Dive planning and OTU's

My dives seem to be getting longer and longer. I am on open circuit and having a problem with the CNS clock. I found this paper and was wondering what some others are doing for their dive planning and maximum OTU's for a single dive on a fresh diver. I have been told to try and keep the OTU's for a single dive below 300 but according to this paper "It had been observed that an
operationally acceptable daily exposure for a “fresh” diver
was 850 OTUs." What are others doing?

http://archive.rubicon-foundation.or...pdf?sequence=1

I worry more about CNS loading than OTU.

To reduce CNS load, made your last stop at 10' rather than 20' where the cave allows.

Some cases, dial down the ppo2. 30% in ginnie Starts to load your CNS for long dives.

Interesting post. That paper is pretty old, but the subject comes up on the forum from to time.

I read all these things and take them much more cautionary than most. Mainly, I'm no 21-year-old specimen of health. I'm old. The thought of an oxygen hit scares me. I could push the limits and probably get away with it, but I have no desire to do that. So as the years go by, I'm making shorter and shorter dives. I can't remember the last time I made a dive over 90 minutes. Certainly not in the past year. In fact, I've become so scared of an oxygen hit that I usually don't even take oxygen into the water any more. I would rather deco on nitrox for twenty minutes than on oxygen for ten.

I'm probably too cautious, and I'm sure there are plenty of people that disagree with me, but that's what this forum is for.

(My life used to be pretty good - now it's great! So I push fewer and fewer limits now. I still ride the crotch rocket, but never hit triple digits any more.)

I'm just curious, are there any documented cases of someone toxing during deco?

As to blowing the CNS clock through the roof, I have a hard time accepting that this would be a good idea. I'm always told that some WKPP people go way beyond 100%, but then in their case I assume everything that is well beyond 100% happens in a deco habitat where toxing may not be so much of an issue.
Also I'm getting told that some rebreather divers break 100% on a regular basis, but then we all know that there are mysterious cases of rebreather dives going wrong too.

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Originally Posted by nakatomi

I'm just curious, are there any documented cases of someone toxing during deco?

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Yes, and he was following the suggested PPO2 of the time. the max PPO2 for deco is about half of what it was back then.

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Originally Posted by FW

Yes, and he was following the suggested PPO2 of the time. the max PPO2 for deco is about half of what it was back then.

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So he was breathing O2 or Nitrox with a far higher PPO2 than 1.6?

How common is the practice of doing deco at say 10' or 15' with a corresponding PPO2 and the CNS loading getting pushed a long way beyond 100%, say 150% or 200%?

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Originally Posted by nakatomi

I'm just curious, are there any documented cases of someone toxing during deco?

As to blowing the CNS clock through the roof, I have a hard time accepting that this would be a good idea. I'm always told that some WKPP people go way beyond 100%, but then in their case I assume everything that is well beyond 100% happens in a deco habitat where toxing may not be so much of an issue.
Also I'm getting told that some rebreather divers break 100% on a regular basis, but then we all know that there are mysterious cases of rebreather dives going wrong too.

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Not always in habitats, especially not lately.

Low ppo2s on the bottom and gas breaks seems to be successful.

I was taught that for every 20 minutes on O2 you are supposed to take a 5 minute "air" break. I always do this but I can not find any research where this would allow you to exceed 100% of your CNS clock. I have run numerous scenarios of a 4 hour dive at 90' on V Planner and unless I want some extreme deco I can't get the CNS % below 100%. What are others doing on dives like this?

Attachment 11440

It must be must be one hell of an OC dive to worry about pulmonary oxtox. As far as I know its common practice to disregard your % for CCR divers on vacation.
Except from pat, you guys were all talking about CNS tox, but obamafan was asking for pulmonary tox, no? If I'm wrong I'd like to play the foreigner card.
BTW, there should be tons of info on that topic on CCR boards.

Ok, bama just posted when wrote my post. Apparently I was wrong than.

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Originally Posted by patpicos

I worry more about CNS loading than OTU.

To reduce CNS load, made your last stop at 10' rather than 20' where the cave allows.

Some cases, dial down the ppo2. 30% in ginnie Starts to load your CNS for long dives.

