Likelihood of getting oxygen toxicity at 1.4 PPO

[Storker]

The thing with pushing po2 is the consequence of a toxic episode. Unlikely to survive

Risk = probability x consequence. The more severe the consequence, the lower the probability I'll accept. And dying is a pretty severe consequence.

But everyone has to decide their own limits.

 

[kelemvor]

Risk = probability x consequence. The more severe the consequence, the lower the probability I'll accept. And dying is a pretty severe consequence.

But everyone has to decide their own limits.

You can reduce the severity of the consequence with a full face mask (or helmet). You can reduce the probability by diving lower pp02, less time, or both. That's how I see it.

 

[PfcAJ]

But what do you consider pushing?

Me personally?

I limit my dives to roughy 1.0-1.2 on the bottom in most cases. Rarely a 1.4, and generally briefly. Deco 1.6, but only when static and comfy. 1.6 time is generally brief except for oxygen deco stops, and I take frequent gas breaks during the oxygen stop.

 

[Seaweed Doc]

Take this with a grain of salt: I just recently nitrox-certified (never had reason to before) and done exactly one dive on nitrox. (Though have a tank full in my garage awaiting the next opportunity...)

Maybe BECAUSE I just went through the course (and was shocked at the 1.4 and 1.6 rule, which was far lower than I had learned in my 1985 OW class), I think there's a bit of confusion. There's CNS toxicity that leads to the 1.4/1.6 rules, and there's the "oxygen clock" type which is pulmonary toxicity. The former causes convulsions underwater and then you die of drowning unless you don't. The latter (if I understand it correctly) "burns" your lungs over time. (My interpretation of "burns" is based on a guess that it's the altered redox potential?) When I learned to dive way back when, I don't think CNS toxicity was fully appreciated, hence less conservative limits.

The drag with CNS toxicity is that response is likely to vary a lot person to person. Are you prone to it or not? Who knows! (Leaving aside the exercise/CO2/drug interaction issues that raise the probability.) If you're not prone to it, you might be fine. If you are, well, not so much.

Pulmonary toxicity strikes me as being related to laws of physics such that you just can't avoid by being genetically lucky. Maybe a bit of variation around the edges from person to person, but not the "all or none" of having a seizure or not at a particular partial pressure.

 

[Lorenzoid]

. . . and was curious to learn whether going past 1.4 or 1.6 would hit you like a rock, or whether that was just an estimate for safe diving like the recreational dive algorithms/tables.

Risk = probability x consequence. The more severe the consequence, the lower the probability I'll accept.

Right. I suspect the "estimate for safe diving" laid out in the dive tables takes into account the severity of the "consequence." A minor case of the bends is relatively easily remedied. Toxing underwater (at least without a full face mask) almost certainly will kill you. Of course, none of this answers the useful question of how much less safe is 1.4 in relation to 1.3, or 1.3 in relation to 1.2, etc.

 

[tbone1004]

Me personally?

I limit my dives to roughy 1.0-1.2 on the bottom in most cases. Rarely a 1.4, and generally briefly. Deco 1.6, but only when static and comfy. 1.6 time is generally brief except for oxygen deco stops, and I take frequent gas breaks during the oxygen stop.

ditto, but on the CCR it's largely convenience. HUDS at 1.0 or 1.1 are usually easier to deal with and the benefits of going to 1.2-1.3 are negligible. I'll deco at 1.6 if I'm not moving, 1.4 if I'm traversing somewhere

 

[Storker]

You can reduce the severity of the consequence with a full face mask (or helmet).

Which brings along a whole 'nother set of issues. AFAIK, they don't teach gas sharing while on a full face mask or a pro helmet, neither in PADI, CMAS nor BSAC rec classes.

 

[boulderjohn]

The current issue of DAN's Alert DIver magazine has some more current information on page #71.

Last year DAN researchers analyzed over 400 U.S. recreational fatalities over a 10-year period to examine the occurence of oxygen-toxicity seizures....[the results included 55 divers on nitrox, with an average mix of 31%.] Only one out of the 55 nitrox fatalities was considered likely due to oxygen toxicity seizures.
​

It is interesting to note that in the only case they found among the "recreational diving fatalities" was the case of "an experienced technical diver" who used double tanks on a dive to 160 feet and apparently believed he did not have a nitrox mix in those tanks. That sounds like a technical dive to me, and the diver was well past a PPO2 of 1.4 for quite some time before he toxed.

 

[kelemvor]

Which brings along a whole 'nother set of issues. AFAIK, they don't teach gas sharing while on a full face mask or a pro helmet, neither in PADI, CMAS nor BSAC rec classes.

It's covered during a FFM class, if you take one. You've got options, the option that's common to all models is to take off the mask and stick your buddy's octo into your mouth. If are risk averse, you will have a backup mask in your pocket to throw on. Most masks also support a second hose for connecting an alternate air source. Could be another tank, could be a pony, etc. You could also or instead use a quick disconnect.

 

[Storker]

As I've already said, everyone has to decide their own limits, given their own level of risk acceptance. And for me, that has to be rooted in my own competence. Being a modest rec diver, I have no other option than to follow the recommendations given by the big agencies (PADI, CMAS, BSAC et al.). And AFAIK they all recommend a max pPO2 of 1.4 bar for planning and 1.6 bar for contingencies (most CMAS 3* certs certify you to max 56m on air. Why 56m? That isn't a round number, neither in meters nor in feet. Well, what's the pPO2 at 56m when your fO2 is 21%?). I'm not competent to do otherwise (and I suspect that most divers aren't either...)