If you had to choose, 80% or 100% for deco gas and why.

[kwinter]

Bottom line is that OxTox is dosage related. That is a combination of amount/concentration/pO2 and time. High pO2 for a short time is probably not going to be instantly fatal. Jim Miller was on the wrong gas for a while before he seized. So were all the others I've heard about at very high pO2s. But there have also been cases of OxTox at 1.4 after prolonged exposure. It's the reason pO2 higher than 1.3 is not recommended after a CCR dive (David Sawatzky study). Even the supposedly eminent Dr. Simon Mitchell agrees, and we hardly agree on anything.

OxTox is real. And while we don't have any definitive measure, CNS clock is a reasonable attempt to provide a guideline. It is not a line in the sand that you should never cross, but it is a warning signal. O2 half life clearance is also not exact, but a reasonable measure to say you shouldn't do that 5000% of CNS clock twice a day for a week.

Watch CNS clock and OTUs, but know what you're looking at and make informed decisions. And do not spew crap about them being worthless and meaningless on a forum that may have a great influence on people who are even less informed than you.


iPhone. iTypo. iApologize.

 

[kr2y5]

1.0 means it happens 100% of the time and 0 means it happens 0% of the time. But I think you know this as your profile lists you as a research scientist. I think you're trying to be obtuse in a flimsy attempt to make a point, when you really don't have one.

On a few occasions, you made strong claims that CNS clock is totally "useless" because there is anecdotal evidence that people did not convulse and drown after exceeding 100%, and you seemed to be arguing (pardon if I misunderstood your point) that you really expect over 100% to mean that a person will absolutely deterministically drown for sure, and under 100% to mean that a person will absolutely definitely stay on the safe side.

The point I have been trying to make is that labeling things as "useless" just because they are not deterministic is uncommon and completely impractical, and furthermore, that it makes even less sense to try to try and justify your position from a probabilistic perspective, as you seemed to be trying to do in the preceding post.

With respect to the obtuse point, the one thing worth realizing is that real-world phenomena are continuous in nature, which means that you have to be careful not to confuse "very highly probable" with "deterministically guaranteed", and certainly not try to draw any general conclusions of any kind based on isolated data points from anecdotal evidence.

Let me illustrate this with an example. Perhaps this morning, you woke up at exactly time A, then you blinked your eyes at exactly time B, the temperature outside was exactly C degrees, the wind was blowing exactly at angle D, and at exactly E miles per hour... as you probably realize, if you tried to "predict" if this exact combinations of parameters will happen, you would likely conclude that the probability is 0, and hence, following the logic you seem to favor, that it is impossible... and yet, here you are, apparently the impossible has just materialized.

Unlike anecdotal evidence that you seem to rely on in your criticism, the CNS clock was apparently based on experiments that involved multiple divers doing multiple dives. Its predictions are not absolute, they are statistical in nature, which means nothing is guaranteed or impossible, it is only more likely as you exceed some threshold, and to even understand exactly how likely it is, you would need many more experiments, with many more divers.

CNS clock appears to be based on the same kind of science that is used to generate weather forecasts, cancer risk assessments, compute NDL times, generate medical diagnoses, or that allows financial institutions of all kinds to make money, there is just less data to give you exact error margins. Still, you probably get better error margins following the CNS clock than what you get based on your gut, so why would you call it useless, and what else would you propose instead that works better at keeping you safe?

 

[PfcAJ]

If you go back a few posts, you'll see what prompted this long drawn out discussion was the assertion that breathing 100% was suboptimal because it affects the CNS Clock. My position has been (and continues to be) that the argument is invalid because the CNS Clock is not a reliable predictor of oxygen toxicity and that 100% oxygen is a superior gas because it contains no inert gas and gets you out of the water faster. Lets make sure that's clear. From what I've seen and experienced, getting bent around what percentage you are on the CNS Clock is a pointless exercise and that the only reliable way to manage oxygen exposure is low bottom gas po2s, no greater than 1.6 on deco, and gas breaks.

The CNS Clock is NOT the same as weather forecasts, cancer risk, or NDLs, etc. Its quite different. Cancer risk is expressed in a % (ie you have a 5% risk of getting cancer after exposure to x or y). Deterministic deco plans are similar (do x plan and you have y% risk of DCS). Now, if we came up with something that DID give us an expected probability of o2 toxicity on a given dive, I would be a happy camper. But at present, we don't have anything even remotely close to that.

Fwiw, I would not predict something like you said as "0". Maybe close to 0, but not 0. If you did a 45min dive at 300ft on air then immediately came to the surface, I would, however, predict your DCS chance to be at "1". Nice try at putting words in my mouth.

The amount of weight you guys are putting into this CNS Clock is astounding. Its simply not a great tool for much of anything because it DOESN'T WORK. If your fuel gauge in your car said you were full, yet you were actually out of gas, you'd say its broken. Likewise, if it said it was empty but you could drive another 500 miles, you'd say its broken. Why bother having a gauge if it doesn't reflect reality? I could get down with it if the gauge was, you know, kinda close. That would be ok. But this particular gauge (CNS Clock) doesn't work for ****.

 

[kr2y5]

Nice try at putting words in my mouth. (...) CNS Clock (...) DOESN'T WORK. this particular gauge (CNS Clock) doesn't work for ****.

