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Thread: What do we really know about O2, CNS and OTUs?

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    Nicholas Smith Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo's Avatar
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    What do we really know about O2, CNS and OTUs?

    I came back from diving the other day with psychadelic flashing snakes across my field of vision. I immediately thought conventid: I'd doubtless overdone my oxygen exposure. It set me thinking about what I really know about hyperoxia, CNS and OTUs. I'd been running a setpoint of 1.3 on 2 dives a day to 70m with runtimes just over 2 hours each - though my real PO2 was around 1.45 or so for much of the leisurely descent.

    Back when I was young and beautiful and first learning trimix on big, heavy doubles, , I was taught to use tables to forecast the CNS clock and OTUs for the dive I was about to do. I have completely got out of the habit because they don't seem that relevant: you can't do a deep dive without popping through 100% on the CNS clock, and if my lungs start to burn I take a day off and re-acquaint myself with my family.

    It really seems to come down to eyes, central nervous system and lungs: I need to keep the set point down to around 1.2, rather than 1.3, to halt the damage to my eyes; I don't think there's significant risk of ox-toxing below 2.0 as long as the exposure is only a couple of minutes; and I really don't think much about whole-body oxygen toxicity anymore. I can't afford an ox-tox hit as I tend to dive solo on deeper dives.

    What do most people do to track and limit oxygen exposure?

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    Re: What do we really know about O2, CNS and OTUs?

    Hi Nick,

    For an Air Dil CCR diver he will be looking at running a high PO2 to reduce the PN2 (narcosis) and he doesn't really have any other options as we do when we start diving with Trimix and the wonderful VooDoo Gas Helium.

    For the bottom phase of a deep dive I run a setpoint of 1.0 considering He is cheap when your diving a rebreather then you are not as concerned as an OC diver who is getting belted every time he fills his big twin set. In saying that we can afford to reduce our narcosis level by not increasing the PO2 but rather by running a higher percentage of He.

    Now you can run a low setpoint (say 1.0) and not have to suffer with narcosis.

    On the ascent phase I bump the setpoint to 1.2 and then when at 15m or so then bump it again to 1.4 to maximise offgassing.

    Run the numbers in your dive planner and see how the numbers come out.

    We did brush over the subject at the Lake when you were over here.

    Regards,

    Lance

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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by Abbo  View Original Post
    I came back from diving the other day with psychadelic flashing snakes across my field of vision. I immediately thought conventid: I'd doubtless overdone my oxygen exposure. It set me thinking about what I really know about hyperoxia, CNS and OTUs. I'd been running a setpoint of 1.3 on 2 dives a day to 70m with runtimes just over 2 hours each - though my real PO2 was around 1.45 or so for much of the leisurely descent.

    Back when I was young and beautiful and first learning trimix on big, heavy doubles, , I was taught to use tables to forecast the CNS clock and OTUs for the dive I was about to do. I have completely got out of the habit because they don't seem that relevant: you can't do a deep dive without popping through 100% on the CNS clock, and if my lungs start to burn I take a day off and re-acquaint myself with my family.

    It really seems to come down to eyes, central nervous system and lungs: I need to keep the set point down to around 1.2, rather than 1.3, to halt the damage to my eyes; I don't think there's significant risk of ox-toxing below 2.0 as long as the exposure is only a couple of minutes; and I really don't think much about whole-body oxygen toxicity anymore. I can't afford an ox-tox hit as I tend to dive solo on deeper dives.

    What do most people do to track and limit oxygen exposure?
    this could be inert gas also. i have a friend who gets his "niggles" when he is close to be bent.
    mel

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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by Lancer4545  View Original Post
    In saying that we can afford to reduce our narcosis level by not increasing the PO2 but rather by running a higher percentage of He.
    The pN2 doesn't trouble me too much: I do a lot of training during the working week with liquid narcosis - the red, the white and the amber varieties, mostly. I suppose I run the pO2 high to keep the p[inerts] lower to leave myself less to offgas, because the sea is always less beautiful and life a little less exciting in the 'decompression zone'.

    I get the principle that 'less is better' with hyperoxia, but because I have never grown out of being a naughty schoolboy, and because I always want deeper, longer and wetter, I want to understand the limits. With due defference to NOAA, most people pay the oxygen clock scant regard, and I wondered a) how much attention people pay to their CNS and OTUs in planning a dive, and b) what limits they tend to use.

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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by scubagrunt  View Original Post
    this could be inert gas also. i have a friend who gets his "niggles" when he is close to be bent.
    mel
    From another poster (DrMike), I remeber him telling of a friend that got bent on VPM, and saw stars when ever he dived.

    I would not rule out decompression issues either, and CNS-tissues at that

    Nicolai

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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by scubagrunt  View Original Post
    this could be inert gas also. i have a friend who gets his "niggles" when he is close to be bent.
    mel
    Astute point: I had written that off because there were no other symptoms. I have been using 30/85 on my new Shearwater, and not paying much attention to the longer decos being advised by the VPM on my VR3. It certainly couldn't hurt to be more careful. Thanks

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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by Lancer4545  View Original Post
    Hi Nick,

    For an Air Dil CCR diver he will be looking at running a high PO2 to reduce the PN2 (narcosis) and he doesn't really have any other options as we do when we start diving with Trimix and the wonderful VooDoo Gas Helium.
    That is if you dont factor O2 as Narcotic, the solubility issue suggest it would be more narcotic, most consider things equal.

    Quote Originally Posted by Lancer4545  View Original Post
    For the bottom phase of a deep dive I run a setpoint of 1.0 considering He is cheap when your diving a rebreather then you are not as concerned as an OC diver who is getting belted every time he fills his big twin set. In saying that we can afford to reduce our narcosis level by not increasing the PO2 but rather by running a higher percentage of He.
    I have been doing and teaching this for years and believe its a good choice I then go to 1.3 for the ascent phase and on the bigger dives come off to O2 at 6m.

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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by Abbo  View Original Post
    Astute point: I had written that off because there were no other symptoms. I have been using 30/85 on my new Shearwater, and not paying much attention to the longer decos being advised by the VPM on my VR3. It certainly couldn't hurt to be more careful. Thanks
    Hi Buddy,

    Hope your well, I would consider changes your GF if diving mid range on helium, the 30/85 is Ok for air dil but maybe 20/80 or 20/85 for general purpose soft decompression. Even thinking about 10/90 for rich He mixes for long RT at depth may be on option.

    all the best

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    Re: What do we really know about O2, CNS and OTUs?

    I am very cautious about oxygen exposures. Unless there is a need, I keep the PO2 to a lower level.

  10. #10
    Nicholas Smith Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo has a reputation beyond repute Abbo's Avatar
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    Re: What do we really know about O2, CNS and OTUs?

    Quote Originally Posted by ScubaDadMiami  View Original Post
    I am very cautious about oxygen exposures. Unless there is a need, I keep the PO2 to a lower level.
    What qualifies as a lower level for you? 1.0? I would assume that appropriate CNS and OTU exposure levels are different with a rebreather, given that elevated pCO2 dilates arteries. Has any research been done giving guidance about what adjustments need to be made to NOAA tables for rebreather divers? Excuse my doggedness in pursuing this, but I am getting the impression that I am not the only rebreather diver who has stopped paying much attention to the ox clock and OTUs.

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