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

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When an Oceanic computer is set at a pO2 of 1.6, the warning goes off at 1.4 and the alarm goes off at 1.6. I have mine set at 1.6 instead of 1.4 because I don't want the warning to be going off all the time at 1.2 Like many, I'm acutely aware of my MOD. I rarely venture above 1.4, and then, not for long.
 
Not convinced that you should be using that word (red/bold above).... Though not something immediately apparent like convulsion, the potential injury is real (and seems to be unpredictable)... Again, always pushing to the limits is a questionable practice.

Although, this is just my opinion.


YMMV
Hi Bob,

To what are you referring? Occasionally diving past the O2 exposure limit on my Oceanic computer, when this is far below the O2 exposure limit on my Shearwater? Or, are you referring to diving within the NDL of my computers? I don't get what am I always pushing to the limit

Good diving, Craig
 
Hey Craig, no debate. We make the decisions. With conflicting info, we can only hope we have chosen the correct set to follow. So much of this is still unknown.

Pushing the limits is where it gets interesting...

I just can't call it "safe". I also am cautious as to someone thinking they can be safe with possibly different devices when reading this.

Again, we each choose how to act.

I always respect what you post.
 
Hi @Astran

The answer to your question is actually quite simple. Oceanic employs an O2 exposure algorithm that works the same way as the NOAA table. It calculates your O2 exposure for a 24 hour period and does not give you credit for O2 elimination until 24 hours after that exposure. So, if you do four dives in a day, it will count all 4 dives as exposure, If you do a dive the next morning, it will be a 5th dive in the 24 hour window. You get no credit for elimination until 24 hour after the end of the dive for the morning before. I have gone over the 24 hour O2 exposure on my Oceanic VT3 on many occasions. This is particularly true when I am doing long drift dives in SE Florida at 4 per day, diving 36% nitrox. My VT3 computer will continue to track nitrogen exposure, it will be on an alternate screen or the nitrogen exposure bar graph. I have a newer Geo 2 that I was using for a backup that I believe tracks O2 exposure like many other computers (see below)

Shearwater, like on my Teric, and Dive Rite, like on my Nitek Q track O2 exposure differently, by giving you a 90 minute O2 elimination half life. For every 90 min you are on the surface, you eliminate one half of your O2 exposure. This carries over less O2 from dive to dive for repetitive dives and essentially eliminates O2 exposure over night. I have never come close to the O2 exposure limit with this algorithm doing 4 or 5 rec or light deco dives per day. For this reason, I will disregard the O2 exposure information on my VT3 and continue diving, paying attention to my nitrogen exposure as always.

Good diving, Craig.
This is from the specs in the owner's manual for my VT3:
upload_2019-10-2_13-16-12.png

This discusses the O2 tracking from my Teric owner's manual. They employ the same NOAA table but credit for the 90 min elimination half life. They do not discuss the derivation of this rule, however:
upload_2019-10-2_13-28-29.png
 
Because, well you know, I sometimes use it for other stuff that I think is more important [than ultraconservative max 02 settings that can be safely ignored], such as NDL limits, gas supply remaining, elapsed dive time, depth, ascent rate, that sort of thing.

Why did I ignore my computers 02 alarms given that both of my computers and my buddies computer all indicated violations?

Because it just didn't seem right. Because we were at about 60' at the time. Because I didn't think our dive profiles were all that much different than dives I've done 100s of times before. That's all I got..

Although it was a somewhat impulsive decision it appears to have been the correct one given the information provided on this thread primarily by @scubadada.

I wouldn't go so far as to say it was the correct decision.

As it turns out, your CNS O2 exposure was likely within limits of different methodology than your computer was using, but you had no way of knowing that at the time because unlike Scubadada, you didn't have another computer you knew was running that calculation. All you knew were three dive computers all were telling you to end the dive.

Scubadada is making an informed decision between two computers running different calculations, one more liberal that the other -- but it was a known calculation, he knows he's within limits on at least one of them and he knows why they differ and what that means. Your choice was between computers you knew were telling you to quit for reasons you didn't fully understand and....the unknown.

Not trying to beat you up, but IMHO, that wasn't a very prudent thing to do. It might be a reasonable thing, going forward, to decide which methodology you believe is best and buy a computer that implements it, but if you don't have it with you, or you aren't calculating CNS halftimes yourself (which is not hard), then best practice is to follow the computer. If I ignore my NDL limits on my computer and find out, only after the fact, that I was within limits using some other algorithm, it doesn't make my decision in the moment a good one. It just means that I was lucky.

Past practice is a fallable guide. CNS hits, like DCS hits aren't that reliably predicted. It's sort of like how many bullets to put in the gun before playing roulette. Plus, unless you were using a different computer, then the fact that it didn't alarm on all your previous trips, and did here, suggests that you were diving more aggressively. And, while I haven't done the calculation, you may have been at or even above recommended whole body limits on the days with a ton of dives, which could have been problematic had you had a DCS hit and needed treatment in a chamber.

Kudos for coming on the board to figure all this out. I guess my one piece of advice, which of course you can take or leave, is that there's always another dive. If my computers were hollering at me to turn the dive, and I wasn't 100% confident that I could safely ignore them, I'd turn the dive and figure it out on the surface.
 
I read that with pulmonary toxicity, once you start feeling the breathing "issues", it'll still be a while before it gets serious. I'm not sure how it progresses in the eyes, though: the first sign is retinal detachment? @Duke Dive Medicine probably knows.
 
I wonder that no one has brought up that CNS hits don't need higher PPO2 to happen. There are documented cases of hits happening at just over 1.3 with other factors coming into play. Repetitive exposure over short periods, individual physiology, individual tolerance, etc. A diver we used to see on Lake Erie when I was doing my tech training in 2008 toxed in a cave at 1.3. He felt it coming on, signaled his buddy, tried to get shallower (not easy in a cave), but to no avail. His buddy watched him die. Couldn't get the reg back in and he drowned.
After that, it became std practice for our group to mix for 1.3 for the working portion of all dives. I still do that today. 1.4 is well researched but it's not the last word.
Just like deco models. The diver has to do their own research, decide what works for them, and determine their own level of conservatism taking individual and environmental risk factors into account.
While there is a lot of science behind all of this, there's still an element of voodoo and educated wild ass guessing involved.
 
I read that with pulmonary toxicity, once you start feeling the breathing "issues", it'll still be a while before it gets serious. I'm not sure how it progresses in the eyes, though: the first sign is retinal detachment? @Duke Dive Medicine probably knows.
HBOT is sometimes used to treat or keep retinal detachment tissue alive but I dont see any evidence it causes or exacerbates them.
 

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