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Nitrox for shallow water artifact diving??

Discussion in 'Advanced Scuba' started by calabash digger, Jun 8, 2021.

  1. ginti

    ginti DIR Practitioner

    # of Dives: 200 - 499
    Location: Lyon, France
    745
    447
    @JimBlay, this entire discussion is about underwater physiology, and its mechanisms are pretty complicated. Today, scientists are not able to describe all the involved phenomena. With no better knowledge, and no cost issues, I would also fill with nitrox, but this forum is a place to understand better, isn't it? So let's try :)

    Here's what I ask you: why would less nitrogen be better?

    Please notice that I am not saying you are wrong - I just do not understand what practical benefits one would have for the kind of dives we are discussing.
     
    JimBlay and calabash digger like this.
  2. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

    2,878
    2,661
    I have a good friend who has treated multiple working divers for DCS from roughly that depth and bottom time. They were extreme sawtooth profiles, but they still defied traditional thinking about the possibility of getting bent from that depth.

    Using nitrox decreases nitrogen uptake, which will increase the no-stop limit. Whether that makes a difference in practice depends on depth and gas consumption, e.g. the NOAA no-stop limit for 36% EAN at 40 fsw is 371 minutes, so there's no possibility of running up against that on a single scuba tank. The benefit there is in decreased nitrogen uptake, which will in turn make a difference on repetitive dives, so it should indeed be considered.

    The NOAA no-stop limit for 36% EAN at 90 feet is 48 minutes. By comparison, the NOAA no-stop limit for compressed air at the same depth is 30 minutes. Provided the diver can make a tank last for 48 minutes at 90 feet, the nitrogen load for both dives would be roughly the same, so the benefit of the 36% EAN is in extending the no-stop limit. But, if the diver's gas consumption rate is such that he or she can only make a tank last for, say, 35 minutes at 90 fsw, the benefit of using 36% EAN would be in decreasing nitrogen uptake, because he or she would be ascending well before the no-stop limit was reached.

    For recreational compressed air/nitrox divers who use dive computers and dive to the edge of the no-stop limits in the computer's algorithm no matter what gas they're breathing, the practical difference is only on ascent/decompression, as the higher O2 partial pressure in nitrox will provide a small decompression benefit.

    Best regards,
    DDM
     
  3. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    27,769
    21,460
    I think you are confused by the fact that tables, like PADI's, often have 35 feet as the shallowest depth on the table. That does not mean there is no NDL shallower than that. It means that you are to use the 35 foot limit for dives up to 35 feet in depth.

    On a 25 foot dive, you are supposed to use the 35 foot limit for your dives. I am not near my tables right now, but if you look at air tables and nitrox tables, you will see that the nitrox tables extend those limits quite a bit.
     
  4. Storker

    Storker ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
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    Concur.

    I have anecdotal evidence to the opposite. Anything that takes you further from the NDL, be it slow ascents, extended safety stops, r-e-a-l-l-y slow ascents after the safety stop or nitrox makes me feel better after the dive.
     
    Lostdiver71, StefinSB, Bowers and 2 others like this.
  5. Storker

    Storker ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    17,202
    13,600
    And to increase offgassing compared to breathing air. But that's often overlooked.

    If I've gone to the NDL, no matter if that was on air or 32%, I'm offgassing more at my safety stop if I'm on 32%.
     
    Ayisha and BRT like this.
  6. ginti

    ginti DIR Practitioner

    # of Dives: 200 - 499
    Location: Lyon, France
    745
    447
    Thanks a lot. I don't know why I wasn't thinking about repetitive dives, which are among the most obvious reasons to use nitrox.

    @calabash digger, so here's what I would do if I were you:
    - write down the duration of my average and my most extreme dives;
    - write down how often I do repetitive dives and the associated surface intervals;

    If decompression limits the duration of your dives, or if you want to reduce the surface intervals, you should go for nitrox. Even if you plan to use nitrox only a couple of times per year, you need to take the course.

