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Bouyancy Mystifies Me

Discussion in 'Basic Scuba Discussions' started by pauldw, Oct 19, 2019.

  1. pauldw

    pauldw Solo Diver

    I hesitate to post this, but I don't grasp how buoyancy works. I do understand how to get positively buoyant, negatively buoyant, and neutrally buoyant; I'm not talking about that. What's confusing is the physics. Help?

    For example, people become more buoyant as they breath out their tank over the duration of a dive. Why? The tank itself hasn't changed shape, so it's displacing the same amount of water over the duration of the dive (if anything, it seems like it would have shrunk slightly over time with less pressure on the interior walls, and therefore displace a teeny bit less water over time). I gather that it's lighter over time, because air weights something and there's less air in it, but if buoyancy is caused by water displacement, how exactly is the amount water being displaced by the effectively unchanging physical size of the tank any different?
  2. FreeFlyFreak

    FreeFlyFreak Solo Diver

    # of Dives: 25 - 49
    Location: California
    The tank displaces the same amount of water but the tank weighs less because of the weight of the air used.
    Hence the tank becomes more buoyant or less negative the emptier it becomes.

    Google Archimedes' principle.

    Its the same as a boat riding higher or lower in the water depending on the weight loaded on the boat.

    In other words. Buoyancy (or lack thereof) is the difference between the weight of an object and the weight of the water it displaces.
    eleniel, markmud and OceanEyes like this.
  3. Ministryofgiraffes

    Ministryofgiraffes Loggerhead Turtle

    # of Dives: 1,000 - 2,499
    Location: Toronto
    ‘Archimedes principle’

    Discussion over. :wink:
    eleniel likes this.
  4. The Chairman

    The Chairman Chairman of the Board

    # of Dives: I just don't log dives
    Location: Cave Country!
    The volume remains the same, yet it becomes lighter, thereby decreasing your overall density.
    eleniel, markmud and FreeFlyFreak like this.
  5. DeepSeaExplorer

    DeepSeaExplorer Solo Diver

    # of Dives: 500 - 999
    Location: Cave Country
    I’ll give it a shot...

    The air stored in the tank has weight, about 6lbs for an aluminum 80. As you breathe and exhaust that air, the tank weighs less.

    As you point out, the tank is a fixed size, so it’s water displacement doesn’t change. Since the tank’s displacement doesn’t change, but it’s now lighter, it is now less heavy in the water. You sense that as positive buoyancy.

    An empty aluminum 80 will float because it displaces more water than it weighs.
    markmud, Jack Hammer and FreeFlyFreak like this.
  6. pauldw

    pauldw Solo Diver

    OK. I may have been missing the obvious. Thanks!
    DeepSeaExplorer and FinnMom like this.
  7. Scuba Lawyer

    Scuba Lawyer Manta Ray

    # of Dives: 5,000 - ∞
    Location: Laguna Beach, California
    It mystified Archimedes too until he stepped into a bathtub one day. :)
    eleniel, markmud, OceanEyes and 6 others like this.
  8. The Chairman

    The Chairman Chairman of the Board

    # of Dives: I just don't log dives
    Location: Cave Country!
    Bouy was that a surprise.
  9. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    The other fun aspect of buoyancy involves calculus....

    Buoyancy is a force, and F=ma, so more buoyancy means an upward acceleration. The integral of acceleration is velocity, so after a delay, the buoyancy gives an upward velocity. The integral of velocity is displacement, so after a delay the velocity produces a upward displacement. This means you don't immediately go up if you breath in or push the button on your LPI. It takes awhile for the double integration to have an effect. By then, you may think you need more buoyancy, when in fact you just need more time for the buoyancy you have to have an effect you can feel.
    pauldw, eleniel, markmud and 2 others like this.
  10. BoundForElsewhere

    BoundForElsewhere Waiting for the zombies ScubaBoard Supporter

    # of Dives: 50 - 99
    Location: NYC
    Oh buoy.
    Norwegian Cave Diver likes this.

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