calculate pressure and flotation

[rasorinc]

Hello, I need help on a flotation question. I can calculate the volume of a tube. However, I cannot calculate how high pressure increases flotation.
EX: a 12" round tube 4' long. If the cap is left off so air can enter it will be 1 atmosphere of 14.5 psi?? and it will float I raise the pressure to 30psi and it floats much higher. There has to be a simple equation for that.....???
Go easy on me my math is out of date. Thanks much in advance. Stan

[LyndonJ]

Pressure increases the volume and then it floats higher for the same displacement. There's no other factors here.

[rasorinc]

Thanks LyndonJ, so if you double or triple atmosphere you raise your flotation equally? Stan

[LyndonJ]

Only if the outside dimensions change. Its just a balloon only less elastic, as the pressure goes up so does the volume.

[tom28571]

Flotation will significantly change only if the tube changes size with pressure. There will be a tiny change for a rigid tube but remember that the 14.7 psi means a column one inch square from sea level all the way to the top of earth's atmosphere. You are working with negligible differences here. Same thing if you draw a vacuum on the tube.

[kmorin]

rasorinc,

At sealevel, in middle latitudes, our atmosphere exerts 14.73 lb.s per square inch on the sea/ground and that is often labeled 14.73 psi(A) for absolute pressure. It represents the wt of that column of the normal mix of the atmosphere which gets thinner and lighter as it is measured closer and closer to the the top (space) as Tom points out.

If you add more molecules of air, say up to 30 PSI [or 43.73PSIA] you've increased the wt of the air inside the fixed volume cylinder. If the tube is a rigid metal tube it will actually sink in the water not raise. Admittedly, the amount it will sink is very, very slight because the added molecular weight of increase in atmosphere you'd compressed into the rigid cylinder- but a rigid tube will sink- not rise. The amount of buoyancy or the weight of the displaced water is not changed [rigid shape] but the mass being floated is increased by adding molecules (they don't weigh much its true- in fact a column of them miles high only weigh's 14.73 lb.) so the object will settle to a higher waterline; unless it can change volume/shape.

Now, if the tube were not rigid, so the increase in the relative pressure inside-to-outside could expand the tube, then the increase in molecules' mass added to stretch the elastic/rubber tube is (much, much) smaller than the increased shape's displaced water increase. So if the tube can expand, without significantly increasing its all up mass/wt. then it will float at a lower waterline- or rise out of the water.

hope this makes sense?

cheers,

[Guest625101138]

Increasing the air pressure in a rigid tube will actually increase its weigt very slightly and it will need greater immersion to support it.

If it was totally evacuated then it would be lighter by 1.2kg/Cu.m. Filling it with hydrogen at atmospheric pressure is almost the same as having it at total vacuum. Helium is a lot safer but slightly more dense than hydrogen.

Rick W

[rasorinc]

I want to thank all of you for your answers. I now understand that increased pressure with out expansion is heavier. I was very much thinking in the wrong direction. I have learned a lot from you answers and will be thinking along the lines of expansion now rather than rigidity and increased pressure. Again thanks much, Stan