air in the bag is equal to the pressure outside the bag and no force is applied.
    You take some more air out of the bag, and it is now down hard around the object in the bag so the volume can't reduce any more.
    You take even more air out of the bag so the pressure of the air left in the bag is at 7.3lb per sq" and the air outside is 14.7lb per sq" the pressure is the same as pushing down on the outside of the bag with a force of 7.3lb per sq".

    If you are curing a veneer tray of say 12"x24" in the bag the outside of the bag will press down on the tray with a force of more than 2000lb. You could achieve the same affect by just putting 2000lb of lead on top of the tray, but you need to make sure it is really flat and uniform to prevent the tray from being damaged. Air automatically applies uniform pressure.

    So in summary the force is applied by the air from the outside, not the vacuum from the inside.
     
  12. Jan 31, 2007 #11

    russ_watters

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    Sea level pressure is 14.7 psi...

    Lets throw some pressure gages and some real vacuum pumps onto this apparatus and see what you get...

    First of all, there are two different kinds of pressure to know:
    -Gage pressure is literally the pressure read by a gage. It is a pressure difference between two different places (often with one of them being the atmosphere).
    -Absolute pressure is the pressure against a vacuum. A mercury barometer measures absolute pressure directly (the column of mercury is held up by the atmosphere pushing against a vacuum inside the mercury-filled tube). And you can simply add the atmospheric pressure to the gage pressure to get absolute pressure in most other cases.

    Now, a quick google tells me that a mediocre vacuum pump will get you down to 1/100th of an atmosphere. Since atmospheric pressure varies by more than that from day to day (for example, the pressure where I am right now is 13.8psi, you can assume for most purposes that a vacuum pump gives you a perfect vacuum (*caveat later). So, hooking a pump up to a vacuum bag with a 1' square piece of laminated plywood on it is the equivalent of piling 14.7*144= 2117 lb of weights on it. Lot of force in pressure, isn't there?

    Now, in the situation I just gave, if you hook up a pressure gage to the vacuum bag (leaving the other end of the gage open to atmosphere), you'll read -14.7psi. Adding atmospheric pressure gives you -14.7+14.7=0 psi absolue pressure. Either way, same thing.

    Now, if you put that vacuum bag into a vacuum chamber and pump it out, a gage between the chamber and the atmosphere will read -14.7psi. A gage between the chamber and the bag will read 0psi. And with 0psi of pressure difference, there is no pressure pushing down on the bag.

    *Now for that caveat: if your vacuum pump for the vacuum bag is inside the vacuum chamber, the pumps act in series with each other (depending on the pump) so while the chamber may have a pressure of .01 atm, the bag may have a pressure of .0001 atm. Multiplying that out gives 14.7*(.01-.0001)= .145psi. Not a lot of pressure, but on a 1 square foot area, it is that's about 21 lb of weight sitting on top of it. But the caveat to this caveat is that the better the pumps, the less of a difference you'll see between the bag and the vacuum chamber. Ie, if your pumps can give you 1/1000th of an atmosphere (still a pretty mediocre pump), you will only have the equivalent of 2 lb of weight sitting on your project piece.
     
    Last edited: Jan 31, 2007
  13. Feb 1, 2007 #12
    An interesting experiment.
    If you get a large dustbin lid (trash can or what ever you call them)
    put some rubber seal like car door trim round the edge.
    Cut a whole in the lid and attach a well sealed tube from your vacuum cleaner

    You can hang off the ceiling. I did this as a kid, a real laugh especially when you turn the vacuum off when your mates not expecting it... :devil:
    but looking at my waistline I probably would need about 10 dustbin lids now :cry:
     
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