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PPo2 Dive Time?

Discussion in 'Advanced Scuba' started by Teljkon, Mar 23, 2014.

  1. Teljkon

    Teljkon Contributor

    71
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    So I have been diving Nitrox for some time know and in all the literature I have looked at I can't seem to find the answer to the question? Dose Higher PPo2 increase your bottom time at lower depth than your MOD for your mix ?? :acclaim: Everything I know says yes but never dose it actually sstate this any where in my texts.
     
  2. Agility

    Agility Solo Diver

    # of Dives: 200 - 499
    Location: Austria
    596
    137
    the increase in NDL is a function of ppN2 and time. The increased ppO2 is only the result of substituting some of the nitrogen in your breathing gas by oxygen.
     
  3. Teljkon

    Teljkon Contributor

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    5
    So am I mistaken in saying that ppO2 or oxygen does not build up in your system over time? Much like over dosing on a drug you either are with in your bodies tolerance for oxygen or your not?
     
    Last edited: Mar 23, 2014
  4. TSandM

    TSandM Missed and loved by many. Rest in Peace ScubaBoard Supporter

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    Teljkon, somebody did you a disservice in your Nitrox class.

    By reducing the amount of nitrogen in the gas you are breathing, a Nitrox mix will always give you less nitrogen loading for any given combination of depth and time. Therefore, your "no deco" time would theoretically be longer at ANY depth.

    However, the MOD is there for a reason -- it's because the oxygen becomes toxic at high concentrations. The major manifestation of this is seizures, which are almost never survivable when they occur underwater. On the other hand, most decompression sickness can be treated, although it is certainly not desirable to suffer it!

    So, from a medical standpoint, you're better off diving air and getting DCS than diving below the MOD of your Nitrox mix and getting oxygen toxicity.
     
  5. Ayisha

    Ayisha DIR Practitioner

    # of Dives: 500 - 999
    Location: Toronto, Canada
    3,208
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    Teljkon, read carefully what TSandM has said. She has written in a very nice way what others could write in a harsh way. If you go below the MOD for your mix, you are risking a sudden end to your dive and very possibly your life due to oxygen toxicity.
     
  6. g1138

    g1138 Marine Scientist

    # of Dives: 500 - 999
    Location: Charleston, SC
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    I think there may be some confusion of terms. I didn't see Teljkon mention going below MOD.
    When you mention bottom time, you're really short handing what should be called your max bottom time or No-Deco Limit (NDL), which depends on how quickly your body is absorbing nitrogen. Your depth effects how quickly you on-gas. So to short hand that, people often say NDL decreases with depth; ie you lose [max] bottom time with depth.

    Nitrox has a different partial pressure of Oxygen and Nitrogen compared with Air.

    There are different processes for getting a tank of Nitrox, I'll just simplify it. You pump a tank with some pure O2, and top it off with air. You'll get a higher PPO2 in that tank because you supplemented it with pure O2. That decreases the PPN2 in that tank. It's a matter of ratios. More Oxygen molecules VS Nitrogen molecules in your tank than in just plain air.

    So with a decreased PPN2, you'll on-gas nitrogen at a slower rate because there's less saturation of nitrogen in your mix. That's the dumb-ed down version.
    So you're thinking of it wrong. The PPO2 in your tank doesn't effect your No-Deco Limit (NDL) directly, what it does is it decreases the PPN2 in your tank. The PPN2 is what effects your NDL.

    You body does absorb and buildup O2 as well. The difference is O2 is used in your metabolism. N2 is not.
    When you become too saturated with O2, ie your PPO2 goes above 1.6 bar, then you get into trouble.
     
    Last edited: Mar 23, 2014
  7. Freewillow

    Freewillow Divemaster

    # of Dives: 500 - 999
    Location: Brussels
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    In fact , there is a form of Lung toxicity that comes with prolonged usage of high oxygen concentration but in practice, the level of toxicity is not reached during recreational dives. This level is called OTU, a term well know to technical divers.

    If you are interested go to the following link Pulmonary Oxygen Toxicity with Technical Divers | SDI | TDI | ERDI

    Here is the beginning of the text that explains it all.

    "In addition to Central Nervous System (CNS) toxicity, technical divers must also be aware of pulmonary, sometimes called whole body, oxygen toxicity. Pulmonary toxicity is only of concern for very long exposures to oxygen (at least several hours). Most tech courses teach that if you look after the CNS toxicity, then pulmonary is not going to be a problem.

    Pulmonary toxicity is tracked using Oxygen Toxicity Units, known as OTUs for short. One OTU is earned by breathing 100% oxygen at one bar for one minute. The most conservative limit sets a maximum of 300 OTUs per day for multi day diving trips. Thirty minutes on oxygen at 6m would give 48 OTUs, well short of the 300 OTU limit, whereas it would give a CNS percentage of 67% which is approaching the limit of 80% of total CNS exposure recommended for technical divers.
    As we can see from the example above, the advice that if you look after the CNS, then the pulmonary toxicity will look after itself seems sound. It is really only with very long decompression dives or long cave dives that there is any risk of pulmonary toxicity. The other time that it can be a problem is during multi day, repetitive rebreather dives. Although open circuit divers will use a partial pressure of 1.4 on the bottom and up to 1.6 during deco, they are not exposed to this maximum partial pressure for very long."
     
  8. fdennis

    fdennis Registered

    # of Dives: 50 - 99
    Location: Middle TN
    32
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    Maybe I'm totally misreading your question - but based on my current understanding of it, a few items of interest:


    1. This is actually kind of a circular question, as the MOD calculation is based on your PPo2. Oxygen can be toxic to your system if its concentration levels get too high. That's the purpose of the maximum operating depth (MOD) calculation - to tell you the maximum depth (and associated pressure) to which your system can be subjected without risking O2 toxicity, given the PPo2 of the gas mix you're diving.
    2. When you say "at lower depth than your MOD" - does this mean you are diving deeper than the MOD calculated for your gas mix? If so, then by definition this means you're subjecting your body's systems to potentially dangerous high oxygen levels, and playing chicken with an O2 toxicity hit. Bad news if you suffer an O2 tox seizure underwater, and nobody's around to make sure your respirator stays in your mouth.
    3. As the PPo2 concentration of your gas mix increases (from EaN32 to EaN36, for example), your MOD - the depth to which you can safely dive - actually decreases. This is because as you descend in your dive, the higher O2 concentration means you'll be approaching the maximum safe PPo2 concentration limits quicker, at shallower depths.

    If you have not previously taken a Nitrox course, I highly recommend you do so before diving Nitrox again.
     
    milbournosphere likes this.
  9. Ricky B

    Ricky B Contributor

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    What TS&M wrote is correct, but it should be understood that what she wrote assumes that the diver (like most rec divers) is not wearing a full-face mask.

    The convulsions triggered by oxygen toxicity are not themselves fatal, not even when underwater. But a diver not wearing a full-face mask will, when suffering from oxygen toxicity, have convulsions and invariably spit out the regulator during the convulsions, which will lead to aspiration of water and drowning (absent immediate rescue and a fair bit of luck). A diver wearing a full-face mask will have no regulator to spit out and so will not aspirate water, and the diver will soon recover from the convulsions not too much worse for the wear (although it's conceivable that the convulsions could cause some other chain reaction of events to be set off). It's for that reason that organizations that have divers working at high partial pressures of oxygen require those divers to wear a full-face mask.
     

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