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Re: [PSUBS-MAILIST] Weight of Water VS. Buoyant Force of Air
What planet is this on?
Why would a a cubic foot of air require more than 62.3lbs to
make it neutrally buoyant in fresh water? Sorry, if I missed
something, this sounds very confused.
Ian.
-----Original Message-----
>From: Brent Hartwig <brenthartwig@hotmail.com>
>Sent: Sep 5, 2008 3:59 PM
>To: personal_submersibles@psubs.org
>Subject: [PSUBS-MAILIST] Weight of Water VS. Buoyant Force of Air
>
>
>Jay,
> If according to my none exact testing method I did several years ago, of a plastic gallon jug full of air, pulled just below the surface of freshwater in a freshwater pool with no chemicals in it, with weights. A gallon of air, which contains exactly 231 cubic inches, exerts 11 lbs. 10 ounces (not sure that was the amount, might of been 10 lbs. 11 ounces of buoyant force in 65 degree F, fresh water, at my altitude.
>
>Also take into account that the weight used to keep the gallon of air down was weighed out of the water. So what weight it has would be different in the water of course. But since we are comparing the weight of freshwater or saltwater vs. the buoyancy force of air in fresh or saltwater, it might be ok to weigh the weights out of the water.
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>So since a cubic foot has 1728 cubic inches in it, there is 7.48 gallons in a cubic foot. Meaning that if my test data is close to correct, that a cubic foot of air would require about 83.119 lbs. to make neutrally buoyant in freshwater, like I explained above. In other words a cubic foot of air will exert about 83 lbs. of buoyant force on the upper areas of the container, which is holding the air down below freshwater.
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>From what I could find, freshwater weighs about 62.4 pounds per cubic foot (1 kilogram per liter.). So this would seem to mean to me that we need to calculate the ambient subs cabin air and then multiply that by about 83 lbs. per cubic foot. Then design the upper areas of the cabin to with stand that with a FOS of ones choosing.
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>I would like it if some one could tell me if my data on the amount of buoyant lift of air in freshwater is reasonably correct. As well as what the number is for air in saltwater or a general specific gravity of ocean waters.
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> Fresh Water Salt Water
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> 62.3 lb/ft^3 64.0 lb/ft^3
>____________________________________________________________________
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>I found this data below in my World Book encyclopedia.
> "Laws of hydrostatics describe the behavior of liquids at rest.
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>One principle of hydrostics state: The pressure caused by the weight of a column of fluid is determined be the height of the column. The shape of the column does not affect the pressure that is exerted by the fluid.
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>Suppose that a pipe extends in a zigzag pattern from the ceiling to the floor. Another pipe of equal diameter extends straight down from the ceiling to the floor. The second pipe is, or course. shorter than the first. If both pipes are filled with water, the longer one will hold more water. But the water pressure at the bottom of both pipe will be equal, because the height of both columns of water is equal.
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>An illustration of the above principle is the buoyant force exerted by a fluid on an object submerged in it. The bottom of an object submerged in water is deeper then the top of the object. Therefore, the column of water pressing on the bottom of the object is taller then the column pressing on the top of the object. Thus, the water exerts an upward force on the object, called a buoyant force. Buoyant forces act to keep boats afloat.
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>Archimedes' principle comes from the principle discussed above. It states: An object placed in a fluid seems to lose an amount of weight equal to the weight of the fluid it displaces. A buoyant force exerted on the object by the fluid causes the apparent loss of weight. Archimedes, a Greek mathematician, developed this principle during the 200's B.C.
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>To illustrate this principle, image that a 1-liter metal can is placed in water. If the can weighs 3 kilograms, it will displace 1 liter of water, which weights 1 kilogram. Thus, the buoyant force equals 1 kilogram. The can will then seem to weigh only 2 kilograms-that is, 1 kilogram less than it's original weight.
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>Pascal's law state: A fluid in a container transmits pressure equally in all directions. Blaise Pascal, A French scientist and mathematician, developed this law during the A.D. 1600's."
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>So if we use Archimedes' principle and we place a gallon or air, that basically weights nothing, in water it will not displace any water, unless forced down below to surface like the air in are subs.
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>Your resident pipe dreamer ;)'Regards,Szybowski
>
>From: bottomgun@mindspring.comTo: personal_submersibles@psubs.orgSubject: RE: [PSUBS-MAILIST] Upward Buoyant Pressures on Ambient Interior SurfacesDate: Thu, 4 Sep 2008 18:58:20 -0400
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>Brent,
>Two things need to be sorted out in this mental exercise. How many cubic feet of air? Convert this into the weight of the equivalent volume of water…this is the force pushing up. Now have many square inches of surface (upward side) is that air applied to? Divide the force (equivalent pounds of water) by this surface area and you have the PSI sensed by the interior overhead. As you descend, the bubble gets smaller is no air is added, buoyancy decreases, and the applied air pressure to the top surface decreases. Water force inside and out are the same for an ambient sub.
