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vacuum bag #249128
06/04/12 09:16 AM
06/04/12 09:16 AM
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pgp Offline OP
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Can the weight of water be used to pull a vacuum?

If I place a small mold inside a vacuum bag at waist height and connect it to an elevated water reservoir which is in turn connected to a tube, will releasing water into the tube evacuate the bag? Any physics to support your answer(s)?


Pete Pollard
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Re: vacuum bag [Re: pgp] #249129
06/04/12 09:26 AM
06/04/12 09:26 AM
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soccerguy83 Offline
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Seems to me that this isn't going to work. In order to have a complete vacuum in the mold bag (top bag) with no air bubbles then you would have to have a completely closed system. By this I mean that there would have to be no air in the reservoir (bottom bag). If there is air in the bottom bag or if you were looking to do this over a bucket or something you would then have air moving up into the top bag to replace the void of the water. In a closed system I don't think you are going to find gravity, your only force, creating enough of a "vacuum" to do what you are looking for. Please someone correct me if I'm wrong, I have no experience in Vacuum Bagging, just throwing out what I think is possible.


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Re: vacuum bag [Re: pgp] #249137
06/04/12 11:55 AM
06/04/12 11:55 AM
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Will_R Offline
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No reason that this wouldn't work except for practicality. You'll have to displace more water than you have air to remove for it to work. It's basic fluid dynamics.

The difficulty as I see it would be the volume of air to be removed relative to the amount of water you are able to "drop". i.e. once you've pulled all the vacuum that you can with the water, it may not be enough to remove the air from the carbon/resin and produce a vacuum of sufficient quantity if at all.

Re: vacuum bag [Re: Will_R] #249140
06/04/12 12:46 PM
06/04/12 12:46 PM
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mini Offline
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Originally Posted by Will_R
No reason that this wouldn't work except for practicality. You'll have to displace more water than you have air to remove for it to work. It's basic fluid dynamics.

The difficulty as I see it would be the volume of air to be removed relative to the amount of water you are able to "drop". i.e. once you've pulled all the vacuum that you can with the water, it may not be enough to remove the air from the carbon/resin and produce a vacuum of sufficient quantity if at all.


In order to pull 1 atmosphere of vacuum with water you need 33 feet. Not very practical. Not sure I understand your description of the system, but as mentioned it must be closed.

Re: vacuum bag [Re: mini] #249142
06/04/12 12:54 PM
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Originally Posted by mini
In order to pull 1 atmosphere of vacuum with water you need 33 feet. Not very practical.


There is that too, lol

Re: vacuum bag [Re: mini] #249143
06/04/12 12:54 PM
06/04/12 12:54 PM
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pgp Offline OP
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Originally Posted by mini
Originally Posted by Will_R
No reason that this wouldn't work except for practicality. You'll have to displace more water than you have air to remove for it to work. It's basic fluid dynamics.

The difficulty as I see it would be the volume of air to be removed relative to the amount of water you are able to "drop". i.e. once you've pulled all the vacuum that you can with the water, it may not be enough to remove the air from the carbon/resin and produce a vacuum of sufficient quantity if at all.


In order to pull 1 atmosphere of vacuum with water you need 33 feet. Not very practical. Not sure I understand your description of the system, but as mentioned it must be closed.


It isn't that I doubt you, but how do you know that? Would a 33' coil work?

I'm considering a very small volume and considering using Luer lock, standard I.V. equipment. In fact, I was thinking of an i.v. bag on a pole.


Pete Pollard
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'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: vacuum bag [Re: pgp] #249146
06/04/12 01:06 PM
06/04/12 01:06 PM
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Pete, just buy a cheap vaccum pump at Harbor Freight.
http://www.harborfreight.com/catalogsearch/result?q=vaccum+pump

Re: vacuum bag [Re: pgp] #249149
06/04/12 01:10 PM
06/04/12 01:10 PM
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pgp Offline OP
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I'm not buying any more gadgets to get used once, then gather dust for decades.

Besides, I'm obsessed with the question...

Last edited by pgp; 06/04/12 01:16 PM.

