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Accumulated 02 following a large number of repetitive Nitrox dives over 3 days.

Discussion in 'Advanced Scuba' started by Astran, Sep 30, 2019.

  1. Madacub

    Madacub Contributor

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    This is kind of precisely my point (maybe the 1.0 PPO2 is exaggerated to make a point). But I'd like to see GJC elaborate on his post # 233.
     
  2. doctormike

    doctormike Medical Moderator Staff Member

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    Its time and exposure. It’s not linear time and exposure. Like most biological systems, there are limits beyond which the tissues react differently. Otherwise, what does - for example - the NOAA CNS toxicity table represent?

    To use your example, the NOAA CNS limit for a single dive at PO2 of 1.0 is 300 minutes. Hard to get large data sets for those dives, but it’s certainly a consideration when planning long CCR dives. Interestingly, there are plenty of people on this board (and probably in this thread) who have far exceeded CNS 100% on such dives and they will tell you about it.

    But it’s not as simple as max PO2 only.
     
    jgttrey likes this.
  3. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

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    CNS and OTUs are tracked differently and are independent of one another. The quote you posted is incorrect.

    If you (hypothetically speaking) accumulated a whole daily OTU dose on a very long shallow dive (e.g ppO2 0.9) in the morning. Then had (work with me here) a long surface interval of say 6 hours (4 CNS half lives). Your deep but short duration higher ppO2 dive in the evening (ppO2 1.4) is not anymore or less likely to have CNS symptoms. Mostly because there's no real data about this kind of exposure except for near repetitive chamber treatments which are usually spaced out over multiple days not just 6 hrs.

    You are probably going to have pulmonary symptoms if you repeat that kind of OTU overload day after day though.
     
  4. jgttrey

    jgttrey ScubaBoard Supporter ScubaBoard Supporter

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    Sort of a general comment made earlier and worth repeating for late comers to this thread....

    The models, rules, algorithms - whatever you want to call them - that we use for oxygen toxicity (both CNS and pulmonary) are just our "best effort" to give guidance about what is likely safe versus what is likely not safe, or at least what is entering into a realm of risk that most people would rather avoid. What these "rules" are certainly not is a precise measure of anything.

    Is the half-time credit for CNS really a half-time? If so, is the time 90 minutes or 120 minutes (does God like to work with round numbers?). How variable is that half time, if it is, for different people? We know, as @doctormike explained that time/exposure are variables in CNS risk, though the relationship is not linear. What is that relationship, exactly? Don't know. We can approximate it based on empirical data, but this isn't like the situation where we can measure it how long it takes to metabolize alcohol (even that is variable, though at least it can be directly measured).

    How is the risk of a CNS or pulmonary hit affected by other factors like whether you vape, are a CO2 retainer, have a lot of inert gas loading? What about how hard you are working or whether you are cold or tired or hungover or on meds? Some people can tolerate 1.3 PO2 day after day without it hurting their eyes, others are sensitive to hyperoxic myopia and cannot. Lots of mysteries.

    I can tell you that I would be far more comfortable hitting 125% on the CNS clock because I did an hour or two of deco at a high PO2 than I would be hitting 95% if I were swimming into a current at 100'. We know exertion plays a factor. We also know that our model does not account for it.

    These formulas - the CNS clock and OTUs - are just approximations work "enough" to serve as the basis for responsible dive planning. They are intentionally conservative -- precisely because we don't know everything. There is no guarantee that following these guidelines will keep you 100% safe, though it is believed that the risk would be quite small. It's a spectrum of risk. At the limits, what works one day may not work the next.

    That is a very long disclaimer and response to some of the posts scattered among the last several pages that seem to imply a certitude about these guidelines that is probably misplaced. It doesn't make a lot of sense to split hairs about oxtox because we don't have a defined hair to split.
     
    Jay, NAM001 and rjack321 like this.
  5. NAM001

    NAM001 Contributor

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    Its been along time since I have been in these aspects of nitrox. I for get which one it is but If im not mistaken its minutes at 1.0 PPO2. not actual minutes. If you have a daily limit of 210 and you are diving at 1.5 PPO2 then you do not have 210 clock minutes you have about 2/3 of that because you are getting 1.5 minutes of exposure for every clock minute you are being exposed. That may not be absolutely accurate for a discription but i think it will suffice. even at 1.03 you gather faster than 1 minute per each clock minute. I dont see why this is that hard to understand. If he said his cert agency at least we could look at that agencies material and find the mis understanding. I too am at this point left with no conclusion other that Astran has incorrectly self taught using some agencies materials or he has self taught using no agency materials and relied on wiki and board threads to get his """ training"""" from. WOW its been a long time this is like ohms law you learn it and never fall back in it again at its basics to under stand why no current with max resistance from the open. You just done have to so do the math any more things become just a given at some point.

    I wish I could find my spread sheet where I could do repetitive dives and it computed the running OTU's and CNS for me. Then perhaps he could see what happened as it was happening..
     
  6. NAM001

    NAM001 Contributor

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    I seem to recal many formulas with both PPO2 ans PO2 something like PO2 x Fo2 = PPo2 in this case at 100 ft PO2 is 4atm
     
  7. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    No, you mean P, not PO2, where P is the total pressure; P=PPO2+PPN2. Dalton's Law.
     
  8. chillyinCanada

    chillyinCanada ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Wasn't wasted on me, thanks all.
     
    seaseadee likes this.
  9. GJC

    GJC Solo Diver

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    For the first Paragraph (I have trouble spitting quotes and answering them in parts)

    This guy is pretty emphatic that the "CNS Clock" "OTU" or whatever you want to call it need to be tracked to avoid CNS hits on repetitive dives. In the comments section he cites 2 instances of CNS hits due to accumulations from previous dives. These are tec divers with accumulations way beyond most rec diving.
    Daily Limits for CNS Oxygen Toxicity


    Second Paragraph

    Correct. We know a lot more about OTU accumulation on the pulmonary system than we do on the CNS system.
     
  10. GJC

    GJC Solo Diver

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    Location: Southern California, USA
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    Saniflush likes this.

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