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Just curious, why are you concerned about CNS? Personally I don't really pay attention to that.

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Originally Posted by PfcAJ

Not always in habitats, especially not lately.

Low ppo2s on the bottom and gas breaks seems to be successful.

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I asked this same question about what the WKPP was doing during my Trimix class and this is what my instructor told me. A buddy and I were discussing this subject this weekend and I thought that was what was being done. I guess it is about like anything else in cave diving as it is a risk / rewards scenario that we all have to decide what to do.

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Originally Posted by bamafan

I was taught that for every 20 minutes on O2 you are supposed to take a 5 minute "air" break. I always do this but I can not find any research where this would allow you to exceed 100% of your CNS clock. I have run numerous scenarios of a 4 hour dive at 90' on V Planner and unless I want some extreme deco I can't get the CNS % below 100%. What are others doing on dives like this?

Attachment 11440

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That dive is clearly impossible to do, just pick something easier.

Seriously, 30% on the bottom, 100% at 20', and 5min breaks every 15min is the way I do it.

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Originally Posted by jj1987

Just curious, why are you concerned about CNS? Personally I don't really pay attention to that.

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He is concerned because he doesn't want to tox in the cavern and become fish food.

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Originally Posted by nakatomi

He is concerned because he doesn't want to tox in the cavern and become fish food.

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There are probably some who wish I would. My girlfriend is one of them right now as I keep going cave diving to much. :)

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Originally Posted by Kevin Leonhardt

That dive is clearly impossible to do, just pick something easier.

Seriously, 30% on the bottom, 100% at 20', and 5min breaks every 15min is the way I do it.

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I know it is just a basic cave dive for you Kevin. :) I do appreciate your advice and enjoyed meeting you at JB a few weeks ago.

Eric

There is a discussion over on CCRX about a presentation made by Dr. Simon Mitchell at EUROTEK regarding CNS exposures. I'll apologize in advance for the cross forum posting but it appears that the discussion on CCRX is the only place the information is currently available online. It's supposed to be published in the near future. It can be found here

As far as pulmonary toxicity goes the NOAA-REPEX table, ie. OTU's, listed in the article linked by Bamafan seems to still be the standard. 850 OTU's for a fresh diver and leveling at 300 by day 11. Even day one has enough OTU's reserved for a chamber ride. If your not planning to get bent you could go significantly beyond 850 on day one ;-)

Thank you for providing us with that link, that is very useful information!

Thanks for the link. I agree with Oliver it was very interesting.

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Originally Posted by jj1987

Just curious, why are you concerned about CNS? Personally I don't really pay attention to that.

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Hi James

Assuming hitting 100% CNS Clock on a single dive is dangerous and that it will kill you if you breach it (over-reacting on this one), I find it much easier to hit the CNS clock limits for my dives versus getting anywhere close to reaching OTU limits. I try to stay below 80% and its often fairly easy to do so w/ the standard gas being pumped in Cave Country and doing my last stop at 10'
Typical cave diving day for me is doing a single dive in the 100-110min range. Rarely doing 2 dives if doing that long w/ deco. I will do 2 90min dives at Peacock on the other hand..

Now, to air breaks..ive learnt the hard way on my last dive where I chose to skip the air break and tough out O2 for 27mins since my buddy was a jerk laughing at me since he cleared 15mins earlier than I did. On the surface, I had an unpleasant dry cough for a few minutes. Perhaps it is time to change my 25/75 GF on petrel :p

I feel like the CNS clock is a very poor model. You can tox with less than 100% of your 'clock' used up, and not tox with WAY over 100% used.

Perhaps its worth considering on multi day nitrox diving, like a week long live aboard or something or a cave trip where you're diving your face off every day. But for single dives, o2 breaks and low bottom ppo2s seem to be the way to go. Personally, I completely disregard the 'clock' thing for single dives.

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Originally Posted by patpicos

Hi James

Assuming hitting 100% CNS Clock on a single dive is dangerous and that it will kill you if you breach it (over-reacting on this one), I find it much easier to hit the CNS clock limits for my dives versus getting anywhere close to reaching OTU limits. I try to stay below 80% and its often fairly easy to do so w/ the standard gas being pumped in Cave Country and doing my last stop at 10'
Typical cave diving day for me is doing a single dive in the 100-110min range. Rarely doing 2 dives if doing that long w/ deco. I will do 2 90min dives at Peacock on the other hand..