Ok fine, I think I understand. You are making extremely strong claims, but that is just for emphasis, you do not mean it verbatim. BTW, to respond to the OP, while my experience is negligible-to-none compared to others, personally I also do favor 100%, and the concerns about increased CNS risk compared to 80% do not quite resonate with me, while the idea that I would have ample personal supply of O2 handy in case the dive needs to be cut short seems convincing.

 

[kwinter]

If you go back a few posts, you'll see what prompted this long drawn out discussion was the assertion that breathing 100% was suboptimal because it affects the CNS Clock.

I don't remember seeing anything about "suboptimal." Using 100% O2 is more efficient and reduces deco time. No one denies that. But it carries with it higher risk that has to be weighed against the benefits. The OP was asking for opinions about whether people prefer 80% or 100%. Some like the greater efficiency and reduced time that 100% gives while they ignore or minimize the risk. Others see the benefit of 100% over 80% as marginal and don't think it is worth the extra risk.

That's why they are opinions, AJ. Everyone has one. But you were the only one to completely dismiss as garbage the entire block of study that has led to the CNS clock and half life tables developed by and for divers including the US Navy. That's fine too, you're entitled to think it's all crap. You're also entitled to breathe argon in your tank and completely dismiss all tables and deco algorithms if you want. The years of study that allow you to dive safely obviously mean nothing. But when you publish the AJ book of safe diving, don't expect me to buy a copy.

 

[danvolker]

On a few occasions, you made strong claims that CNS clock is totally "useless" because there is anecdotal evidence that people did not convulse and drown after exceeding 100%, and you seemed to be arguing (pardon if I misunderstood your point) that you really expect over 100% to mean that a person will absolutely deterministically drown for sure, and under 100% to mean that a person will absolutely definitely stay on the safe side.

The point I have been trying to make is that labeling things as "useless" just because they are not deterministic is uncommon and completely impractical, and furthermore, that it makes even less sense to try to try and justify your position from a probabilistic perspective, as you seemed to be trying to do in the preceding post.

With respect to the obtuse point, the one thing worth realizing is that real-world phenomena are continuous in nature, which means that you have to be careful not to confuse "very highly probable" with "deterministically guaranteed", and certainly not try to draw any general conclusions of any kind based on isolated data points from anecdotal evidence.

Let me illustrate this with an example. Perhaps this morning, you woke up at exactly time A, then you blinked your eyes at exactly time B, the temperature outside was exactly C degrees, the wind was blowing exactly at angle D, and at exactly E miles per hour... as you probably realize, if you tried to "predict" if this exact combinations of parameters will happen, you would likely conclude that the probability is 0, and hence, following the logic you seem to favor, that it is impossible... and yet, here you are, apparently the impossible has just materialized.

Unlike anecdotal evidence that you seem to rely on in your criticism, the CNS clock was apparently based on experiments that involved multiple divers doing multiple dives. Its predictions are not absolute, they are statistical in nature, which means nothing is guaranteed or impossible, it is only more likely as you exceed some threshold, and to even understand exactly how likely it is, you would need many more experiments, with many more divers.

CNS clock appears to be based on the same kind of science that is used to generate weather forecasts, cancer risk assessments, compute NDL times, generate medical diagnoses, or that allows financial institutions of all kinds to make money, there is just less data to give you exact error margins. Still, you probably get better error margins following the CNS clock than what you get based on your gut, so why would you call it useless, and what else would you propose instead that works better at keeping you safe?

How about this.... The CNS clock is like a PADI Instructor....it might work for you....or, it might not

 

[rjack321]

ppO2 of 1.6 is the same anyway, 30ft or at 20ft. A max of 45mins even if you accept the CNS clock concepts as valid. The only way you reduce the magic "CNS clock" is by getting to a lower ppO2 through ascending, which is avoiding the full benefits of the deco gas and trying to use the reduced pressure to offgas (in the extreme case you're on backgas). You can ascend from 30ft to 20ft and remain on the 80% for that lower ppO2 of 1.28
Or you could ascend from 20ft to 10ft on 100% and wait for it... have a ppO2 of 1.3

Whoopdie friggin do.

The only reason I could see using 80% is in a cave where a 20ft stop is ridiculously impractical (e.g. a high flow restriction) but a 30ft stop is peachy. In the ocean (and for some reason the 80% proponents all seem to be ocean divers) I see no good reason to have nitrogen, the gas most likely creating my ceiling in the first place, in my final deco gas.

 

[TONY CHANEY]

. In the ocean (and for some reason the 80% proponents all seem to be ocean divers) I see no good reason to have nitrogen, the gas most likely creating my ceiling in the first place, in my final deco gas.

Not me! I do caves as well but use 100% when diving them for the reason you stated.

 

[waterpirate]

The answer is no. None of the information or entertainment offered in this thread was compelling enough for me to change my application of deco gas choice or my belief in the cns clock. The only thing I learned was that the concept of the clock has no real scientific basis other than when you adhere to accepted limits your likely to not have a major event.
Eric

 

[jadairiii]

Wow, another 80/20 thread! For those that missed the 90’s here is a recap:

1. You should be good on air deep
2. 80/20 is a better choice deco gas
3. All tech diving is really solo diving
4. Be prepared to leave your buddy in an emergency
5. Nothing wrong with bungee wings

And my ALL time favorite……

1. “that” does not work “here”

That will save you all kinds of time re-reading the Cavers and Tech Diver forums and prepare you for when you’re buying drinks for that 300 pound tech instructor while he preaches to all those unsuspecting novices what an expert he is and the fact that he cant really talk about his time in special operations…….