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Diving air to 60m

Discussion in 'Advanced Scuba Discussions' started by John Bantin, Jul 17, 2019.

  1. Centrals

    Centrals Barangay Pasaway

    # of Dives:
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    Have you been to anywhere else where AIR is the only option? I have been to a few and as far as I know it is still the only breathing mix available in those places.
     
    shoredivr likes this.
  2. mafi

    mafi ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Copenhagen, Denmark
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    For like a millionth time - helium decreases
    • WOB
    • CO2 retention
    • narcosis
    Those are undisputed facts.

    No one is saying that you are safe in Trimix. But at 60m but, you are definitely safer on Trimix than on air. You might be a great diver (I certainly hope if you are doing such dives) and relatively safe after doing this a thousand times, but it's beyond me how could you state that you'd be safer on denser and more narcotic gas.

    I am not arguing about helium price, availability or convenience. Those are all great reasons not to use it. But saying you are safer on air is just bogus.

    If I may ask, have you looked at the gas density resources we share with you?
     
    leadduck and Rechno like this.
  3. fsardone

    fsardone Solo Diver ScubaBoard Supporter

    # of Dives: I just don't log dives
    Location: Rome, Italy
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    It is. There is noticeably much more effort in moving the gas in CCR, since OC all you need to do is keep the demand valve open by maintaining depression in your mouth. In the reb you need to overcome the viscosity and inertia of the gas in the entire loop and, according to the configuration and your trim, any pressure imbalance due to the counterlungs/lungs relative position.

    As such while OC I am happy to dive 50ish meters in air (benign conditions), CCR (conditions notwithstanding) I use some He starting from 30ish.
     
    leadduck likes this.
  4. mac64

    mac64 Barracuda

    # of Dives: 5,000 - ∞
    Location: Ireland
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    First thank you for the resources and video. I watched the entire Simon Mitchell video and the one thing he emphasized was the variation in individuals, he clearly stated that some people will subconsciously allow CO2 to build up by not ventilating enough.but others won’t 18.30min to 21.30min on the video. I have never got as much as a minor headache while diving on air in 50 years The only reference to depth he made was 10atm.
     
  5. helodriver87

    helodriver87 DIR Practitioner

    # of Dives: 200 - 499
    Location: Alabama
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    If it's the only option, it's the only option. I've been to a couple places like that. Still doesn't make it ideal.
     
  6. leadduck

    leadduck Barracuda

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    I think this is the general problem here: risk factors do not simply sum up, they influence each other, and one cannot draw a red line for single factors. 50m on air OC will be OK for me under benign conditions, but on a cold water night dive at an 80m deep wall with currents I wouldn't do 50m on air, probably not even 40m.

    Training agencies do make recommendations such as "40m is enough", but every diver with a little experience knows that
    (a) this is a gray area, problems with narcosis and WOB increase gradually with depth, so it's a personal trade-off between convenience and risk safety margins and there's no scientific hard limit
    (b) hence any depth limits on any gas are only recommendations,
    (c) no surprise training agencies do not even agree on recommended limits; for some 50m deep air is part of the training program, for most 40m on air is fine, some recommend to never dive air and require trimix below 30m
    (d) the total risk of a dive depends on many other factors, hence one cannot even determine a personal trade-off between convenience and risk without knowing other factors (overhead, currents, temperature, visibility, ...)
     
  7. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    I think it's important that you understand the physiologic consequence of what you have just said (and I'm not implying that you don't). For others reading along,
    a lack of respiratory response to elevated CO2 is NOT protective. Not getting a headache does NOT mean you are more resistant to hypercarbia.
    What it means is that at depth, after effort, with hypercarbia, but WITHOUT symptoms or an urge to breathe harder, you are closer to a point of hypercarbic toxicity, and you don't even know it.

    It is possible that your success thus far means that you are one of the non-responders, and have just been lucky. Your actions to avoid effort at depth are exactly the right thing to do. I don't see that one will always be able to avoid it, however. Keep in mind that you may be nearer the edge than you know...

    My 2psi...
    Diving Doc
     
    RyanT and helodriver87 like this.
  8. leadduck

    leadduck Barracuda

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    Note that the publication (Mitchell: Respiratory Physiology of Rebreather Diving) shows experimental results where between 6g/l and 7g/l half of test dives couldn't be finished.
    But these were working dives and I don't see the parameters (how much work, for how long), so we don't know how hard it was and how relevant the result is for recreational divers. Maybe divers had to pedal ergometers like in the NEDU deep stop study, and half of them couldn't do that for 30min breathing air at 45m? OTOH in the NEDU deep stop study they pedaled ergometers set to 65W for 30min at 51m/170ft on air and the paper doesn't mention any dive failure due to exhaustion or unconsciousness. So the other rebreather CO2 retention test was probably much more strenous than the NEDU study.
    Now if we assume that the open water deep air diver does not swim hard against a current for 30min at 50m but will be wise enough to go shallow before, it's quite plausible to me that he's never ever felt a problem nor even noticed increasing WOB because he never came close to the high CO2 levels seen in this working dive test on rebreathers.
     
    Dark Wolf and rsingler like this.
  9. mac64

    mac64 Barracuda

    # of Dives: 5,000 - ∞
    Location: Ireland
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    But my rate of breathing does change depending on what I’m doing. What I thought Simon Mitchell was saying is that some people don’t respond to CO2 build up and their rate of breathing stays the same hence CO2 starts to build. Have I missed something?
     
  10. fsardone

    fsardone Solo Diver ScubaBoard Supporter

    # of Dives: I just don't log dives
    Location: Rome, Italy
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    I cannot speak for @Dr Simon Mitchell, but from a few exchanges I had with him (I am quite interested in O2 retention) I understood that being CO2 tolerant only moves further up the concentration at which you have a response to it.
    So the only thing that happens is you can walk closer to the precipice whitout feeling it but it is still there.
    So this is also what @rsingler was saying: you (or any tolerant person) might be closer to the edge that you (he) know(s).
    Be careful ... a deep CO2 hit is a non recoverable loss of control on respiratory rate unless you have a huge volume of gas ... and based you the discussion your reserves appear to be thin.

    Cheers
     
    RyanT likes this.

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