Now, to air breaks..ive learnt the hard way on my last dive where I chose to skip the air break and tough out O2 for 27mins since my buddy was a jerk laughing at me since he cleared 15mins earlier than I did. On the surface, I had an unpleasant dry cough for a few minutes. Perhaps it is time to change my 25/75 GF on petrel :p

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I was more curious why you assume that? If it's just a gut instinct then fair enough, I have just never seen any science to support the 100% CNS limit in diving like what FL cave divers do (with back gas breaks).

Its more referencing what we are taught in class and following it to the letter.

Remember in rec courses where reaching NDL was drilled into you as the devil...and now you have done tech diving....its laughable..

On a serious note, oxygen limits is not something I want to play guinea pig much! I've already seen a friend being pulled out of a cave from toxing. That was graphic enough for me.

One of the doctors here can chime in, but my understanding is that there are TWO separate issues with oxygen toxicity.

Neurological toxicity and pulmonary toxicity. The immediate PPO2 being the majority factor in Neuro toxicity (convulsions, ect) and the OTU clock being the majority factor in pulmonary toxicity (damage to lung,cardiac, venous tissues).

Pulmonary toxicity being the cumulative damage that occurs to tissues thru long term exposure (days/weeks) to high PPO2. If exposure is limited to one 24 hr period, there is a much higher OTU that is tolerated. But for repeated exposures, several times a day over multiple days, much lower OTU tolerances are used.

AND... the OTU schedule used on the dive site should be conservative enough to allow Table 6 recompression (if needed) without causing pulmonary damage.

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Originally Posted by FW

Yes, and he was following the suggested PPO2 of the time. the max PPO2 for deco is about half of what it was back then.

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Was a time that O2 at 30' was acceptable, as well as doing dives at Devils on 36 to 38% too.

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Originally Posted by gschaut

One of the doctors here can chime in, but my understanding is that there are TWO separate issues with oxygen toxicity.

Neurological toxicity and pulmonary toxicity. The immediate PPO2 being the majority factor in Neuro toxicity (convulsions, ect) and the OTU clock being the majority factor in pulmonary toxicity (damage to lung,cardiac, venous tissues).

Pulmonary toxicity being the cumulative damage that occurs to tissues thru long term exposure (days/weeks) to high PPO2. If exposure is limited to one 24 hr period, there is a much higher OTU that is tolerated. But for repeated exposures, several times a day over multiple days, much lower OTU tolerances are used.

AND... the OTU schedule used on the dive site should be conservative enough to allow Table 6 recompression (if needed) without causing pulmonary damage.

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There you go. This is what I meant. From what you guys are posting you seem to confuse those two. That is what you should know from a basic Nitrox course. From exceeding your clock % over extended periods of time you will screw your lungs. Getting cramps you will get ONLY from high ppo2 at a point in time, not because of a high clock %.

I am in no way an expert & didn't stay in a Holiday Inn last night. In the past I would have a higher working PO2 and push the PO2 past 1.6 as was standard practice. Then the limit of 1.4 for the working part of the dive and 1.6 for deco was brought in and I adopted it. I have had two hard DCS hits and have been checked for a PFO (I don't have one). I would get skin bends after decompression dives and sometimes even on regular depth decompression dives such as devils. Simon explained to me that even on shorter dives "OTU's" can become an issue and suggested lowering my working PO2 down to 1.0. He also suggested when running on CCR to run it like OC on deeper decompression stops by jacking the PO2 up at a stop then letting it drop as it would on OC which I adopted. Since doing this I have not had a single case of skin bends even with some rather deep (well past 300 feet) and rather long (well over 6 hours) dives.