>R/Jay
>
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>Respectfully,
>Jay K. Jeffries
>Andros Is., Bahamas
>
>Talk sense to a fool and he calls you foolish.
> - Euripides (484 BC - 406 BC)
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>From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Brent HartwigSent: Thursday, September 04, 2008 6:34 PMTo: PSUBSorgSubject: [PSUBS-MAILIST] Upward Buoyant Pressures on Ambient Interior Surfaces
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>Upward Buoyant Pressures on Ambient Interior Surfaces Grand Greetings SMMOM's ;)' I've been pondering this question for some time now, when I was designing a couple if different ambient subs in my head. Doug, I believe brings up some good points below about the higher PSI inside the ambient subs being a little higher. He also made a point recently about the PSI being a little higher from the constant flow of air, coming into the cabin for breathing. I was thinking that there is one more thing item we need to consider. That being that air is really buoyant, as we all know. So one of the main reasons that 1 ATM subs are so heavy, is that they need to counter act the buoyant forces that the air in the interior is exerting upwards. So that means to me that all the air in the ambient subs cabin is mostly pushing upward on any surface that's there. This is why I was thinking that a hatch and/or any windows and frame work can handle that amount of buoyant pressure that is trying to lift the sub, and will do so, if you release a drop weight. For some large ambient cabin spaces, I would think that pressure might be substantial. Any thought? I wouldn't be surprised if I missed some thing.
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>Your resident bubble dweller ;)'Regards,Szybowski
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>To: personal_submersibles@psubs.orgSubject: Re: [PSUBS-MAILIST] depth indicatorDate: Fri, 29 Aug 2008 09:55:57 -0400From: djackson99@aol.com
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>I think the correct answer is "almost" no difference. Take a 4 ft pipe. Cap one end and anchor the open end to the bottom of a lake so that the cap is just below the surface. Now fill the pipe completely with air. At the open end of the pipe the air pressure is equal to the water pressure and a depth indicator will read the same if it is inside or outside the pipe. But at the top of the pipe the water pressure outside the pipe is zero and the air pressure inside the pipe still indicates the pressure for 4ft of water. So my dive computer outside the view port will indicate the depth of the view port, and inside it would indicate an additional 3+ feet, to were I release air into the surrounding water.
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>Best of LuckDoug www.submarineboat.com
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>-----Original Message-----From: Jay K. Jeffries <bottomgun@mindspring.com>To: personal_submersibles@psubs.orgSent: Fri, 29 Aug 2008 5:01 amSubject: RE: [PSUBS-MAILIST] depth indicator
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>In ambient sub senses the same pressure inside or out so there is no difference having the gage inside or out.
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>R/Jay
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>Respectfully,
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>Jay K. Jeffries
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>Andros Is., Bahamas
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>Talk sense to a fool and he calls you foolish.
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> - Euripides (484 BC - 406 BC)
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>From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of djackson99@aol.comSent: Friday, August 29, 2008 1:08 AMTo: personal_submersibles@psubs.orgSubject: Re: [PSUBS-MAILIST] depth indicator
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>Hmmmm... Not the weight of the sub. It should indicate the depth of the vent where the air is released into the surrounding water and likely a little deeper since the constant flow of air may add a few psi to the ambient cabin. Air at the top of the cabin is at the same pressure as the air at the bottom of the cabin where it is vented to the surrounding water. So it does not matter where the gauge is inside the cabin.That also makes the air at the top of the cabin at a pressure higher than the water surrounding the top of the cabin. So a hatch in the top of an ambient sub seals naturally if it opens inward. Completely opposite to a 1ATM sub.In my case the dive computer is going to ride along outside the forward view port. There is no room for it inside and with it outside it will automatically start logging the dive when it submerges.So what are you thinking of building?Doug Jwww.submarineboat.com
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>-----Original Message-----From: David Bartsch <dbartsch2236@hotmail.com>To: personal_submersibles@psubs.orgSent: Thu, 28 Aug 2008 11:11 pmSubject: [PSUBS-MAILIST] depth indicator
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> In an ambient submarine, if one uses a dive watch type depth indicators to give an indication of how deep He is, does this display the actual depth or will it be off a little having the additional weight of the sub working on it? One of those HMMMM thoughts... David Bartsch
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>Be the filmmaker you always wanted to beearn how to burn a DVD with Windows. Make your smash hit=
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