Pete Pollard
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'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: vacuum bag [Re: pgp] #249155
06/04/12 01:33 PM
06/04/12 01:33 PM
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Originally Posted by pgp
I'm not buying any more gadgets to get used once, then gather dust for decades.

Besides, I'm obsessed with the question...


Well, on the surface, absolute vacuum will only generate atmospheric pressure on the item in the bag...which equates to about 14.5 psi. You can try to pull "more" vacuum on it but once you have removed all of the air, there is nothing else to pull. So, the vacuum bag can only match the differential pressure between the outside and the inside of the bag...i.e. 14.5psi atmospheric and 0psi inside the bag = 14.5psi total...and you can't generate less than 0 inside the bag because there's nothing left to draw out.

That said, you CAN increase the external pressure on the bag to increase the differential and the pressure on the item inside the bag. This is the principle of an autoclave. You first draw a vacuum on the bag, insert the bag into a pressure vessel, pump up the pressure inside the vessel and put a lot more pressure on the part. Hall Spars builds masts this way (and the Autoclaves normally employ heat as well).

You ~could~ also take your vacuum bag and lower it into water to create more differential in the pressure in the same way the autoclave works.



Jake Kohl
Re: vacuum bag [Re: pgp] #249156
06/04/12 01:38 PM
06/04/12 01:38 PM
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Daytona Beach Florida
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Pete,
You could always go this route.
www.spacebag.com

Re: vacuum bag [Re: pgp] #249158
06/04/12 01:42 PM
06/04/12 01:42 PM
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Richmond, Va
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soccerguy83 Offline
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Originally Posted by pgp
Originally Posted by mini
Originally Posted by Will_R
No reason that this wouldn't work except for practicality. You'll have to displace more water than you have air to remove for it to work. It's basic fluid dynamics.

The difficulty as I see it would be the volume of air to be removed relative to the amount of water you are able to "drop". i.e. once you've pulled all the vacuum that you can with the water, it may not be enough to remove the air from the carbon/resin and produce a vacuum of sufficient quantity if at all.


In order to pull 1 atmosphere of vacuum with water you need 33 feet. Not very practical. Not sure I understand your description of the system, but as mentioned it must be closed.


It isn't that I doubt you, but how do you know that? Would a 33' coil work?

I'm considering a very small volume and considering using Luer lock, standard I.V. equipment. In fact, I was thinking of an i.v. bag on a pole.


That is a basic SCUBA concept, every 33' you go down the pressure increases by 1atm, so no a coil will not work to create that additional atm you would need a column of water. To elaborate on Jake's idea, the idea of a one-way valve on the bag and attaching the bag to an anchor in 33' of water comes to mind (try this on something replaceable first) with the valve pointing up so you don't get an air bubble.


Brian C.
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Re: vacuum bag [Re: Jake] #249160
06/04/12 01:42 PM
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pgp Offline OP
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Originally Posted by Jake
Originally Posted by pgp
I'm not buying any more gadgets to get used once, then gather dust for decades.

Besides, I'm obsessed with the question...


Well, on the surface, absolute vacuum will only generate atmospheric pressure on the item in the bag...which equates to about 14.5 psi. You can try to pull "more" vacuum on it but once you have removed all of the air, there is nothing else to pull. So, the vacuum bag can only match the differential pressure between the outside and the inside of the bag...i.e. 14.5psi atmospheric and 0psi inside the bag = 14.5psi total...and you can't generate less than 0 inside the bag because there's nothing left to draw out.

That said, you CAN increase the external pressure on the bag to increase the differential and the pressure on the item inside the bag. This is the principle of an autoclave. You first draw a vacuum on the bag, insert the bag into a pressure vessel, pump up the pressure inside the vessel and put a lot more pressure on the part. Hall Spars builds masts this way (and the Autoclaves normally employ heat as well).

You ~could~ also take your vacuum bag and lower it into water to create more differential in the pressure in the same way the autoclave works.



Is there a standard? X amount of resin per square meter of cloth, for instance?

IF .5 atmospheres is all that is required for most applications, I suspect vacuum bagging operations are designed to handle relatively large amounts of air in commercial operations.

Last edited by pgp; 06/04/12 01:45 PM.