Everyone is different with their personal limit of CNS. Finding the limit can have dire consequences. I have had dives that are way over the CNS clock limit and have pushed OTU's as well. Repetitive dives over days are a bigger factor then one big dive from my experience. The lungs have to be able to exchange gas to off load on decompression, keep the PO2 low on the working part of the dive to preserve the lungs for decompression. When doing deeper dives with deeper stops jack the PO2 up for off gassing then let it fall to give the lungs a rest. I normally use 275, 130, & 50 feet simply because it matches up well with the profiles I run for OC BO & the gases that are kind of standard.

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Originally Posted by bent

From what you guys are posting you seem to confuse those two. That is what you should know from a basic Nitrox course. From exceeding your clock % over extended periods of time you will screw your lungs. Getting cramps you will get ONLY from high ppo2 at a point in time, not because of a high clock %.

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I hear that a lot and do not think it is true.

The risk of a CNS oxygen toxicity event is a factor of an elevated PPO2 and exposure time, thus the CNS percentage as a tracking mechanism.
The OTU's are meant to track the pulmonary and possibly the ocular toxicity risks, but both CNS and OTU incorporate exposure time as a major factor.

Here is an excerpt from:
http://"http://www1.maths.leeds.ac.u...ba/baker3.pdf"

"Two oxygen toxicity parameters are typically “tracked” in technical diving calculations. The first is
pulmonary oxygen toxicity which primarily concerns the effects to the lungs of long-term exposures
to oxygen at elevated partial pressures. The second is central nervous system (CNS) oxygen toxicity
which primarily concerns the effects to the brain of short-to-medium-term exposures to oxygen at
elevated partial pressures. Both oxygen toxicity parameters are a function of the partial pressure of
oxygen ( PO2) in the inspired breathing gas and the time of exposure. "

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Originally Posted by Bobby

...& didn't stay in a Holiday Inn last night.

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IT's a Holiday Inn Express, Bobby. How can you expect to have any credibility?
https://www.youtube.com/watch?v=ZlCLuIwuVgQ

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Originally Posted by nakatomi

I hear that a lot and do not think it is true.

The risk of a CNS oxygen toxicity event is a factor of an elevated PPO2 and exposure time, thus the CNS percentage as a tracking mechanism.
The OTU's are meant to track the pulmonary and possibly the ocular toxicity risks, but both CNS and OTU incorporate exposure time as a major factor. "

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I hear you. I am only repeating what I read, I sure don't have the expertise to add anything to that. I just posted that because people seemed to be confusing those two.

Ollie, you probably already have it, but in case you don't. You should get the "Ehm", he has got really good sections on oxtox that were backed up by actual testing. Don't think it is available in English though. You can borrow it if you want. It's the best book on diving physiology I have come across.

There must be a bunch of people on here that have done dives with the cns clock going through the roof. Where are you guys?

Ok, just read Bobbys post. Your lights are awesome btw!

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Originally Posted by bent

Ollie, you probably already have it, but in case you don't. You should get the "Ehm", he has got really good sections on oxtox that were backed up by actual testing. Don't think it is available in English though. You can borrow it if you want. It's the best book on diving physiology I have come across.

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Thank you Bent! I believe I have that book in my collection, although I can't access it right now as I'm out of the country.

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There must be a bunch of people on here that have done dives with the cns clock going through the roof. Where are you guys?

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I fear that some of them may be on that Excel sheet over on that .uk web site and that's exactly what worries me.

I'd just check on CCR boards like rbw. There should be threads on that for sure.

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Originally Posted by bent

There you go. This is what I meant. From what you guys are posting you seem to confuse those two. That is what you should know from a basic Nitrox course. From exceeding your clock % over extended periods of time you will screw your lungs. Getting cramps you will get ONLY from high ppo2 at a point in time, not because of a high clock %.

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Pulmonary toxicity is really difficult to get. You need hours upon hours of exposure to high levels of O2. In the ICU we'll keep ventilator patients on 100% O2 for days if necessary. It's not something we like to do but oxygenating the brain is more important than protecting the pulmonary tissue. And it takes days for that to be an issue. Under pressure you have less time but even a few hours shouldn't be an issue. It's the neurological toxicity that is a concern then.

