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Thread: Breaks

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    Breaks

    Iím preparing a dive plan for a quite deep and therefore quite a long dive. Without getting bogged down in the actual dive plan there is a deco (time) need for a setpoint of 1.3 or 1.4 and as a result the CNS clock hits about 300%.

    Assuming that on this dive one could easily be on a setpoint of 1.4 for 9 hours what are peoples theories on Ďair breaksí? When I say air breaks I am referring to the concept of taking a break from typically breathing 100% O2 for extended periods. Iím thinking about transferring that line of reasoning to a whole CCR dive.

    To my non medically trained brain it seems reasonable that if we should take an air break when breathing O2 for long periods then there still exists a need to have a break from a pO2 of 1.4 after a certain amount of time. If this is the case, what should the break schedule look like?
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    Re: Breaks

    Steve,

    The "Catalina" hyperbaric treatment table, which is a modified US Navy Table 6, shows a run time of 11 hours and 52.4 minutes. In that period, they show 20 minutes on O2 and 5 minutes off cyclically. Remember, however, that this is 100% O2 at 60 fsw equivalent, or 2.8 ATA.

    "A currently useful procedure employs systematic alternation of hyperoxic exposure intervals with relatively brief normoxic (emphasis mine) intervals to increase markedly the total duration of tolerable exposure to a selected level of hyperoxia within a period of 24 hours or longer." Bennett and Elliott, Physiology and Medicine of Diving, 5th edition, page 403, paragraph 2.

    This would seem to indicate that dropping the ppO2 to 1.0 for short periods would extend your O2 tolerance into the time range that you need.

    The hard part for you will be determining where in the water/pressure column you want to set your breaks, what gas will give you the drop in ppO2 that you want, and how best to supply it. (My base assumption here is that you will be going deeeeep!:D )

    Rob
    Last edited by ROB DAVIE; 13th January 2006 at 05:36.

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    Re: Breaks

    Mate,

    That's not an easy question to answer. Studies on (pulmonary) oxygen toxicity in man have shown that a 20:5 schedule (ie 20 mins on O2 and 5 off) will double the time taken to reach a given decrease in FVC (forced vital capacity). You are talking about 1.3 to 1.4 ATA's for 9 hours which is about 750 UPTD - maybe just enough to cause pulmonary symptoms at the end of the dive and not very different at all to a standard TT6 and a long way short of causing any major trouble.

    There is no human evidence but good animal evidence that air breaks help prevent CNS toxicity and we use them routinely for medical patients on this assumption.

    I don't understand exactly where the NOAA limits come from (these are what I think the CNS clock is based on). The risk of an O2 convulsion is probably quite low for the dive you describe, but could presumably (after all you look a bit like a hamster don't you?) be reduced with air breaks according to whatever schedule - especially in the shallower stops. What impact this will have on the duration of the stops is too hard for my little brain.

    Dave T

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    Re: Breaks

    Back from depth I'm usualy around 1,5-1,6 b PpO2 and when the CNS hit 200% I'll do a 5 min low ppo2 break at 0,4 b (flush the loop with deep dil), before going in the habitat at -13m I'll do again a 5min low ppo2 break, and then in the habitat I'll break 20:5 again on low ppo2.

    If no habitat and O2 used in hot water from -9m I'll will also break 20:5 on low ppo2

    There is no scientific evidence behind all of that, this is purely experimental, I've been doing that for the last two years but not enough times to be statisticaly valid ;)

    Regards

    Jerome

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    Re: Breaks

    Lots of people go way over the oxygen clock limit these days by using the break from the high PPO2 method. On open circuit, I originally used to go with 20/5 because that is what most people did.

    Later, I changed to 12/3. I find that this works well for me though I had no problems with 20/5. I like it because it is just about two cycles of gas through the body (takes about two minutes per cycle) but does not prolongue the exposure to either high PPO2 or inert gas while on break (though that's not much worry since you are likely offgassing mostly at this point in the dive). It also works well for me on shorter dives since it always seems like a lot of my profiles would require about 25 minutes on O2, and I always was wondering where I should draw the line between just doing the whole lot or only doing a short oxygen spike after spending what seemed like a lot of time (five minutes) on "back" gas. So, dividing the O2 spike a little more evenly seemed better for me if the spike was long enough to call for a break in the first place.

    Now that I will be rebreather diving, things could change again. I hope to be doing much longer dives when I go deep than when I was diving open circuit. I hope I figure out an efficient and safe path soon. (No worries, I will not be deep diving again for quite some time now that I have made the switch)
    Last edited by ScubaDadMiami; 14th January 2006 at 07:48.

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    Re: Breaks

    Hell, i'm no authority but if only for a stastitic, i, (we, my dive buddies and i) stick to, EAN50, 25' on 5' off, and O2, 15' on and 5' off.

    The time on the air breaks are considered as part of the deco time total!

    And for an examle, the times are manipulative Eg. if i were to do 20' on O2 i might do 10' then 5' on bottom mix then 5' on O2 = 20' TOTAL.

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    Re: Breaks

    Quote Originally Posted by ScubaDadMiami
    I like it because it is just about two cycles of gas through the body (takes about two minutes per cycle)
    This is new to me. What do you mean by this and where did you get this information from?

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    Re: Breaks

    The DCIEM tables call for 30 on O2 and 5 on air for surdO2. I think a little more conservative approach for in water dco would be required, but the tables indicate that an optional air break for 5 after 30 can be used... but the break is not counted as good deco time.

    roll the dice and see?? I hate decompression!!:p

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    Re: Breaks

    I re-read the post.... those are some LONG times on 1.3 and 1.4 why not drop your set point. If you are on bottom that long, and have the support for a dive like that, then the extra deco from the lower set point should not be a problem....
    9 hours underwater...wow... only on hose gear for me:D

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    Re: Breaks

    Quote Originally Posted by cameron
    I re-read the post.... those are some LONG times on 1.3 and 1.4 why not drop your set point. If you are on bottom that long, and have the support for a dive like that, then the extra deco from the lower set point should not be a problem....
    9 hours underwater...wow... only on hose gear for me:D
    I one were to drop the setpoint the deco would be even longer. Buggered if you do, buggered if you don't
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