ppO2 for nitrox, why so low?

[boulderjohn]

I think the last posts are indicative of a problem I mentioned earlier. Was the death clearly a result of oxygen toxicity, or was some other factor involved? The description of the July 2007 case sounds like it could also been immersion pulmonary edema.

 

[boulderjohn]

NOAA’s table allows up to 45 minutes at 1.6.

I cannot remember what thread it was in, but I believe we recently learned that the history of time limits associated with oxygen exposure are pretty much guesses.

 

[TONY CHANEY]

I cannot remember what thread it was in, but I believe we recently learned that the history of time limits associated with oxygen exposure are pretty much guesses.

I agree. But when someone post that O2 tox will happen @ 1.6 ata we had better think about some of the deep air record dives by ones like Hal Watts, A. J. Muns, Neal Watson, and others. Take for example Bret Gilliam’s record dive to 452 on air. If my math is correct then that would be 3.086.

 

[Gareth J]

Why, when everyone has any discussion about issues that are about physiology issues, decompression, O2 tolerance, etc, do people expect a black and white answer.

The human body is not a machine, not identical individual to individual. Even developing data on a specific individual,the result is not repeatable day to day.

It is not possible to state if you do x profile you will be guaranteed dci free. Add 1m or 1 minute and you are guaranteed to be bent.
Dive with a PO2 of 1.6 you are 100% guaranteed safe, a PO2 of 1.61 you are guaranteed to have a hit.

There are far to many variables. Most theory / best practice is based on a mix of theory, analysis, and practical experience. It is heavily weighted to minimise risk.

I don't think any one would be happy if you said its 50:50 that you will be Ok, or 90%, or 99% etc.

Point 1.
Physical fitness is a variable in the amateur diving community.
Age is a variable in the amateur diving community.
Environment is a variable.
How our physiology behaves day to day a variable.

Data is limited because of the ethics of human experimentation.
Despite this, the Navy's of the world have carried out tests.
One of the repeated results is the variability of the results, even with the same individual.
In the case of O2 tolerance, when the US Navy was carrying out tests. they could put the same super fit individual through the same tests, on occasions the individual could withstand very high PO2's BUT, on other occasions they would fit on PO2's below 2.0. There was no particular 'trigger' that they could determine for this variability.
There where triggers they could introduce that would increase the likelihood of a fit (CO2, Heavy work, Stress, Fatigue, Cold, Heat etc).
If they kept the PO2 below 1.6 (1.4), then it appeared statistically unlike that the diver would fit.

Along time ago I was taught a PO2 of 1.6 is the maximum..... In IDEAL conditions.
For any adverse, remove 0.1. - work, cold, fatigue, stress, etc. So a PO2 for 1.4 for the 'dive phase', 1.6 for the rest (decompression) phase.

Now, my maximum PO2 is 1.3, but then I'm on CCR. In an emergency, I allow a higher PO2 for off loop bailout, because the PO2 will crash as I ascend. For that brief instance, of high PO2, the risk of a CNS hit is very low.

Yes, you can do a dive with PO2 of 2.0 and have a safe dive, today, tomorrow, but eventually your luck will run out. Thats your problem, except of course, it becomes your buddies problem, and the rest of the dive team. Potentially, my problem when someone decided there are to many accidents.

 

[caruso]

"Most"
People do tox at 1.6, especially when combined with exercise and CO2. They almost always lose their regulator and drown.

I've never read a single article or study that supports your opinion that most divers tox out at 1.6 and lose their regulators and drown.

Not one. Since you have stated such an absolute fact you must have a source?

I for one would really like to see it but I doubt it's going to happen.

 

[kotik]

I've never read a single article or study that supports your opinion that most divers tox out at 1.6 and lose their regulators and drown.

Not one. Since you have stated such an absolute fact you must have a source?

I for one would really like to see it but I doubt it's going to happen.

Not to answer for anyone who can answer for themselves, but I believe you are misunderstanding rjack321's statement. I don't believe he's trying to say that most people tox at PO2 1.6

The way I read the statement, he is only saying that it has happened... nothing about the frequency. I have no citations to give because I'm lazy, but I do believe there are cases reported where at least a plausible cause of an accident was O2 tox from PO2 at or below 1.6. For at least some support of this, you could look above at Kensuf's post #30, looks like he has some memory of one such event. As someone mentioned, O2 tox is hard to pin down after the fact, since the person that we're speculating about is not in a position to tell the story themselves.

As for rjack321's statements about frequency. He quoted the word "most" with emphasis from someone else, as in: "Most" people do not tox at 1.6ata. That is certainly true.

His other statement is that if someone does tox and goes into convulsions, they almost always are going to be dead because of it. That is also true.

Personally, I just don't see any reason to push high PO2's for the average Joe Diver.

Yes there are guys who went out to set records that hit some crazy numbers. Those are also extremely knowledgeable divers, and they went straight down and came straight back up, with the benefit of a lot of accumulated experience in expending the absolute minimum of energy during the dive. These guys are very different than the average person that just took a nitrox class. Even so, there are plenty of stories about attempts at these records where expert divers ended up dead.

There is just no good reason to play around with high PO2's in casual recreational diving. Whatever perceived benefit is very marginal, and the consequence that's possible is the worst there is. Your life is not going to be any better because you dive at 1.8 or 2.0, but it can damn well be over because of it.

 

[caruso]

Not to answer for anyone who can answer for themselves, but I believe you are misunderstanding rjack321's statement. I don't believe he's trying to say that most people tox at PO2 1.6.

Could be. The post is written in a rather ambiguous fashion.

 

[Jack Hammer]

Maybe 8 or 10 years ago I recall seeing the results of a Navy study that was older at the time. It had known numbers of toxing events for different PO2s, I believe all young healthy Navy divers in hardhats. The numbers were around nil at about 1.4 and climbed as the PO2 went higher. There were quite a few at 2.0 and above.

For those unaware, pure O2 was used as deep as 100'/30m quite some time back and regularly to 50'/15m around WWII. There were incidents and the depths were changed to shallower.

I dont have a copy and may have messed up the details, that is how I recall it. It was most likely linked on TDS when it was active. Maybe someone else recalls or has better details.

 

[mainedvr]

I think Kevin puts it very well, don't screw with CNC toxicity limits, they are not forgiving.


Another point to remember, is a fitting diver in a chamber during treatment is manageable. A fitting diver in water has a very high risk of turning into a fatality!

All the more reason for me to eat more junk food and work out less jk

 

[TONY CHANEY]

Not to answer for anyone who can answer for themselves

You could have stopped at that. I really value each and every diver’s opinion but when someone comes out post “facts” I’ll call them on it. So please give rjack321 a chance to post his resources. I am not a “Know it all,” and will be the first to say that I am wrong if I am.