Here is the biggest problem with OTUs and multiple days of exposure to high levels of Oxygen. This is especially applicable to cave and rebreather divers who come to cave country and dive, dive, dive. I see divers who make multiple dives over a period of days and unlike CNS you do not get a surface interval credit with pulmonary O2 tracking unless you let the clock completely reset. So here we have a diver for instance that loads up day one to 600 or 700. That is probably fine. IF THEY DO NOT DIVE THE NEXT DAY. The problem comes in when they do, and then they dive the day after, and the day after that. Then they get bent that last day of a week long trip and have to go to the chamber and get hit with additional O2 for hours on end. Now they are frying their lungs. A big part of the OTU limits was keeping in mind the ability to be able to survive a chamber ride without significant damage to the lungs due to high Oxygen exposure. Many OC divers do not pay as close of attention to OTUs as they should in part because most OC divers do not ever get up to those limits. The average boat trip is two dives at 45 minutes each. The average vacation cave diver may do 2 or 3 times that in water time every single day. I see divers go back and forth to the dive shops into the eveneing to refill tanks over and over again. With cave divers doing multiple long dives in a day for a week straight or CC divers doing 4-6 hour dives over a period of days this becomes a very real issue. Believe me I have seen divers go to the chamber after having reached that OTU limit and they experienced the signs and symptoms of pulmonary O2 toxicity not upon exiting the water, but upon being treated for the hit. Food for thought.

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Originally Posted by RN

Pulmonary toxicity is really difficult to get. You need hours upon hours of exposure to high levels of O2. In the ICU we'll keep ventilator patients on 100% O2 for days if necessary. It's not something we like to do but oxygenating the brain is more important than protecting the pulmonary tissue. And it takes days for that to be an issue. Under pressure you have less time but even a few hours shouldn't be an issue. It's the neurological toxicity that is a concern then.

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Rob,
I disagree with your statement. What works in a hospital has no relationship to what is happening or works in the water. My experience is that even on short shallow decompression dives of a few hours pulmonary can and will become an issue. In hospitals the pressure is not above 1.0 and the patient does not have a substantial inert gas load that requires off loading within a short limited time. On dives 2-2.5 hours at less than 100 feet I would commonly get skin bends. After dropping my working PO2 down to 1.0 I have not had a single case of skin bends even with trimix dives running double digit run times. This advise was given to me by Simon who does work extensively in the field of recreational decompression theory.

My experience is that on single long dives CNS load well beyond the standard accepted limit are not an issue. Multiple dives over multiple days however can and will be an issue within the presently accepted CNS teaching. When Dr. X was Basic Cave and we lived up North it was common for us to do multiple dives over multiple days. From my past experience including working on live aboard dive boats with divers doing no decompression dives over multiple days with elevated PO2's I have seen many CNS issues.

On single extended run dives my primary concern is pulmonary, CNS is not a concern (for me). When doing multiple dives in a day we track CNS & do not give surface credit for 24 hours. Just like years ago we would use higher than 1.6 PO2 for decompression I believe that the present teaching of Advanced Nitrox & Decompression Procedures needs to updated to our present understanding. Those courses were developed from the NOAA standards which are very limited in their application & do not relate well with present time types of dives that have become common.

By lowering my PO2 for the working part of the dive I feel better getting out of the water, which is intuitive data. I also have not had a single case of skin bends, which is empirical data. Before I made a concerted effort to lower my PO2's I would occasionally dive places like LR that pushed my PO2's back up to 1.4 on the working part of the dive on OC. The skin bends would then come back, since lowering them to 1.0 on all dives both OC & CCR I have had zero incidents. This is a small data set however it is enough for me to believe that it is spot on.

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Originally Posted by Bobby

Rob,
I disagree with your statement. What works in a hospital has no relationship to what is happening or works in the water. My experience is that even on short shallow decompression dives of a few hours pulmonary can and will become an issue. In hospitals the pressure is not above 1.0 and the patient does not have a substantial inert gas load that requires off loading within a short limited time. On dives 2-2.5 hours at less than 100 feet I would commonly get skin bends. After dropping my working PO2 down to 1.0 I have not had a single case of skin bends even with trimix dives running double digit run times. This advise was given to me by Simon who does work extensively in the field of recreational decompression theory.

