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Why the high PO2 on CCRs?

Discussion in 'Rebreather Diving' started by Genesis, Sep 13, 2002.

  1. Genesis

    Genesis Great White

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    Am I missing something here?

    If you were designing a CCR, would you not want it to maintain something close to a normoxic mix regardless of depth, and definitely under PO2 of 0.5, so that there would be no O2 "clock" (not to mention CNS tox) issues?

    If you were designing a rebreather, wouldn't you set it up so that it kept the PO2 at any given depth within the range of 0.21 - 0.50 at all times, or at least to the best of its ability did so?

    What am I missing here, or am I misreading things when I see that many of the CCRs (like the Inspiration) tend to keep PO2s above 1.0 at depth...
     
  2. padiscubapro

    padiscubapro Cave Instructor

    # of Dives: I'm a Fish!
    Location: New York
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    actually most of us run about 1.3

    the higher the po2 the longer the no deco time.. @1.3 you get a max single exposure of 3 hours!

    i have done many 3 hour dives with the deepest part of the dive @130 for 1 hr and another 30 @ 100... I'll cruise a reef @ 50 fir another 1.5 hours and no deco required...
     
  3. Genesis

    Genesis Great White

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    with no deco required?

    How's 'dat?

    PO2 of 1.3 @ 130 feet is about 26% O2. The EAD is still 120' on that percentage..... which definitely doesn't give you much NDL time.... (like 10 minutes!)

    When you get done with the first hour and a half you certainly do have a serious deco obligation if I'm doing the math right... Do you carry enough bail-out gas to meet that obligation if something goes wrong with the CCR at that point?

    That's a heck of a lot of gas in an OC bailout, is it not, especially considering that you'd have deep stop obligations being down that long....

    Assuming you hold a PO2 of 1.3 at 50 fsw, you'd be running about a 50% mix (actually 51.7%, but who's counting at that point)

    Yes, you'd offgas a lot during that hour and a half, but do you really have no deco obligation remaining at the end of that 90 minutes?

    I'm curious about the OC bailout requirements to do this safely...

    (I'm not a rebreather diver, but am trying to understand them... I understand the gas consumption advantage, but am trying to get my arms around the elevated mix....)

    Would it not be better to run the lower PO2 at the bottom, and then crank it up when you do your deco? That is, something adaptive to get you the offgassing advantage when shallow and doing your deco, but removing the O2 clock issue when deep?
     
  4. deepblueh2o

    deepblueh2o Angel Fish

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    :confused:
    I am a rebreather diver and I don't understand the math either. I just moved up from semi closed to fully closed. At 150' on SCR I needed 30 cf of 80% for my deco, 30 min at depth, then 28 minutes to the surface using, z-planner.
    My ccr is running at 1.00 because of the error comfort it allows, anything goes wrong, I have a little time to find it before it REALLY goes wrong. Unless you have a VERY expensive computer, ccr is done on tables. You are docked for max depth and ppo sort of like the old PADI days, sort of like, not like.
     
  5. deepblueh2o

    deepblueh2o Angel Fish

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    Just reread my post. I'm not pointing fingers, I'm asking too! I'm way too new on a ccr, and I have just moved up to v-planner. It is truely disgusting to find out that over $1500 worth of diving computers are now just glorified bottom timers.
     
  6. Genesis

    Genesis Great White

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    for a number of reasons.

    CCRs intrigue me.

    (Its the engineer in me - who knows, some day I might try to build one :))

    The problems that some designs seem to have intrigue me too. What I understand of the Inspiration, for example, lead me to believe that it should be a HIGHLY reliable system. It appears not to be in actual practice.

    The basic concept, and some of the adaptations you could make to solve some of the potential problems (e.g. use helium as a diluent instead of air, thereby giving you effectively no narcosis penalty) sound even more interesting, providing you could find a way to have backup if something goes wrong with the thing.