Pete Pollard
Blade 702

'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: vacuum bag [Re: pgp] #249161
06/04/12 01:46 PM
06/04/12 01:46 PM
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South Carolina
Jake Offline
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Ahh, but now I see that you just want to generate the vacuum with a column of water.

The coil won't work - you need 33.5 vertical feet in a column of water to match 14.5 psi. It's the only way to get the gravity to add up.


Jake Kohl
Re: vacuum bag [Re: pgp] #249163
06/04/12 01:59 PM
06/04/12 01:59 PM
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pgp Offline OP
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Umm, I'm probably using the wrong approach. I need to know how much resin my piece needs for maximum strength, then I can just roll it out by hand.


Pete Pollard
Blade 702

'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: vacuum bag [Re: pgp] #249164
06/04/12 02:01 PM
06/04/12 02:01 PM
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Home is where the harness is.....
Will_R Offline
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Straight out of Felder and Rousseau, "Elementary Principle of Chemical Process"

1atm = 33.9 ft H20 at 4 deg C.

Jake, I disagree. If the pressure (vacuum) gauge reads lower (less) than 0 psia then you are exerting more than 1 atm of pressure on the item in the bag. The benefit of the autoclave is that you can reach the high activation temperatures required for prepeg AND put it under pressure.

Re: vacuum bag [Re: pgp] #249165
06/04/12 02:07 PM
06/04/12 02:07 PM
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pgp Offline OP
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"Straight out of Felder and Rousseau, "Elementary Principle of Chemical Process"

thx


Pete Pollard
Blade 702

'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: vacuum bag [Re: Will_R] #249170
06/04/12 03:05 PM
06/04/12 03:05 PM
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Jake Offline
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Originally Posted by Will_R
Straight out of Felder and Rousseau, "Elementary Principle of Chemical Process"

1atm = 33.9 ft H20 at 4 deg C.

Jake, I disagree. If the pressure (vacuum) gauge reads lower (less) than 0 psia then you are exerting more than 1 atm of pressure on the item in the bag. The benefit of the autoclave is that you can reach the high activation temperatures required for prepeg AND put it under pressure.


I think we're saying the same thing - you can't have a bag out in the open space (at atmospheric pressure), at full vacuum, and have more than one atmosphere on it. You can't achieve more vacuum than complete vacuum. You CAN increase the pressure outside the bag in order to increase the pressure inside the bag by using an autoclave to get better than atmospheric pressure.


Jake Kohl
Re: vacuum bag [Re: Jake] #249173
06/04/12 03:21 PM
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OK smarty pants......why do beer cans explode after you shake them? What is it about Mentos that makes coke bottles blow up? Those of us on the wrong side of the bell curve need real world knowledge! crazy


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Re: vacuum bag [Re: Jake] #249178
06/04/12 04:21 PM
06/04/12 04:21 PM
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Originally Posted by Jake
Ahh, but now I see that you just want to generate the vacuum with a column of water.

The coil won't work - you need 33.5 vertical feet in a column of water to match 14.5 psi. It's the only way to get the gravity to add up.


If you have a column of water descending 33 feet creating a vacuum space above it, then the water at the surface of that column will, in essence, boil. The vacuum space will have a considerable amount of water vapor in that volume. Will the presence of that "dampness" have an adverse effect on the curing process?

Re: vacuum bag [Re: David Parker] #249197
06/05/12 06:55 AM
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Jake Offline
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Originally Posted by David Parker
Originally Posted by Jake
Ahh, but now I see that you just want to generate the vacuum with a column of water.

The coil won't work - you need 33.5 vertical feet in a column of water to match 14.5 psi. It's the only way to get the gravity to add up.


If you have a column of water descending 33 feet creating a vacuum space above it, then the water at the surface of that column will, in essence, boil. The vacuum space will have a considerable amount of water vapor in that volume. Will the presence of that "dampness" have an adverse effect on the curing process?


Ahhhh...great point! Because the water will boil/evaporate, you will never achieve a consistent maximum vacuum that way - the water vapor will continue to fill the vacuum space and reduce it somewhat.


Jake Kohl
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