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Simple Theory Question

Discussion in 'Advanced Scuba Discussions' started by time2sail, Oct 4, 2018.

  1. time2sail

    time2sail Regular of the Pub

    # of Dives: 100 - 199
    Location: South Florida
    Let me apologize in advance if this has already been discussed ad nauseam in another post.

    Two divers diving at the same depth, let's say 100', diver 1 starts the dive with 80CF of gas, diver 2 starts with 120CF of gas. Both get back on the boat with 15CF of gas remaining. Seems to me that diver 2 would have more nitrogen build up and may possibly have a greater deco obligation. Or would diver 2 be off gassing quicker due to a higher SAC? Of course this would never happen because we are taught to get back on the boat with 500psi remaining...which would mean there'd be more gas left in a 120 than in an 80, but I tend to like to do all my calcs based on actual CF.
  2. sigxbill

    sigxbill Tech Instructor

  3. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    It sure seems that way, doesn't it? But that is not what happens.

    Nitrogen on-gases and off-gases depending upon the gradient between the gas in the tissues and the gas being breathed. This happens by diffusion--the gases essentially migrate around wherever they please, meaning that if there is more nitrogen in the air being breathed than in the tissues, more gas will migrate into the tissues than out of it, thus giving the tissues more nitrogen. It isn't being forced in there like water going through a hose.

    So at a certain depth, the air you breathe has a certain amount of nitrogen in it, and that determines whether you are ongassing or offgassing. It doesn't matter how fast you are breathing it.

    The only exception to this is if you are breathing heavily due to exertion. In that case, your blood will be pumping more vigorously, carrying nitrogen more efficiently. In that case, the difference is due to the increase in blood flow, not the rate of the breathing.
    TrimixToo, PatW, markmud and 2 others like this.
  4. dmaziuk

    dmaziuk Orca

    Think rates rather than amounts: if one diver is twice the size of the other, it'll absorb twice as much nitrogen -- to get to the same gas loading. That's how the model sees it, that is. :D
    dberry likes this.
  5. JohnnyB108

    JohnnyB108 Angel Fish

    # of Dives: I just don't log dives
    Location: Oregon
    Gas theory is still huge with the study of tissue absorption. Different tissues absorb less or more than others. As similarly it is for divers of varying exertion, breathing and size. That’s why it’s all still called dive theory. Every certifying agency sets a model standard that keeps everyone in the safe area regardless of the actuality of individuality. Remember dive tables, then the progress to computers? You’d dive your table and every subsequent dive based on your deepest dive regardless of how long you were at that depth. There are still a lot of studies being done in gas theory for divers.
  6. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    you mean the model as in deco model? it just calculate time and depth only? I dont think it caters for size or sac
  7. dmaziuk

    dmaziuk Orca

    Right, more accurately: pressure. You need twice as much gas to fill twice the volume to the same pressure, ergo a twice larger diver needs to breathe in twice the gas to achieve the same gas loading.

    Protect yourself from DCS: get fat.
    JohnnyB108 and Sh0rtBus like this.
  8. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    im sure we could spin this out for another 60 posts
  9. TrimixToo

    TrimixToo Regular of the Pub

    # of Dives: 200 - 499
    Location: New York State
    As this is the advanced forum, I have to say that I think there are probably other perfusion-related reasons than exertion (e.g., diver warmth at different dive phases) that can affect inert gas transfer.

    Nonetheless, in a nutshell this excellent explanation is another reminder to me about why, were I to start over again (hey, at least one of them was born by 1972), I'd like to have learned from John Adsit or Trace Malinowski or someone like them. I flirt with the idea of becoming a destruct...er, that is, instructor from time to time, but these guys are really good at it from what I can see here and their reputations.

    And, for the record: No, I've never met either of them as far as I know.
    boulderjohn likes this.
  10. RyanT

    RyanT Solo Diver

    # of Dives: 500 - 999
    Location: Maryland
    The rate of diffusion is influenced by the steepness of the concentration gradient (among other things). So if a diver inhales a lung full of air, the initial point where the fresh burst of gas reaches the lungs is when the concentration gradient (difference) between the inspired gas and the blood stream will be the greatest. As the nitrogen in that breath diffuses into the blood, the steepness of the concentration gradient declines and diffusion in that breath of gas will begin to occur more slowly. So in theory, diver X who breathes substantially slower than diver Y, will have lower nitrogen loading as they should be taking advantage of the lower concentration gradient (diffusion rate) at the end of the breath. My guess though is that the actual difference in nitrogen loading between the two divers is small enough to have no practical significance. And indeed, I'm not aware of any evidence that suggests that higher breathing rates increase nitrogen loading in any kind of meaningful way. In addition, even a very slowly breathing diver will fail to completely equalize the gas distributions between the lungs and blood stream, so much of the inspired gas will never even reach the blood stream anyway.
    JamesBon92007 likes this.

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