My experience is that on single long dives CNS load well beyond the standard accepted limit are not an issue. Multiple dives over multiple days however can and will be an issue within the presently accepted CNS teaching. When Dr. X was Basic Cave and we lived up North it was common for us to do multiple dives over multiple days. From my past experience including working on live aboard dive boats with divers doing no decompression dives over multiple days with elevated PO2's I have seen many CNS issues.

On single extended run dives my primary concern is pulmonary, CNS is not a concern (for me). When doing multiple dives in a day we track CNS & do not give surface credit for 24 hours. Just like years ago we would use higher than 1.6 PO2 for decompression I believe that the present teaching of Advanced Nitrox & Decompression Procedures needs to updated to our present understanding. Those courses were developed from the NOAA standards which are very limited in their application & do not relate well with present time types of dives that have become common.

By lowering my PO2 for the working part of the dive I feel better getting out of the water, which is intuitive data. I also have not had a single case of skin bends, which is empirical data. Before I made a concerted effort to lower my PO2's I would occasionally dive places like LR that pushed my PO2's back up to 1.4 on the working part of the dive on OC. The skin bends would then come back, since lowering them to 1.0 on all dives both OC & CCR I have had zero incidents. This is a small data set however it is enough for me to believe that it is spot on.

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I've haven't heard of skin bends being associated with PO2 at depth. The few cases I've experienced involved air dives and insufficient decompression.

It is not something I read. I was chatting with Simon at a conference about my experience. After establishing that I had tested negative for a PFO he explained how & why lowering my PO2 on the working part of the dive. It took a while to get my head around it because, at the time, my line of thought was higher PO2 = less loading of inert gases. Now it is lower PO2 = less pulmonary stress allowing more efficient off gassing on decompression.

Sent from my XT1080 using Tapatalk

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Originally Posted by RN

Pulmonary toxicity is really difficult to get. You need hours upon hours of exposure to high levels of O2. In the ICU we'll keep ventilator patients on 100% O2 for days if necessary. It's not something we like to do but oxygenating the brain is more important than protecting the pulmonary tissue. And it takes days for that to be an issue. Under pressure you have less time but even a few hours shouldn't be an issue. It's the neurological toxicity that is a concern then.

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Not sure I'd agree. During the research phase of development of saturation diving pulmonary toxicity was a real problem that had to be quantified, so most of the research you see in diving medicine textbooks comes from that era. Essentially the maximum PO2 that will be tolerated indefinitely without evidence of toxicity is about 0.5 ata. Above that the rate at which manifestations develop rises quite steeply. At 1 ata (100% in your ICU) even people with normal lungs will have objective evidence of decreased vital capacity after 6 hours.

In diving the effect may be harder to discern but I have only experienced subjective symptoms of pulmonary toxicity during and after dives of 8 hours or more, and only troublesome symptoms over about 10 hours, with PO2s always exceeding 1.0 (usually up to 1.3 during decompression).

The CNS clock is a reasonable measure of total oxygen exposure. It has no demonstrable correlation with risk of CNS toxicity, which is not surprising given the quasi-random nature of CNS toxicity. There is definitely a threshold PO2 below which CNS toxicity is just about unheard of regardless of time of exposure (say 1.2 ata), yet it is possible on a long dive in which the PO2 never exceeds 1.2 to push the CNS clock up to 200-300+%. OTUs and the CNS clock are measuring the same thing in different ways.

Bobby I think that's a fascinating point. It seems backwards that lowering the PO2 on the working part of the dive will result in LESS decompression stress, but the way you just reasoned it out really piques my interest - to the point I'm going to alter some diving habits and see what happens.

I've had what I assume are skin bends on a few decompression dives, and always connected it with not being conservative enough. But I also know that extending the decompression sometimes leaves me with a dry cough which I suspect is a symptom of some level of pulmonary toxicity. I have extremely minor non-exercise induced asthma, so I just wrote it off as being a little more susceptible to the Oxygen and dry tank air.

Now thinking of it counter-intuitively, I can really see how lowering the PO2 during the working phase could minimize the initial pulmonary stress, which in turn makes the decompression more efficient, and could actually decrease the likelihood of getting bent, even though I'm loading a little more inert gas for a given amount of bottom time.

