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Scuba Nitrox Incident for Analysis

Discussion in 'Near Misses & Lessons Learned' started by jagfish, Feb 22, 2021.

  1. jagfish

    jagfish The man behind the fish ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Kanagawa and Florida
    Scuba Nitrox Incident for Analysis
    This apparent true incident was taken from Scuba Board almost 20 years ago. I use it in my nitrox course before the final exam as a discussion exercise for students in small groups. Here, I describe the incident, please post your analysis in the comments, and I'll post my video analysis next week.

    A great deal can be learned from the incident analysis. We get to learn from the mistakes and bad luck of others, without having to experience the "pain" of the actual incident. In this spirit, I am grateful for all incidents that are posted, and the generosity and humility of the posters. We analyze these events not to mock the inexperience, poor decisions, or bad fortune of others, but to make ourselves better divers.
    OrcasC205 likes this.
  2. John C. Ratliff

    John C. Ratliff Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Beaverton, Oregon
    Okay, Jim, I’ll bite. I’m a diver for decades, and have been in the safety profession for over 30 years. So here goes.

    A couple of things from the video:
    —he was using (supposedly) 36% nitrox. That means the gas was 36% oxygen, the rest (baring contaminants) being nitrogen. That would be 64% nitrogen.
    —depth where symptoms happened, 75 feet (23 meters); he felt that even though he was receiving the required gas from the regulator, he wasn’t getting enough oxygen.
    —he had a spaced-out buddy, who couldn’t or would not help.
    —upon heading for the surface, he felt better at 30 feet (9 meters)
    —Once he returned to the bottom, things were fine, and he resumed the dive without surfacing or needing the buddy’s safe second (octopus), assuming that the buddy had one (not sure, equipment configurations were not detailed).
    —He asked about stratification of the gases within the cylinder (separation, he said).

    I’d like to point out that the gas wasn’t tested for composition. So we don’t know the actual mix. Also, there was some panic in this description, as he said he felt the need to surface even with the risk of lung over-expansion or the bends (no mention of how long they had been at depth.

    Because the dive resumed without incident at the 75 foot level (23 meters), I’d place this to his lack of experience, and probable hyperventilation. He got anxious, and then started breathing with quick, short, shallow breaths, which really did not pass oxygen to the circulation from the lungs, as the lungs did not receive “new” air from the regulator. Then, as he began ascending, his breathing stabilized, and he got more oxygen into his lungs from the deeper breathing he was using on the ascent. Relaxing more, with his breathing under control, he felt more at ease and resumed the dive by swimming back down to his buddy at 75 feet (23 meters).

    Concerning the air “separating,” to my knowledge that simply doesn’t happen. Yes, they are slightly different densities, but the Brownian Motion would cause these molecules to evenly distribute throughout the cylinder.

    There was no regulator malfunction, as the dive resumed and it breathed normally.

    So the lessons learned are that breathing is important, and a diver needs to breath more deeply and slowly than people on the surface, especially when exercising. We used to call this the “hyperpnia-exhaustion syndrome,” but that has been discontinued from what I can see.
    Hyperventilation syndrome - Wikipedia

    This, by the way, can also happen on the surface. Here’s an incident which happened to my buddy on a dive last summer here in Oregon.

  3. yle

    yle Instructor, Scuba

    # of Dives: 500 - 999
    Location: Southern California
    Concerning the air “separating,” to my knowledge that simply doesn’t happen. Yes, they are slightly different densities, but the Brownian Motion would cause these molecules to evenly distribute throughout the cylinder.


    If oxygen and nitrogen could separate, all the oxygen in the atmosphere would be hanging out at the bottom and the nitrogen would be at higher altitudes. I always find it fascinating when people suggest gases can somehow "stratify" the way liquids can, when the same people have been immersed in, and continuously breathing, a 79/21 mix for their entire life.
  4. Edward3c

    Edward3c Instructor, Scuba

    That’s a good advert of why we ware BCs, to provide buoyancy on the surface.
    John C. Ratliff likes this.
  5. Angelo Farina

