Nitrox max oxygen exposure question

[Jackie Treehorn]

Yeah, this kind of statement is unfortunate, and it's generally the attitude expressed in the PADI nitrox course materials. Along the lines of "We're required to tell you about this, but you can forget all about it because it doesn't matter."

Fortunately instructors are able to enhance the course with their own experience, and provide hypothetical scenarios for students to consider. I'm not sure what "dive your brains out" means, but when I talk about pulmonary oxygen toxicity in the nitrox course, I put together a series of five dives that, while aggressive, are possible and result in hitting the maximum on the DSAT table. It seems that a lot of students that want to take the nitrox class also are interested in more information about diving. I'd rather satisfy their curiosity with examples than tell them "oh, you don't have to worry about that."

We also go through the details of how their computers keep track of their "oxygen clock", and I encourage them to keep an eye on it anytime they dive (even with air) just to get an idea of how much exposure they have on a typical day. Then they can decide what is significant to them, or not so significant.

You should go pull the reference. I'm not posting it here as I'm assuming its frowned upon, but the quoted phrase was a preface to what someone taking the Tec40 should now have to be focusing on. What I quoted, although accurate, wasn't really part of the actual training... just kind of setting the stage for the lesson.

The PADI nitrox course focuses on what it should... and that is real situations where they could get hurt. All the other scenarios being described is trivia for a recreational diver.

You have to start somewhere, Nitrox is an entry level course. Overwhelming students with a bunch of extra non-essential stuff risks them not getting the main takeaway... which is don't exceed your MOD.

People wanting that type of information should be diving more, and then enrolling in AN/DP or Tec40/45.

 

[The Chairman]

You have to start somewhere, Nitrox is an entry level course. Overwhelming students with a bunch of extra non-essential stuff risks them not getting the main takeaway... which is don't exceed your MOD.

This is an excellent observation. Kudos.

 

[yle]

You should go pull the reference. I'm not posting it here as I'm assuming its frowned upon, but the quoted phrase was a preface to what someone taking the Tec40 should now have to be focusing on. What I quoted, although accurate, wasn't really part of the actual training... just kind of setting the stage for the lesson..

Thanks for the clarification... I did think it was an actual quote you had posted, and I'm not sure if it makes it worse that I had no problem believing it was an actual quote from the training material!

 

[Jackie Treehorn]

Haha! Yes, it is an actual quote.

But again, its not part of the actual course lesson, more of the background to set the stage. It's very much similar to how PADI does their documentation... Which I kind of like (I've been exposed to both PADI and TDI).

 

[Storker]

You have to start somewhere, Nitrox is an entry level course. Overwhelming students with a bunch of extra non-essential stuff risks them not getting the main takeaway... which is don't exceed your MOD.

Even though PADI's nitrox class is so simple that if you remember just a bit of your high school physics it's a walk in the park, you could simplify the course even more:

1. Analyze your gas
2. Use 32%
3. Max 30m, or you might have some rather serious problems
4. Done, here's your card. Have fun

Me, I rather enjoyed learning about pulmonary oxtox, running oxygen clock scenarios and finding out myself that for my kind of diving pulmonary oxtox would never be an issue. But then I'm the kind of dude who worked the tables and calculated gas consumption at different depths already during my OW class. And the "dumb it down" approach would have left me thoroughly dissatisfied with the class.

 

[GF99/99]

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.

To actually answer the two questions you have asked:
1. No most people do not ignore the guidelines but is oxygen toxicity something you need to worry about recreation open circuit diving probably not because you will never come close to the limits even doing 5 nitrox dives a day liveabords. However these guideline are in place for a reason, yes they are conservative but its for your safety since everyone body reacts differently to pulmonary oxygen toxicity. I'm not supper up to date on SDI nitrox course standards but I think they teach not to exceed 80% CNS as do most other diving agencies around the world. This 80% is a conservative guideline that should be followed to keep you from having oxygen toxicity issues underwater. Convulsions under water is pretty much guaranteed drowning/death. Your computer should track your CNS % even if your doing multiple dives a day and using different nitrox mixes through out the week (Its the same concept as your computer tracks your accumulated nitrogen loading even over multiple dives). And some of the more advanced computers like the Shearwaters will tell you your end CNS based on your PPO2 and TTS.

