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

[KenGordon]

Perhaps I'm confused in this regard but it is my understanding that setting my P02 to 1.6 allows me to use a richer EAN mix for a given depth, thus extending my NDL bottom limits and/or shortening my surface intervals. And yes, because I am diving a more aggressive P02 setting I will probably stay longer at depth and thus run my 02 exposure higher than I would have at P02 1.4 but so what? No harm done, longer dives with less depth restriction, and in the end it's all good.

If that were true we’d all be diving 100% and life would be a lot simpler.

Don’t dive an actual ppO2 of 1.6. That would be using 32% at 40m. Your CNS clock then hits 100% after 45 minutes, rather than 180 minutes for 1.3. The probability of a fit increases worse than linearly with ppO2. Also, since you are actually the working part of the dive you will be generating more CO2 which turns out to make fitting more likely.

As I understood suggestion for telling the computer that the max ppO2 is 1.6 was because it will give an annoying early warning at .2 below the limit. You should not ignore the computer telling you that your current ppO2 means you have passed the maximum operating depth of your gas. That may lead to serious issues, fitting, drowning, etc. Nitrox courses teach that susceptibility to fitting is variable, so just because it was ok on Tuesday doesn’t mean it will be ok on Thursday.

Also you will find that a rich mix vs a more moderate mix isn’t a huge advantage in NDL.

You might want to find a Nitrox course that properly covers this. The BSAC ones do but are probably not available. For sure TDI Advanced Nitrox does, but that might be going a bit far. I just looked at the TDI Nitrox standards pdf but it isn’t obvious whether it is covered or not.

 

[uncfnp]

@Astran If you haven’t yet read this...

OXTOX: If You Dive Nitrox You Should Know About OXTOX — DAN | Divers Alert Network — Medical Dive Article

You gives you a bit of the history behind the limits and discusses the high variability between dives and divers when it comes to O2 exposure. It discusses ppO2, exercise, temperature and even gas densities as variables.

As you know I am very conservative on NDL but pretty liberal with O2. But even I would not ignore an alarm for oxygen exposure. Sure the odds are heavily in your favor but unlike rolling the dice for NDL, losing with oxtox isn’t fixed with a ride in the chamber and there really isn’t such a thing as a mild hit. More like one and done...forever.

And at risk of further mudding the water, diving high oxygen exposures produces tissue damaging free radicals beyond the body’s ability to counter easily. Excess free radicals are not a good thing.

 

[Astran]

You should not ignore the computer telling you that your current ppO2 means you have passed the maximum operating depth of your gas.

If you are referring to my specific dive profiles, the 02 alarm was not indicating a violation of the MOD for the gas mix I was using.

 

[tursiops]

If you are referring to my specific dive profiles, the 02 alarm was not indicating a violation of the MOD for the gas mix I was using.

That's only because you were set at 1.6, which is too high for the working part of a dive.

 

[Astran]

Also you will find that a rich mix vs a more moderate mix isn’t a huge advantage in NDL.

Interesting. I must admit I never really looked into the actual differences, I just assumed it would be enough to make it worth setting the P02 higher (and assuming the slight greater risk) and dive the slightly richer blend to maximize my allowed bottom time in regard to NDLs. Truth is I typically run out of gas before I get there, except on repetitive deep dives which isn't all that often.

 

[Astran]

That's only because you were set at 1.6, which is too high for the working part of a dive.

I flipped to the P02 screen when I got the warning and it was at 1.03 and continued to drop to about .95 for the remained of the dive, which was at about 65' at that time.

I don't believe I violated what would have been the MOD had I been set at P02 1.4 during any of the dives I did over the 3 days.

 

[tursiops]

I flipped to the P02 screen when I got the warning and it was at 1.03 and continued to drop to about .95 for the remained of the dive, which was at about 65' at that time.

I don't believe I violated what would have been the MOD had I been set at P02 1.4 during any of the dives I did over the 3 days.

This is written proof that you do actually do NOT understand the difference between going too deep and going too long, with respect to oxygen toxicity. For your sake, you DO need to understand how these are different and what your alarms mean.

 

[scubadada]

Hi @Astran

Appears none of your dives was over the 1.4 MOD for the gas you were using. Not sure if you did that on purpose, or was accidental. However, the two pathways to oxygen toxicity are breathing a high pO2, not necessarily time dependent, and cumulative exposure, discussed in detail earlier in the thread. The Oceanic computers give a warning 0.2 lower than the setpoint. When set at 1.6, you must pay attention to the warning, rather than the alarm to generally stay below a pO2 of 1.4

Nothing like knowing you own computer

 

[Astran]

This is written proof that you do actually do NOT understand the difference between going too deep and going too long, with respect to oxygen toxicity. For your sake, you DO need to understand how these are different and what your alarms mean.

I am (and was) well aware that the P02 setting on the alternate screen was not related in any way to the MOD of the breathing gas that my computer was set for and the alarm was indicating cumulative 02 not a violation of the MOD.

 

[Astran]

Hi @Astran

Appears none of your dives was over the 1.4 MOD for the gas you were using. Not sure if you did that on purpose, or was accidental.

I was aware in advance of the dives that I would not exceed the MOD but it would be close on a few of the two dives. In fact I had no control over which blend I was using as the rental tanks were supplied by the Dive Ops and I didn't want to go to the trouble to rent tanks with a higher blend elsewhere (on the day we were diving EAN 32 on a wreck that sits at around 115').