    Angelo Farina Marine Scientist

    # of Dives: 1,000 - 2,499
    Location: Parma, ITALY
    I agree with the hypothesis of short breathing causing the symptoms.
    However the physiological explanation given does not convince me.
    First of all, it is impossible to get hypoxia at a depth of 23m, where the ppO2 is more than 0.6 bars.
    The bad breathing affects CO2 retention, not O2 availability.
    In practice, breathing with fast and short inhalations makes it difficult to eliminate CO2, causing hypercapnia. This causes the symptoms and the sensation of not getting enough air.
    Hypoxia usually has no symptoms.
    Furthermore, an high concentration of CO2 stmulates a faster breathing, causing a vicious loop.
    This is NOT hyperventilation, which is actually the exact opposite: people who ventilate too much, causing the CO2 concentration to drop (hypocapnia).
    However the conclusion is the same: a guy who was not properly trained to breath in a controlled way felt into dyspnea (lack of air, "affanno" in Italian) and needed to ascent to shallower depth for gaining back control of his breathing.
    Nitrox has absolutely no relevance at all in this incident, it had occurred identically breathing air.
    Teaching breathing control was of utmost importance when I started diving, as at the time, in the seventies, beginners were trained using pure-oxygen CC rebreathers. These devices required to breath in a very slow and deep way, for avoiding hypercapnia.
    When these ARO systems were abandoned, in the eighties, the 3 months long training about breathing control was also removed from training, allowing for much shorter courses.
    But this exposes novel divers to a severe risk of loosing control of their breathing, as it happened here...
  6. chillyinCanada

    chillyinCanada ScubaBoard Supporter Staff Member ScubaBoard Supporter

    How's your buddy? Is he OK? Was he having a heart attack or something? How long had it been since his last dive, before preparing to dive with you that day?
    John C. Ratliff and VikingDives like this.
  7. Wibble

    Wibble Contributor

    # of Dives: 500 - 999
    Location: UK
    Hypoxia would result in him passing out. You can't tell if "you're not getting enough oxygen" -- otherwise there's no need for 3+ O2 sensor cells on CCR.

    Hyperoxia can result in 'feeling strange' -- COVENTID -- which could be misinterpreted

    "Can't breathe" is a symptom of hypercapnia or high work of breathing (WOB) -- gas not turned on fully? When at depth and working hard could suck hard on the oxygen.

    What was the gas analysis; have to assume that 36% is accurate.

    Clues on gas separation? Breathing high PPO2 until the gas mixed?
    Angelo Farina likes this.
  8. OTF

    OTF Contributor

    # of Dives: 200 - 499
    Location: same ocean as you
    • Impossible to know what really happened here, but this sounds like narcosis, hyperventilation, CO2 buildup, good old fashioned overthinking panic, or possibly even CO. As posters above have said.

    • Unless the gas was blended in some super bizarre way (mixing pure nitrogen with O2??) there's no way the mix was accidentally hypoxic. Nitrox is blended by ADDING oxygen to regular air. I guess it would be possible to confuse his tank with someone else's deep trimix tank but that seems awfully unlikely. Even without labels the trimix would probably be in doubles or a sling-rigged bottle easily recognizable as something different. But let's run with that unlikely theory for a minute.

    • At 75 feet even a quite low oxygen mix should be survivable. A stolen 10/70 trimix bottle would give him a PO2 of 0.33, MORE oxygen than he was breathing on the boat deck.

    • If he was somehow SURE that he was accidentally on a hypoxic mix at depth, surfacing would actually be the worst move. PO2 would decrease and he would pass out. Getting on a different gas supply would be the only solution. But unless he looked at the tank marking and realized he accidentally took someone's hypoxic mix on his rec dive there would be no way to know why he was feeling oxygen deprived and other causes for the feeling are much much more likely than bad mix.

    • This video didn't start until 1/3 through it's run time.
  9. JohnN

    JohnN ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Oar--eee---gun
    I'm surprised you were considering nitrogen narcosis as a theory. If the person was really diving EAN 36 (which we have to assume), the equivalent air depth at the bottom was only 54 feet. If you're getting narc'd at that depth, you need to consider a different hobby.

    My bet is the person got excited, started hyperventilating (shallow breathing) and had some CO2-induced panic.
  10. OTF

    OTF Contributor

    # of Dives: 200 - 499
    Location: same ocean as you
    Nitrox DOES NOT reduce narcosis. You need to add helium for that.

    Although yeah 75' is a little shallow to be seriously narc'd and CO2 hyperventilation/panic can feel almost exactly like the original incident described.

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