    About the fatigue issue, most evidence is anecdotal, not empirical, so I guess it's up to you what to believe. I tend to trust scientists: if they didn't manage to clarify that nitrox reduces fatigues, I don't see any reasons to believe it.
     
    calabash digger and JimBlay like this.
  7. JimBlay

    JimBlay Divin' Papaw ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: South Florida
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    I'm not sure what exactly you want in a response. This is all Decompression 101.

    Here is my thinking:

    Less nitrogen is always a good thing as a diver. Nitrogen is not metabolized and does NOTHING but saturate various tissues leading to the need for those tissues to then desaturate. This is the whole basis for decompression and NDLs that we learn from the very beginning in our training as divers.

    Will the increased nitrogen in air relative to nitrox cause an issue at these shallow depths? As long as you follow the established dive tables/algorithms and ascent rates, no, it shouldn't. However, ANY increased nitrogen causes increased tissue loading which must then be unloaded resulting in "stress" on the various tissues within the body. Shouldn't be enough stress to cause issues as long as all established guidelines are followed, but tissue stress is tissue stress. Why introduce more than necessary on the body?

    I will always choose to reduce my nitrogen loading whenever possible even when diving well within NDL.
     
  8. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
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    Hopefully this will explain things a bit more.

    1. As soon as you begin to descend on a dive, your tissues begin to absorb gases as the pressure increases. If at the end of the dive your tissue pressure is too great in comparison to ambient pressure, especially upon surfacing, you are in danger of getting DCS.

    2. Tissues are able to have a certain amount of pressure greater than ambient and still be OK. Otherwise you could not ascend at all. Different tissues have different maximum tissue pressures compared to ambient pressure.

    3. It is generally believed that you can surface at sea level (1.0 ATA) with a 1.5 tissue pressure.

    4. As you ascend or do a safety stop, tissues that have a pressure higher than ambient pressure will off-gas, and tissues with a pressure lower than ambient will still on-gas. If you stop at a specific depth, they will on-gas or off-gas until they reach that ambient pressure. That is called reaching equilibrium. Once a tissue reaches equilibrium, it stays there until the diver changes depth.

    5. The faster tissues will reach that safe level quickly, often during the ascent. Slower tissues off-gas more slowly and need more time.

    6. Staying at safety stop depth allows faster, off-gassing tissues to get to 1.5, and the alower, on-gassing tissues cannot get above 1.5. The diver can then safely surface.

    7. A diver in the 20-35 foot depth range will be in a range of 1.7-2.0 ATA. If tissues reach equilibrium at that pressure, they are in a gray area of being at risk upon surfacing. The closer to 2.0, the more problematic, so that is why there is a NDL for divers in that range. If a diver stays too long at that depth, the slower tissues will have become saturated enough that they will not have time to off-gas during the ascent.
     
    jale, shoredivr, rjack321 and 3 others like this.
  9. ginti

    ginti DIR Practitioner

    # of Dives: 200 - 499
    Location: Lyon, France
    745
    447
    First of all, I basically always dive nitrox, for two reasons:
    - I often dive with GUE divers, and standardization involves a lot of advantages that I am not going to cover here (it's way OT);
    - when I dive with people who prefer air, I use nitrox just in case my plans after the dive change, and because it is easier for me to deal always with the same gas (easy to remember NDL, etc.)

    But I have good reasons to do it. Even if I believe that nx has so many advantages, I need to be clear with other people who are asking about its use regarding advantages, disadvantages and neutral aspects.

    Now, besides the above reasons, I do not see others that are always valid.

    Let's assume that one day I have to do a single dive, within recreational limits. After this dive, I do not have to do any kind of activities that involves environmental pressure changes. In this case, having more or less nitrogen after the dive doesn't make any difference to me. What do you think?

    EDIT: I understand that you consider that stress something negative; I agree with you, but it's our way to see it. At the end of the day, millions of people have used air without any health issues in the past and are using it even today.
     
    calabash digger likes this.
  10. BRT

    BRT not a soft touch

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    14,396
    If staying at 25' it is ongassing until saturated. The expectation is that the human body can withstand saturation at that depth and a quick return to the surface. Human bodies may differ.
     

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