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Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air.

Discussion in 'Advanced Scuba Discussions' started by Boston Breakwater, Jul 7, 2020.

  1. wstorms

    wstorms ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Netherlands
    for air:
    pO2 max 1.4 bar --> MOD 56 meter
    pO2 max 1.4 bar + pN2 < 4 bar --> MOD 40 meter
    pO2 max 1.4 bar + pN2 < 4 bar + gas density max 5.2 g/L --> MOD 30 meter

    Mostly a matter of definition. For me, the MOD is the max depth at which I can breath the gas, not the target operating depth. Based on that, I would go with 56 meter, but I can understand why people have different answers
  2. Dutch-Diver

    Dutch-Diver Garibaldi

    # of Dives: 1,000 - 2,499
    Location: Germany
    1.4 / 0.21 = 6.66667 bar ambient pressure which is 56m or about 184 ft.
    if you go to 1.6 it will be 1.6 / 0.21 = 7.72 bar which is 67m or about 220 ft.

    Time at the max depth is the most important factor and it´s different for everyone and not allways the same.
    So lot questions that make it difficult say when it will hit. advice: stay away from these limits.

    Anyway long before the Oxygen causing you trouble the Nitrogen will do.
    While being in 56m of depth with air over a longer time is not healthy.
    markmud and Boston Breakwater like this.
  3. JimBlay

    JimBlay Divin' Papaw ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: South Florida
    For me I'm limited by narcosis. I won't dive air below 120' or so. I don't do any dives that deep these days but if I did and wanted to remember it I’d go with 21/35 trimix.

    Same MOD though as air. I’d use 1.4 since it would be a technical dive and most of the bottom portion of the dive would be near the MOD. MOD of 21% at 1.4 is 187’.

    I dive a MOD of 1.5 on recreational Nitrox dives with the majority of the dive being much less than 1.5.
  4. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    @halocline the argument for helium below 100ft that @PfcAJ has basically nothing to do with ppO2. @Boston Breakwater saying something off topic is not where that post went sideways.
    We have 2.5 variables that weigh into MOD for any particular gas
    First and foremost is ppO2 which is heavily emphasized in nitrox courses due to oxygen toxicity and is unfortunately the only variable that is really taught in most courses. That ppO2 limit is personal preference and can go anywhere from 1.0 to 2.0. 1.6 is the accept MOD that is used for marking bottles. In that case, Air has a MOD of 220ft.

    Once we start getting into trimix classes, narcosis starts becoming very important, and when we start to talk about deep air it is obviously very important. A combined ppO2+ppN2 less than 4 is my personal preference. Research is saying that O2 is just as narcotic to us as N2 and where best mix for trimix used to only set N2 to some limit, I think it is much more prudent to set the combined partial pressure to 4. That happens to be 100ft, and that is the point where symptoms of hubris, dark narc, memory issues, and overall processing power/situational awareness become an issue. Again, you can set that limit to whatever you want, but the agencies normal recommendation is to set ppN2 to a max of 4.0, or set a combined ppO2+ppN2 to 4.0. In this case the MOD of air would be 132fsw or 99fws.

    The half variable. I say half because it is really a subset of the narcosis variable, and that is gas density. Why does gas density matter? When you are trying to breathe, and it gets harder to breathe, you have a natural tendency to start breathing shallow which leads to CO2 retention in the lungs. CO2 is 20x as narcotic as nitrogen and can lead to all sorts of issues. CO2 is a scary effing gas and downright deadly. This is part of the reason that many divers consider DPV's a required piece of safety equipment when diving to trimix depths in order to limit the production of CO2 by minimizing exertion. It's the reason that many well known divers have died, most notably David Shaw, and I firmly believe is the real reason that most people get narc'd, not nitrogen. Unlike N2 though, it doesn't clear immediately after you get to a shallower depth, and it has lingering effects *my least favorite is a horrific headache at the base of your skull that lasts for over a day*. Based on that, the MOD for air is something that you have to choose for yourself. For me? It is variable. I'm much less reluctant to dive air to 180ft on a lazy drift dive in a bathing suit than I am to do a 180ft cave dive or even to go that deep in our local lakes which are cold and dark. I stand with AJ that below 100ft I want to have helium, though I don't consider it a hard and fast rule. With CCR now I have a lot more helium in a lot sooner than I would on OC and it's noticeably better.

    I'll posit these questions to you @Boston Breakwater
    Why are you curious about this subject when the industry has said that you shouldn't breathe air deeper than 100-130ft depending on the agency?
    If you're curious because there is some sort of dive you want to do and are thinking about doing it on deep air, where is the dive, how deep is it, and what are you wanting to do on said dive?
  5. halocline

    halocline Solo Diver

    What agency has said you shouldn't breathe air deeper than 100ft and when did they say it?

    All I said was that for my tastes, switching to trimix for dives over 100ft is too conservative, and there have been many many many thousands of recreational dives on air deeper than 100 ft to support my opinion. The major dive industry agencies have for decades described recreational diving limits at 130 ft. This predates nitrox use and continues in the present.

    I understand that technical divers with access to expensive helium and/or CCRs may prefer to use helium deeper than 100 ft, but that hardly makes it the industry standard.
  6. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    GUE and UTD are two such agencies. UTD said it from its inception because its founder came from GUE.
    KWS and Boston Breakwater like this.
  7. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    Technically none of them since the agencies that say helium starts at 100ft don't breathe air, as EAN32 is their standard gas. As @boulderjohn said, it's the DIR agencies, so GUE/UTD/ISE/etc. all say trimix below 100ft. GUE was the first to say it, and likely said it close to their inception. The others that spun out of GUE like UTD/ISE who were both founded by former GUE big wig instructors will have been since their inception. I'm sure there are some others out there, but those are the "big 3" DIR agencies.
    I believe everyone else is 130ft except for the specific narcosis management courses from agencies like PSAI.
  8. 100days-a-year

    100days-a-year Solo Diver

    # of Dives: 5,000 - ∞
    Location: NE Florida
    P02 is a hard fact. Narcosis is pretty subjective and variable.
    Been past 1.6 on air and did not like it, narcosis was noticeable as I was working.
    My experience is that in 40fathom grotto on air in cooler waters wearing a thicker wetsuit the narcosis was worse. In warm clear waters with no current I stayed above 180' for the most part. I have buddies who are more conservative and a couple who are less.
    I have quite a few videos of me at work at different depths, pretty easy to see any changes in behavior.
    As far as oxtox, never experienced any symptoms so I try to stay below 1.4 as I'd hate to have my first be my last.

    And as always my disclaimer " Some people are impaired at 1 atmosphere "
  9. barth

    barth DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Netherlands
    When using a CCR, GUE says use trimix deeper then 15 meter or 50 ft.
    grantctobin likes this.
  10. Dominik_E

    Dominik_E Barracuda


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