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My first (and unpleasant) rebreather experience

Discussion in 'Rebreather Diving' started by Deefstes, Mar 3, 2010.

  1. battles2a5

    battles2a5 Divemaster

    # of Dives: 200 - 499
    Location: Charlotte, North Carolina
    You are right. He obviously has the resume so I guess I really can't second guess him either. Just seems like an odd choice for the job.
  2. Gill Envy

    Gill Envy Rebreather Pilot

    # of Dives: 500 - 999
    Location: Seattle, WA
    I'd have to heartily disagree that automation makes a rebreather recreational, I personally think Automation is inappropriate for many beginners. I would content that automation is best suited for the most advanced rebreather diver, those who truly need it and for those who have the greatest level of self discipline, those who can experience hundreds of hours of flawless function and still maintain the level of vigilance necessary to catch sudden malfunction where the time to react is down to a matter of seconds. I truly believe that for lack of better data, that the preponderance of information now available points to manual injection of o2 to be the "safer" bet for the beginner.

    dead horse beaten to a pulp...

  3. caveseeker7

    caveseeker7 Rebreather Pilot

    Thought I chime in, too. I've done three rebreather intros with three different instructors, spending time on four different units. I enjoyed all three of them, as well as the time I got to spend with those RBs in and out of the water.

    Deefstes, sorry to hear that for you it was such a bad experience.

    I agree that you most likely had CO2 hit, though with a lingering headache the next day and vomiting after the dive I wouldn't call mild. Just, for the lack of a better word, a CO2 hit, neither mild nor severe.

    And while I generally agree with TS&M's statement regarding understanding of the gear you dive I honestly believe that in this case it doesn't really apply. Tryouts are for people not trained on the equipment in particular, so laying blame on the OP seems rather unfair.

    If anything I applaud you for taking responsibility. That in and by itself may well show you have a better state of mind than some people diving those contraptions.

    In this case I believe the instructor should have taken responsibility, though. For one thing it looks like the theoretical part of the intro lacked some rather important information. The main risks of rebreathers come from the fact that you can breath off the loop even if the gas you're breathing will not support life. Hypoxia, Hyperoxia and Hypercapnia, as well as their symptoms should have been covered. As in "if anything feels wrong, bail out"! Especially on a unit that is designed for just that and only that. Covering those basics is an instructor's job.

    Since everything points to Hypercapnia the unit was obviously not good to dive. No matter if that was due to an exhausted scrubber, water in the absorbent (increases the work of breathing) or a combination of both. The proper pre-dive and staying within manufacturer's recommendations should have prevented that. This is also the instructor's job, or at the very least should be done under his direct supervision.

    One statement there somewhere in the thread I disagree with, though. The instructor does not need to be in the water. Actually, it's a lot easier to observe divers in the pool when watching from the side of it. The last thing you want is to miss an unconscious diver in the water. On my first two intros the instructors were poolside, on the third the instructor was in the water with spotters poolside, for that very reason.

    A few words regarding the unit itself:

    The pO2 limits of the Discovery are 0.4 ata and 1.3 ata.
    The unit will maintain a lower setpoint of 0.5 ata at the surface and 1.2 ata at 15 meters and below. In between, the setpoint is dynamic in between those two settings.

    While most units maintain a low setpoint of 0.7 ata the Discovery did seems to have kept the setpoint the way it is programmed to. Exhaling periodically into the water and thus automatically adding diluent into the loop will reduce the pO2 and keep the solenoid busy injecting. Under those circumstances a pO2 of 0.4 ata is reasonable.

    The Discovery has been certified for 180 minutes at 40m, water temp 4°C, and a breathing rate of 40 liters per minute producing 1.6 liters CO2 per minute as per CE requirements. That's the same rating the Inspiration received, which most people wouldn't classify as a recreational rebreather.

    The duration of the O2 supply and the scrubber are pretty much matched so that they're replenished at the same time, for simplicities sake, rather than the diver having to track and replenish both separately. The O2 supply is limited (to 135 bar tank pressure), not the scrubber duration.

    The scrubber duration is affected by both ambient temperature and ambient pressure (depth), so by all means it should last longer than 180 minutes in a pool, more so a heated one. On the other hand exceeding manufacturer's recommendation is always a gamble.

    I wonder how long the O2 lasted, if they changed/refilled the oxygen tanks on those units. If they did it was pretty dumb not to change cartridges at the same time.

    Anyway, what makes this unit recreational is the limits the electronics set, with alarms going of past 40 m depth, when getting close to deco obligations, the lack of manual controls that force you to bailout in concert with the electronics. IMHO not a bad idea for recreational diving, but there can be better options for a well trained, experienced diver in a non-recreational environment.

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