Bouyancy Characteristics

[WildBill]

Hello All, I have a question for the room.   If water is approx 8 lbs per gallon, (assuming gravity is relatively constant around the globe), and Atmospheric Pressure is in the mid teens (bar), then would a 232 cu inch solid mass, say.... Styrofoam....which would weigh maybe 2 lbs, exert the same 6 lbs of lift from ANY depth? Obviously depth pressure will come  into play at extreme depths, compressing the mass, but, what if it couldn't be compressed?  Any comments are appreciated.
                                              Thanks, Bill.

[WildBill]

Thanks P-Motion,
      So, If I understand this correctly, If a diving bell were able to withstand the compression at say 1 mile deep, the uniform pressure on it would be approx 2300 psi correct? Now, since the bell maintains its size(or displacement), I must assume that the waters density hasn't altered, because it won't compress. Is this correct? I Suppose the correct question in this example would then be, Does it require the same amount of lift effort to lift the bell from 5280 feet as a it does from 50 ft.

[WildBill]

  WildBill,
       To lift a diving bell, it would depend on how much compressed gases are in it.

This I don't undertand. If a hull structure maintains its physical size, or displacement, wouldn't it then only depend on the amount of ballast within the bell making it heavier? internal pressure should have no bearing on lift should it? or am I missing something?

[mscoffman]

Both you guys were right in the first place. An object is buoyed up by the weight of the fluid
(water) it displaces. So styrofoam floats upward in water by the weight of the volume of water it
displaces, independent of depth. This is due to the fact that fluids and correctly designed solids
are called incompressible. As you know solids are incompressible only within certain ranges and
it turns out the same is subtly true with liquids (they are only somewhat incompressible)

Notes:

Obviously gases are compressible and therefore the gasses volume is directly proportional to
*absolute* pressure (and temperature). The atmosphere has a weight of 30inches or mercury
or 33ft of water. A vacuum can therefore suck water 33ft up a pipe at sea-level. A diving bell
shell protects it's contents from high pressure as long as it stays intact. That deep sea
submersible was filled with gasoline as it's floatation device for the reasons given above.

An object that changes it's displacement at depth has to lift the whole column of fluid above it,
even though it's upward buoyancy changes only by the weight of the fluid it displaces.

:S:MarkSCoffman

[WildBill]

Thank you very much too both of you guys!! That definately clears up my understanding of that!! Back to the drafting table