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Snorkel Breathing

  1. May 30, 2014 #1
    Hi,

    I have seen multiple explanations as to why there has to be a maximum length for functional snorkels.

    1. You inhale the same air that you exhale. (I.e. the so called "dead space" is too large).

    2. The extra pressure, provided by the depth, on your chest is too overwhelming for your muscles to perform work against.

    I'm sure both are factors, but which would dominate?

    I made a quick estimate for 2. which maximizes realistic length at about 0.5m (all below the surface).

    About 1. Wouldn't partial pressures equalize quickly enough through diffusion? Or is the gradient too small?

    1. and 2. are possibly coupled. Because if diffusion would be enough, number 2 would be somewhat redundant -unless- expansion through muscle work is required to increase diffusion area or decrease diffusion length or alter pressure (if it plays a role). If passive diffusion is not enough, convection would presumably be the main purpose of muscle work?

    #1 could possibly be overcome through breathing out through the nose while breathing in through the mouth.
     
  2. jcsd
  3. May 30, 2014 #2

    berkeman

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    Staff: Mentor

    Speaking as someone who has done the experiment (as a kid) in a pool, #2 is the only factor, IMO. If you use a small diameter hose, the dead space is not that large compared to your lung capacity.
     
  4. May 31, 2014 #3

    Borek

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    Staff: Mentor

    But if you use small diameter hose, it takes much more effort to breathe.

    Plus, you may be not aware of the fact you are breathing mostly the same air from the dead space.
     
  5. May 31, 2014 #4
    I get the part where the dead space has to be smaller than the "lung capacity" IRV + TV = IC (about 3.5 l) in theory, but then it must be the time to equilibrium that sets the limit. There's both a temperature gradient and a concentration gradient between the exhaled air and the atmosphere. Maybe Ficks law could be used to estimate it, but it requires an estimate of the diffusion coefficient.

    In my mind I've always pictured the Maxwell-Boltzmann distribution to justify a very quick diffusion process, where there are few collisions, but it doesn't seem to hold here?

    X2604-S-43.png

    TV is about 0.5 l, while the dead space present in the conducting airways (eg. bronchi, trachea) is about 0.15 l .
     
  6. May 31, 2014 #5
    The dead space problem could be easily overcome using 2 tubes and 2 flapper valves so that one tube only lets air down and the other only lets air up. The pressure problem is a real limiting factor.
     
  7. May 31, 2014 #6
    The second tube can be omitted. The air out valve is sufficient. But as it is impossible to blow out water in this configuration there must be an additional mechanism to keep water out of the tube.
     
  8. May 31, 2014 #7

    berkeman

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    I used a piece of common garden hose as a 10 year-old kid in the backyard pool, which is I guess a medium-diameter hose... :smile:

    I agree with this approach. Then the main problem is the water pressure, which becomes very apparent as you try to descend more than a meter or so...
     
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