I never would have connected the dots like that. Thanks.

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Originally Posted by Bobby

Rob,
I disagree with your statement. What works in a hospital has no relationship to what is happening or works in the water. My experience is that even on short shallow decompression dives of a few hours pulmonary can and will become an issue. In hospitals the pressure is not above 1.0 and the patient does not have a substantial inert gas load that requires off loading within a short limited time. On dives 2-2.5 hours at less than 100 feet I would commonly get skin bends. After dropping my working PO2 down to 1.0 I have not had a single case of skin bends even with trimix dives running double digit run times. This advise was given to me by Simon who does work extensively in the field of recreational decompression theory.

My experience is that on single long dives CNS load well beyond the standard accepted limit are not an issue. Multiple dives over multiple days however can and will be an issue within the presently accepted CNS teaching. When Dr. X was Basic Cave and we lived up North it was common for us to do multiple dives over multiple days. From my past experience including working on live aboard dive boats with divers doing no decompression dives over multiple days with elevated PO2's I have seen many CNS issues.

On single extended run dives my primary concern is pulmonary, CNS is not a concern (for me). When doing multiple dives in a day we track CNS & do not give surface credit for 24 hours. Just like years ago we would use higher than 1.6 PO2 for decompression I believe that the present teaching of Advanced Nitrox & Decompression Procedures needs to updated to our present understanding. Those courses were developed from the NOAA standards which are very limited in their application & do not relate well with present time types of dives that have become common.

By lowering my PO2 for the working part of the dive I feel better getting out of the water, which is intuitive data. I also have not had a single case of skin bends, which is empirical data. Before I made a concerted effort to lower my PO2's I would occasionally dive places like LR that pushed my PO2's back up to 1.4 on the working part of the dive on OC. The skin bends would then come back, since lowering them to 1.0 on all dives both OC & CCR I have had zero incidents. This is a small data set however it is enough for me to believe that it is spot on.

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I did mention that the time limitations under the pressure experienced in diving does play a factor. My comments are more in response to the statement that you will "screw your lungs" with high CNS. Yes, there are other issues involved but I think the effect on the pulmonary tissue will be the least of the issues.

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Originally Posted by apitkin

Not sure I'd agree. During the research phase of development of saturation diving pulmonary toxicity was a real problem that had to be quantified, so most of the research you see in diving medicine textbooks comes from that era. Essentially the maximum PO2 that will be tolerated indefinitely without evidence of toxicity is about 0.5 ata. Above that the rate at which manifestations develop rises quite steeply. At 1 ata (100% in your ICU) even people with normal lungs will have objective evidence of decreased vital capacity after 6 hours.

In diving the effect may be harder to discern but I have only experienced subjective symptoms of pulmonary toxicity during and after dives of 8 hours or more, and only troublesome symptoms over about 10 hours, with PO2s always exceeding 1.0 (usually up to 1.3 during decompression).

The CNS clock is a reasonable measure of total oxygen exposure. It has no demonstrable correlation with risk of CNS toxicity, which is not surprising given the quasi-random nature of CNS toxicity. There is definitely a threshold PO2 below which CNS toxicity is just about unheard of regardless of time of exposure (say 1.2 ata), yet it is possible on a long dive in which the PO2 never exceeds 1.2 to push the CNS clock up to 200-300+%. OTUs and the CNS clock are measuring the same thing in different ways.

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I think we're saying the same thing. I just didn't go into detail. My response was specifically geared to bent's statement. I wasn't trying to say high CNS is not an issue on pulmonary tissue but rather that it's the least of the issues.

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Originally Posted by Kelly Jessop

Was a time that O2 at 30' was acceptable.

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Might be worth some research.

Guys as Long as you stay Close to CNS Limits and not or only slightly exceed 100% CNS there is no Chance whatsoever to get close to any OTU issues no matter how long and often you dive..
BTW on a rebreather I use a PO2 of 1.0 for the working Phase of the Project but not for reasons of OTU or CNS even if it helps though. I do it because it is simply the closest to claibration Point and it does not Change deco Obligation significantly as Long as you ramp it up during deco..