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The partial pressure of gases below sea level?

  1. Nov 5, 2009 #1
    Hello everyone,

    The partial pressure of the atmosphere 33 feat benath the surface is doubled the normal amount. (2x760 mmHg). I can understand how lower you go down higher the pressure but I don't understand how you can measure atmospheric pressure under the sea. I mean under the sea you got water around you not atmosphere. Are the gases in atmosphere dissolved in this water. My question is mainly related to divers under the sea. They get double the atmospheric pressure of nitrogen and nitrogen narcosis could occur. So how do you get double the N2 in the atmosphere when you are surrounded by the sea. Also why do these effects occur when divers are using an oxygen tank. The gases inside the oxygen tank is normal atmospheric pressure right. Thanks :smile:
  2. jcsd
  3. Nov 5, 2009 #2


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    Staff: Mentor

    Scuba divers use tanks of compressed air. This air goes through the special kind of valve, called diving regulator - so that air that is feeded to the diver has exactly the pressure of surrounding water. So its pressure depends on the depth and is not constant.
  4. Nov 5, 2009 #3
    So the pressure inside the lungs is equal to the pressure outside the body. Is this because pressure outside pushes on you and lung increases the pressure by same amount to neutralize this. Please feel free to explain this more. So can lungs regulate pressure with out a tank. When you climb up a mountain does the lung regulate pressure to keep up with the low pressure outside. How does the lung do this and how does the lung neutralize the pressure acting all around your body.

    Thanks a lot for the help!!
  5. Nov 5, 2009 #4


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    Staff: Mentor

    More or less. In fact lungs are open to the air (just like an open bottle) so there is no way for the pressure in the lungs to differ much from the surroundings. When you are breathing you are creating some over- and underpressure to move the air, but these differences are small, and they are relative to the externall pressure, whatever it is.
  6. Nov 6, 2009 #5


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    They are just using atmospheric pressure as a reference pressure. At approximately 33 feet, the hydrostatic head pressure of the water is equivalent to 1 atm or approximately 14.69 psia (760 mmHg). Since atmospheric pressure is also 14.69 psia (760 mmHg), you have P_atm + P_hyd = 2x760 mmHg.

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