Nitrox max oxygen exposure question

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ls1dreams

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Hi all,

I was recently nitrox certified. I understand the NDL/MOD implications for individual dives, but am a little confused on continued oxygen exposure over multiple dives.

I realize oxygen is not the same as nitrogen accumulation, but I'm trying to debate this guideline:

SDI states that you should not exceed 2.5hrs of the MOD limit for EAN dives in any 24hr period.

Now, of course, plenty of people do liveaboards or multi-dive days where 5 dives in a day may accumulate up to 5+ hrs of dive time. Not all of this time is at the MOD, of course though.

Do most people mostly just ignore this guideline given that they stay within a MOD using the 1.4 guidelines? Or do you look at accumulated oxygen time somehow on your dive computers?

CNS feels generally pretty low risk here to me because I'm guessing most dives will only be at MOD 50-75% of the time, but on 5 dive days at 30-40m it feels closer to me.
 
CNS Oxygen Exposure Tables

If you follow agency guidelines (which you should because they are conservative and safe) you should absolutely track your CNS exposure, although with recreational nitrox you will not likely hit the clock limit. Most computers will track it for you as well, or you can calculate it after each dive using the SSI CNS tables.

I asked a similar question more than a year ago in the thread above, although I was more concerned about understanding how computers did the calculations. There is a very good paper that someone posted to the thread that helped my understanding quite a bit.
 
Thanks - looks like I can simply watch CNS % on most dive computers, but should be low risk with the surface intervals & reasonable half-life.

Curious how many DC's give SI credit.
 
When I took my nitrox certification course, agencies were much more uptight about it, and the test was pretty tough, with lots of math. A lot of the math had to do with figuring long term exposure for pulmonary oxygen toxicity. When I did the problems that got a hypothetical diver near those limits, I thought, "Well, that is certainly not going to be a problem for me."

I assume a lot of people came to the same conclusion, and I assume that is why that topic is barely mentioned in current certification classes.
 
Oxtox comes in two flavors: CNS and pulmonary. In principle, the two are independent. The former is acute, the latter is chronic. At depth, what you should worry about is CNS. Through the whole dive and the subsequent dives, what you should worry about is pulmonary.

Don't mix them up.
 
NOAA tables use exposure in a 24 hr rolling period. It is not difficult to exceed this limit with recreational diving with 4-5 dives in a 24 hour window. My Oceanic computer uses this method. Other computers give you O2 elimination with a 90 minute half life. It is essentially impossible to exceed this limit in rec diving. My Teric uses this method. I choose to follow the method with the elimination half life.
 
I use a Teric these days, and a Perdix/Petrel before that. Even on a week-long liveaboard with 27 dives on 32% I've never come close to the limit, so I don''t worry about it. Using rich mixes for deco would be a different story completely.
 
I use a Teric these days, and a Perdix/Petrel before that. Even on a week-long liveaboard with 27 dives on 32% I've never come close to the limit, so I don''t worry about it. Using rich mixes for deco would be a different story completely.

How many dives a day were you doing? I feel like it might get close if you are diving EAN32, 5 dives a day at reasonably deep depths (90-100')
 
Typical Caribbean liveaboards do 5 dives daily. Even if you drop down toward the MOD, though, you aren't going to be there for long, especially on a single tank. You typically spend the last part of the dive puttering around on the reef under the boat.
 
https://www.shearwater.com/products/teric/

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