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Thread: PADI Nitrox and CNS Clock

 

  1. #11
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    Thanks for replies... I'll give you the background of my nitrox course. I originally had my aow course booked in. I enquirer about what to do after and the owner of the lds told me to do my nitrox, which he said was only another $99 on top of my course. I paid the extra amount and I was handed a thin quite thin book from padi. I was going to go to the night course but I work shift work so I arranged to come in by the shop and run through a few things. I was given a multiple choice exam, it was graded then and there I analysed two bottles of nitrox. End of story. Knowing I want to progress further I have done my own research and hence the questions.

    Being in the emergency services I have a bit of an idea of twa (total weighed averaged) and short term exposure limits in regards to chemical exposures, so if I can get some decent info I should be right to wrap my head around it.

    ---------- Post added August 24th, 2013 at 08:09 AM ----------

    Oh no disrespect to the shop owner in that comment he has been very helpful in other aspects and accommodating, I just feel that it was a gimme getting that cert and it could have been taught a lot better

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    Quote Originally Posted by johnnyblackau View Post
    ..... So is anyone game to try ad explain it to me? How to calculate cns % for dives and how to track it over multiple dives also can someone explain otu and how they are worked out and tracked ?
    Sure. here you go

    Alberto (aka eDiver)

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    The current advanced nitrox text from TDI is pretty decent, it might be worth ordering a copy to look through.
    Last edited by rongoodman; August 23rd, 2013 at 08:17 PM.

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    The "new" padi nitrox course just like the "new" DM course massively dumbs down the theory. This is where the quality of instructor comes in. The stuff is still in the course, can still be taught and should really still be taught. A good instructor will cover all of that as normal. A poor/lazy one just teaching the bare minimums to the letter does not.
    My suggestion - find a better instructor.
    Anyone taking offence at anything in my posts - tough. It's only an internet forum. Stop being over-sensitive. The real world isn't as warm and fuzzy.
    Remember, underwater only YOU are responsible for YOUR own safety. Nobody else is.

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    Okay so i've managed to read up on a few things that have filled in a lot of the gaps! thanks alberto!

    2 more questions:

    1: OTU - I played with teh multi deco software and when it produces the plan it shows otu units. How many is considered the limit and do these clear and have a half life like the CNS clock does?

    2: CNS Clock - Is it right to think that if you rack up say 9% after dive 1 at 9am and add another 17% after dive 2 at 11am. would your clock be back to 9% at 9am the next morning?

    thanks

    ---------- Post added August 26th, 2013 at 02:19 PM ----------

    Sorry to last post with wrong info... I was confused re times to clear the clock. I read it is 1 hr after a dive it will be reduced 50% from that dive? Is that right if so when is it 24hr later is that if you exceeded your 100% ???

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    I'd forget completey about whole body oxygen toxicity. You have to produce some seriosly odd CCR profiles over a few weeks to come near a limit. Its simply not possible on OC.
    Anyone taking offence at anything in my posts - tough. It's only an internet forum. Stop being over-sensitive. The real world isn't as warm and fuzzy.
    Remember, underwater only YOU are responsible for YOUR own safety. Nobody else is.

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    Can you clarify my understanding of when the cns clock is cleared?

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    When we used to teach the oxygen clock calculations with tables in the PADI nitrox course, the guideline was that exposures accumulate over a 24-hour period, so 24 hours after your last exposure you would have cleared the cns clock. Now dive computers actually calculate offgassing according to their algorithm. So computer users will see the oxygen exposure percentages drop during surface intervals.

    BTW, for those amongst my students who are interested, I get out a copy of the old nitrox manual and the tables to teach them how to figure out their oxygen clock exposure. But the fact of the matter is that it's information that the vast majority of ordinary recreational divers don't need, considering that its extremely unlikely that any of them will ever exceed oxygen exposure limits. And for those who had to learn to figure it out with those tables in order to take the old nitrox test, I'd be willing to bet that virtually none of them would be able to remember how to use those tables once they started diving with a computer. I never see anybody diving nitrox without a dive computer even though we still see lots of air divers who don't have dive computers.

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    Quote Originally Posted by johnnyblackau View Post
    Next year i'm considering doing the adv. nitrox course with my LDS. At themoment I have my padi nitrox cert (which i was quite dissapointed in). It failed to touch on CNS clock and how to calculate your oxygen exposure. It really showed a reliance on leaving that up to the computer.

    So my head will be in the right spot prior to doing the course has anyone got some advice on how to calculate CNS clock, exposure and what are OTU and can someone explain how they fit in?

    Blackie
    This thread has gone all over the place except for what the OP asked!
    Short answer: CNS clock is in standard NOAA tables. That's your "confusion/seizure limit". Separately, OTU's are a "lung damage calculator", and are accumulated generally over days.
    1) Pick a max CNS P02 and don't dive past it. Most guys stick with 1.4; short exposure to 1.6 is tolerated without seizures. Review NOAA CNS Percentage Exposure Table and Navy Decompression Tables in this regard. You are max'ed out at a PO2 of 1.4 after 150 minutes (100% CNS clock). A CNS clock of 100% is reached at PO2 1.6 in 45 minutes. Multiply inspired O2 by the atmospheres at depth to find your PO2. See the Surface Interval Credit Table to determine how fast the CNS clock dials back. It recovers very quickly: a 100% CNS clock decreases to 7% in 6 hours. A 100% CNS clock is at less than 1% in 12 hours. After which (from a BRAIN standpoint alone) you can go right back to high oxygen exposure! That's why certain Navy dive tables include air breaks - to allow the CNS clock to recede after periods of high PO2 exposure.
    2) OTU's are a separate subject: PULMONARY oxygen toxicity. Basically, oxygen burns your lungs. We see it in the Intensive Care Unit all the time with a major injury on a ventilator with 95% O2 for days. That's a PO2 of .95 x days, increasing the patient's lung failure beyond some original trauma - the cure is as bad as the disease in this case. Same thing happens at higher PO2s in a shorter interval with diving, hence: OTU's.
    An OTU is breathing pure oxygen at sea level (PO2 1.0) for 1 minute. Longer exposures or higher concentrations of O2 at depth give you higher OTU's. For this discussion, see http://www1.maths.leeds.ac.uk/~read/scuba/baker3.pdf (only read the first 3 pages). This gives you the basic theory. For use of OTU's, see Pulmonary Oxygen Toxicity with Technical Divers | SDI | TDI | ERDI for a very brief discussion, and http://njscuba.net/zzz_gear/deoc_tol..._high%20o2.pdf for a more complete discussion. This last will tell you that from a pulmonary standpoint, 600 OTUs is tolerated with mild lung irritation in a single day, and 2,800 OTU's can be tolerated over 9 days. This is old data and some additional research will show you the latest. Going back to our ICU patient, we see that on 95% O2 for 24 hours, he/she is exposed to 1368 OTU's. In less than 2 1/2 days, the 2,800 OTU threshold is reached. Someone requiring max O2 to stay alive will have measurable lung damage in a week. Don't go there as a diver with air or Nitrox at depth over a week's dive holiday.
    couv likes this.

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    Thanks for that last post.... It was hhonestly the way that really cleared up what otus are and how of and cc dives are affected. Thanks for that. What the formula or table to calculate the time at which a particular cns level will degrade over time?

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