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Thread: CNS O2 level

  1. #1
    TadPole
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    CNS O2 level

    Hi
    I am a fairly new diver, and I have just bought my first dive computer (Aladin 2G). It has a CNS O2 level reading in percentages.
    What is it actually telling me, and what should I be paying attention to? How am I suppose to work with this information?
    Thank you

  2. #2
    Megalodon
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    O2 CNS is about half of the information in a Nitrox course. Simply put, at partial pressures greater than .21 atmospheres (.21 ATA) prolonged exposure to oxygen under pressure has a CNS (Central Nervous System) effect (Toxicity) How fast it will affect you is dependent on the time and depth you dive. Even air can potentially become toxic at depth. Your O2 CNS percentage is the maximum allowable exposure to oxygen at depth. If you're diving air, this I not a huge issue, but with nitrox, it is.

    As an example, a dive on air to 60' for 55 minutes will give you a .6 ATA would be 10% by the DSAT table so you'd have to do 10 dives to 60' on air to get 100% Allowable exposure. That same dive on 40% Nitrox would be 30%.

    I once saved a man in Wichita just to watch him dive...(inventor)

  3. #3
    TadPole
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    Thank you. I knew what the O2 CNS is all about, it's just that I didn't know how to work with the percentages. When diving strictly by the dive tables, the tables take that all into consideration (especially with air). Therefore it confused me a bit. When using a dive computer, is this something that I have to monitor while diving, or is it just extra info that the computer is giving me? The way I understand it, practically speaking, as long as I don't reach 100% I'm OK, and if I dive according to the computer I shouldn't come close. Is that a correct assumption?

  4. #4
    Megalodon
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    My apologies, are you Nitrox certified?

    If you're diving air, you don't have to worry abut your Allowable Oxygen Exposure (AOE), your computer will still track it, but it really doesn't come into play. If you're diving air on tables, there is no reason to calculate your Oxygen Exposure. If you're diving Nitrox, you need to track your AOE, either by computer or using tables. PADI Nitrox tables (since these are the ones I know) only show your NDL and nitrogen loading, The DSAT Exposure table is used to calculate your oxygen exposure percentage

    Most Nitrox courses recommend keeping you allowable exposure less than 80%. The PADI Nitrox course teaches a maximum of 80% without figuring any reduction in the level during surface intervals, most computers will give you "credit" for surface time.

    I once saved a man in Wichita just to watch him dive...(inventor)

  5. #5
    TadPole
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    Got it. Thanks.

  6. #6
    Barracuda
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    Navy summed it up pretty well, I would add to read the manual for the computer to see exactly what it is telling you from the percentages.
    any puddle...anytime...anyplace!

  7. #7
    Barracuda Founding Member
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    All good info.

    Allowable Oxygen Exposure in the realm of what most people will see when diving is pretty irrelevant. PPO is more important. It would be hard to push AOE of recreational dive mixes and dive times.

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