Also I think your understanding to the guidelines and CNS loading is a little off. Yes you should following your MOD and and for recreational diving 1.4 atm is a good PPo2 guideline to follow but I think you are forgetting that your CNS loading is also time dependent. So if you dive at a constant 1.4 atm and follow your MOD you can still reach your O2 exposure limit because if you did this for 150min you will reach the recommend exposure limit. Yes in recreational diving and even most technical diving this is hard to do but it is possible. But as you can see you would need to spend 150min breathing 1.4atm O2 that is basically impossible to reach OC recreational diving.

If you are wondering this really only become something closed circuit divers need to look out for. Most CC diving is done by setting the PPo2 you want to dive at (basically you are breathing the perfect nitrox mix the entire dive). Since your on a rebreather general your scrubber time or bail outs are your limiting dive time factors so you can do some really long dives if you want. This is when you need to start looking at CNS loading say you are diving at a 1.2 set point if you want to stay within your CNS guidelines you can only do a 210 minute dive. When you start doing long cave dives or deep dives below 100 meters you start to rack up a lot of deco and if your bumping up your set point to 1.4 on deco you can really start to rack up the CNS clock so this is when divers make a calculated risk to go over the 80% but on dives like this you would have support divers meet you in the shallows and once you start going over the 100% mark you would take air breaks. Plus after a couple hour dive on a rebreather its actually really refreshing to take air brakes and to hand off bail outs.

 

[sea_ledford]

I've heard it called Hypoxic Myopia.
A LOT of oxygen over a long time and it is generally a temporary case of mild near sightedness.
I've heard of people keeping an eye chart and if the vision starts to drop off, time to take a break.
This is fringe stuff. Repetitive scientific diving type of profiles. Not the recreational vacation profiles.

That is the way I have heard of it as well. The only significant case that I have heard of was from Howard Hall, when they were making some IMAX movie and doing multiple 300+ foot dives per day for multiple days in a row. I want to say six days, but I could be misremebering that. He had to wear glasses for about 6 months afterward, if I remember correctly.

 

[HKGuns]

Point of reference. I did two 40 minute EAN32 dives last Saturday to a max depth of 47 feet, obviously no-where near MOD for that blend.

My perdix calculated CNS of 9% at the end of dive 2.

I found this link interesting after reading this thread.

Shearwater and the CNS Oxygen Clock - Shearwater Research

 

[sea_ledford]

Well, I got all my numbers wrong!

From Howard Hall - SDI | TDI | ERDI | PFI


Hyperoxic-induced Myopia
BG: I understand you have had some vision problems,»

HH: I accumulated more than 90 hours underwater during our first 22 days of shooting out here in Cocos. Towards the end of that trip I began to notice significant problems with my eyes and by the time we were on our way home, I couldn’t see well enough to get around the airport terminal. Michele called DAN as soon as we got home and no one knew what the problem could be. Finally we found a doctor in Pensacola, Florida who knew what was causing my vision problems. Dr. Frank Butler, a Captain in the Navy who works a lot with the SEAL Teams, said I had hyperoxic-induced myopia. It’s a condition that is not uncommon among patients treated for burns or skin disorders in hyperbaric chambers under high partial pressures of oxygen for long periods. Although it had never been seen in divers, to his knowledge, he was sure it was induced myopia. Hyperbaric patients are generally treated daily for 90 minutes or so at oxygen pressures well over 2 atmospheres, much higher exposures than we are getting on our rigs. But 22 days at four hours a day at 1.3 atmospheres of oxygen was enough to cause my vision problems. He suggested that hyperoxic induced myopia has not been seen in divers because nobody has put in so much time at 1.3 atmospheres for so many days straight. The good news is that it usually goes away after a few weeks once out of the water. It took three weeks for my eyes to normalize after that first trip.