    My gut feel and back of the envelope look makes it appear that the real limiting factors with a CCR are practical ones that limit you not so much due to the CCR design but rather due to the requirement that you be able to bail out if it breaks on you at depth for some reason.
     
  7. Genesis

    Genesis Great White

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    from my point of view is that it would APPEAR that all the data necessary for the rebreather to do deco calculations and such is in the unit!

    Other than perhaps mass airflow (to get some kind of adjustment for your working rate, which may not even be necessary)

    It would ALSO appear that you could almost "push-button dive" with one of these, if it was designed correctly. Of course you have the problem that if it breaks at depth you're in big trouble, but the need to carry backup (tables, timer and gauges) has always been there, no matter how you're doing it in the first place.

    Finally, there are some TRULY intriguing possibilities here for accelerated deco; for instance, the unit could conceivably do deco on elevated FO2 intentionally, while keeping PO2 relatively low during bottom time, especially if you are using something like helium as a diluent...

    I know I'm just rambling and musing here... but ever since I heard about CCRs the design concepts and possibilities have intrigued me...
     
  8. boomx5

    boomx5 DIR Practitioner

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  9. deepblueh2o

    deepblueh2o Angel Fish

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    Stop! One at a time, your making my brain hurt. The first thing that popped into my mind when I read your reply was,'how much money do you want to spend?' You can do it all if you have the money.Yes, we do use exotic gas. Mine is home built and I can clear my loop and switch gas on the fly for quick deco. Your O2 usage changes depending on load, and can do so quickly.
    The CCR2000 will actually ventalate you with a slight side to side motion at depth.
    I'm not a electronics pro so I don't know what's out there. How much do you trust your life to electronics? How many backups are enough?
    There are easy to make semi closed rebreathers out there that are used as bailouts. The 'joker', in europe comes to mind. Very small, used by cave divers are a backup bailout.
    Most of the inspiration deaths I here about are traced back to pilot error. Anyone can take the course, at this level you make a mistake, it kills you. And some of them do.
     
  10. madmole

    madmole Manta Ray

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    I'm also slightly confused as I own an Inspiration and was at 45m twice this weekend (132 ft) (Salsette and P555) and I definately cant stay 1 hour there at a PPO2 of 1.3 bar

    On both dives using a VR3 with real time PPO2 monitoring and deco calcs I did 25 mins bottom time. Dives were terminated when Time to Surface was 10 mins

    Deco on both dives was 2 mins @ 24, 2 mins @ 12 and 2 mins at@5. I do a slow ascent and clear these normally before reaching them. Buddy was diving a nexus so had 6 mins at 6m (Bend and mend computer!!!)

    The limiting factor IS the bailout you are willing to carry and how bored you want to be on the stops. For both these dives I carried
    3l O2 main supply, both dives used 30 bars of this:D
    3l Air diluent, 7 bar used on each dive (leaking Drysuit dump)
    3l Air bailout mounted to side of Inspiration cover
    3l 50% Nitrox Deco mix

    The later two are also rigged with hose to enable plug into unit

    This combo gives me enought OC bailout to do worse case stops plus 1/3 reserve and about 10 hours duration in SCR mode!!!!!! (DDPlan was used to cut worse case and bail out tables)

    For longer dives I take 2x3l Air (mounted on the case) and a side slung 7l 50%. I will be doing my Mixed gas training on the Insp ASAP

    All my case mounts are on pony releases so can be handed off to a buddy

    The elevated PPO2 really does help shorten deco requirements and you feel a lot better for it. At 1.3 PPO2 the MAx exposure is 3 hours which is the same duration of the scrubber. So thats what sets your max dive time. If I was diving several times a day for a week or 2, I would drop the PPO2 to something lower to limit my CNS and OTU take up

    But some dives are great. The Inspiration WILL give you 3 hours at 20m (60ft) as a No Stop Dive:)

    And doing a 18m dive 3 hours after the morning 45m one I had 700 mins NST, 2 hours scrubber left and still enough gas in the unit for the full 2 hours:) :) :)
     

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