The presence & pressure of gas's

Discussion in 'Stability' started by ted655, Apr 22, 2008.

  1. ted655
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    ted655 Senior Member

    Not being an engineer, I often ask stupid questions.
    In general, can someone explain the properties of gas's in an enclosed container? Taking a 1 cu. ft. sealed box/tube, how is is it's buoyancy affected by the insertion of different gas's? The box, with air, no pressure? The box, pressurized with 100# of compressed air? The box with a vacuum? Nitrogen, helium, hydrogen?
    I'm not able to think this thru on my own. Is displacement a simple factor of size X 62/64, or does contents in a sealed vessel affect it's buoyancy to a useful degree.
    do pontoons, (as a for instance), carry more weight if air is compressed or another gas is inserted? Will a football weigh less if another gas is used to inflate it? Will it fly farther?
    .
    I'm thinking as a useful application of boxes /tubes installed on cabin tops & decks as self righting implements & wondering about manipulating the buoyancy of such tubes or boxes:) .
    Thanks, Ted
     
  2. Guest625101138

    Guest625101138 Previous Member

    The density of air at standard temperature and pressure is 1.2kg/Cu.m. The density of saltwater is usually taken as 1025kg/Cu.m. Roughly 800 times more dense than air.

    Air compressed to 100psi (say 7 atmospheres) without heating will be 7 times more dense than air at 1 atmosphere or approx 8kg/Cu.m.

    A vacuum weighs nothing. However the structure required to support a vacuum has to be able to withstand around 15psi so this will add weight to your tubes.

    Hydrogen has a density of .09kg/Cu.m at standard conditions so close to nothing. Very buoyant in air hence why it is good for balloons.

    I cannot see any benefit in using gasses other than unpressurised air. For example if you had 1Cu.m of tubes weighing 25kgf fully immersed then the uplift would be 1025kgf less 25kgf for tubes less 1.2kgf for air equals 998.8kgf. If you replace the air with hydrogen then the uplift increases to 999.91kgf. Not much gain for all the effort it would take to fill and seal in hydrogen.

    A good solution for this type of requirement is foam core with a thin fibreglass skin. The polystyrene foam commonly used has a density of 30kg/Cu.m so you can get an uplift of about 990kgf with something that is simple to construct and retains buoyancy even if the skin is damaged.

    Rick W.

    at
     
  3. alan white
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    alan white Senior Member

    What he said.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Displacement is a direct function of cubic volume. The contents have no bearing, except in the weight they'll subtract from the volumetric efficiency (as buoyancy) of the immersed mass. In other words a cubic foot of air, sealed into a box is more efficient as buoyancy (floats higher), then a cubic foot of foam, weighing 3 pounds (for example), which must be subtracted from the amount of buoyancy the sealed box provided on volume alone.
     
  5. TAH
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    TAH New Member

    Fugacity,,,,,,,and not a bad name for a boat.
     
  6. ted655
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    ted655 Senior Member

    Thanks. It is making more sense now. I guess I am still confused about submarines & PFDs. The displacement of a self inflating vest is a fixed figure. The materials are present at all times. When triggered by moisture, the vest inflates and the buoyancy greatly increases. Submarines rise & fall by blowing water from ballast tanks, yet they act independently of the atmosphere. I see now, the subs actually displace tank water , releasing high atmosphere air to lower pressures, then, (I assume), compressing back into heavier air mass.
    The PFD is more confusing. It's container is allowed to decompress but does not displace any water.
    I'll puzzle on the information some nore, Thanks.:)
     
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  7. charmc
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    charmc Senior Member

    Rick is right on the big advantage of foam; an air space must be air tight to be effective, while marine foams are "closed cell", meaning they trap air in millions of tiny "compartments" so damage to outer skin has little or no effect on flotation.

    Although lowest in cost, polystyrene may not be the best for safety floatation foam use. Here is a good article on foam recommendations for marine floatation:

    http://www.glen-l.com/weblettr/webletters-7/wl55-flotation.html
     
  8. charmc
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    charmc Senior Member

    Ted,

    This is oversimplified but reasonably accurate: The mass/weight of a PFD is fixed, but the volume of water displaced is variable. Before the PFD cartridge is triggered the vest is essentially flat, having length and width, but hardly any depth, therefore very little volume. As the cartridge triggers, the PFD acquires a "third dimension" because it expands. Now the same mass/weight displaces a much greater volume, so it floats.
     
  9. ted655
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    ted655 Senior Member

    :idea: Duh! No moss grows on me. NOW, i get it. volume is the part I overlooked
    Thanks for the foam link also. Foam is a BIG subject in my little group of amateur :) boat builders.
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    except in some particular cases. Like nitrogen or CO2 to prevent corrosion or to protect from fire. Argon for thermal insulation...
     
  11. Guest625101138

    Guest625101138 Previous Member

    Agreed but the discussion is on buoyancy - not fire suppression or insulation.

    Argon is also useful for inert gas welding. Nitrogen prevents vegetable oil from going rancid. We could go on for ever about the virtue of all types of gasses but the topic is buoyancy.

    Rick W.
     
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  12. lazeyjack

    lazeyjack Guest

    Always learning from your posts ole son:)) thanks
     

  13. Guest625101138

    Guest625101138 Previous Member

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