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80/20 or 100% O2 for decompression

Discussion in 'Ask Dr. Decompression' started by SharkFan, Dec 6, 2000.

  1. SharkFan

    SharkFan Guest

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    Should decompression be accomplished on pure O2 or on an 80/20 mix?

    Some of the tech agencies teach deco on 80/20 because a sudden change in the weather or loss of buoyancy control due to emergency conditions can make deco on 100% O2 dangerous. They are also beginning to argue that breathing 100% O2 for extended periods can have harmful effects on lung tissues.

    George Irvine argues that decompression should be accomplished with 100% O2. His arguments, although presented as a "Baker's Dozen" actually boil down to just a half dozen palpable arguments:

    1. Decompression with mixes less than 100% is inefficient.

    2. Decompression with 80/20 allows divers to compensate for poor buoyancy control with a longer decompression.

    3. The 80/20 mix is actually incorrect for 30 feet as that provides a pO2 of 1.52.

    4. The 80/20 mix provides a pO2 of only 1.28 at 20 feet and 1.04 at 10 feet, which he claims is "worthless for decompression."

    5. In a diving emergency that causes decompression to be shortened, 80/20 does not provide as large a safety margin as 100% O2.

    6. If buoyancy control is a problem at 20 feet, why isn't it a problem at 30?

    In response to arguments that breathing pure O2 has measurable negative effects on the lungs, Irvine's followers argue that you can take occasional breaks from breathing the pure O2 by breathing a little backgas. This argument appears to negate the principal argument in that breathing backgas provides a large return dose of nitrogen.

    T'anks.
     
  2. danreind

    danreind Guest

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    Hi Sharkfan,

    I'm not going to pretend that there is a right answer. Consider for a moment that during decompression two competing incentives are occuring. The shallower you are, the better your body can get rid of excess inert gases. However the downside is that the shallower you are the more bubbles want to grow, and if you go deep enough the bubbles will shrink. So during decompression one must find a balance between these two effects. But oxygen is essentially "burned up" by the body and doesn't really count for table calculation. So with pure oxygen you want to be as deep as you safely can be.

    In the case of oxygen there are toxicity concerns, there is a maximum "safe" partial pressure for breathing (though it is a fuzzy line), usually drawn between 1.2 and 1.6 atm. Ideally one would breath this much O2 throughout the decompression, and that is exactly what we do with rebreathers. In the case of 100% vs. 80% O2 there is a trade-off. The 100% maximizes inert gas removal but forces you to be shallower, while with 80% O2 inert gas removal is reduced slightly but you may decompress deeper for the same O2 limit. For the lung tissue concerns it isn't a question of the fraction of oxygen, but of the partial pressure of oxygen breathed - again the limit is reached deeper for 80% than 100%. Part of the 100/80 dilemma is a residue of our attachment to 10 foot intervals.

    Either approach will work, but one point to consider is what happens if you were forced to breath your O2 on ascent after a catastrophic loss of breathing gas. At 60 ft on pure O2 your ppO2 is 2.8, while on 80% it is 2.2. Both are dangerous but 2.8 is *really* dangerous, the risk really ramps up fast at high levels. Here a little inert gas goes a long way.

    Also for point #6, Boyle's law is why buoyancy control is worse at 20 ft than 30 ft. Also I've very often seen divers gain buoyancy as their tanks drain, only to become very buoyant when they reach the shallows where the neoprene expands again leading to a runaway ascent.

    Cheers,

    Dan Reinders
     
  3. SharkFan

    SharkFan Guest

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    Thanks, Dan.

    As for your response to #6, I think I oversimplified Irvine's argument. Remember, as I noted in #2, Irvine believes the 80/20 is merely compensation for poor buoyancy control skills. One response to this has been the rough seas argument. You're diving on the Andrea Doria, and you come up to find your self in 8 to 10 foot seas. Now, even for the best diver, maintaining depth becomes somewhat difficult. Irvine's response is two-fold:

    1. Don't dive in rough seas (which I consider a foolish response; sea conditions can be unpredictable).

    2. If a sudden sea state change makes buoyancy coutrol problematic at 20 feet, does it not also become so at 30 feet?

    Personally, I think your answer is still correct. Buoyancy control is more difficult at 20 feet in this case for the same reason that buoyancy control is *always* difficult at 20 feet.
     
  4. Dr Deco

    Dr Deco Loggerhead Turtle

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
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    My thanks to Dan for the comments made earlier, especially concerning buoyancy control (which is not a decompression physiology problem).

    It is certainly true that decompression is less effective if there is nitrogen in the breathing mix. The question is by how much is the effectiveness reduced. Twenty percent nitrogen can be shown to not be a so bad, and I would hardly term it “worthless for decompression.” Likewise, it is not inefficient. Deco times are simply a bit longer. It might be necessary to know exactly what was in mind; decompression stops at thirty feet are not in the usual recreational diver realm.

    The effect of oxygen on the lungs is not particularly great within the durations normally seen for recreational diving. When I worked in diving research in Germany (at the DFVLR from 1977 to 1980), we employed very high oxygen partial pressures (time average 1.9 ATM. over a period of several hours with no adverse effects to the dive subjects. {Powell, Thoma, Fust, and Cabarrou. Gas phase formation and Doppler monitoring during decompressions with elevated oxygen. Undersea Biomed. Res., 10 (3), 217-224 (1983).} High oxygen partial pressures are usually a problem of central nervous system toxicity and do not engender pulmonary (lung) difficulties.

    Is this a "tempest-in-a-teapot" issue??? :confused:
     
  5. Norm

    Norm Barracuda

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    Very good posts. Extremely interesting points. I personally got into the oxygen deco thingee before I knew of George's ideals but I have to agree with a lot of what he says. Much of it depends on the deco needs you have. If you are talking safety stops verses decom from a DEEP dive, that can be two different things. Coming up from a deep dive, going to 50/50 until you get shallow enough to use 100% is the only way to go. I began this line of thinking this past summer and it works! (The USN has become extremely interested in it as well.) Otherwise, just to make a dive safer, I suppose the nitrox thing would be just fine. I would love to read more about your research with oxygen exposure sometime Doc. Thanks.
    Norm
     
  6. Remora

    Remora Nassau Grouper

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    Hey Shark Fan!

    When we are doing deeper stuff we usually have 100% rigged on 14' hoses from the boat. We dive with 80% on the left and travel mix on the right.
    If upon ascent the sea conditions do not allow for a safe deco at 10' we will complete the deco obligation on 80%at 15-20ft. We plan the dive for both cases and write the deco obligations on our bottom timers. We also practice deep stops.
    You should be weighted so you can easily hang at 10' with no bouancy problems.
    Sombody once told me that no matter who says what at the end of the day you are the person behind the mask. No one else will be there when the SHTF.

     

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