CNS O2 exposure

[DevonDiver]

I don't know what you're reading there, but I see the 24-hour allowed exposure clearly mentioned several times in the excerpts you posted.

I am reading the Instructor Manuals - Teaching Notes from the PADI and SSI nitrox courses.

Both courses acknowledge the existence of 24-hr exposure limits, but clearly indicate that that are "not really an issue" in recreational diving. Further to that, the PADI course notes specifically indicate the surface interval credit is "unnecessarily complex" and that "you don't calculate surface interval credits".

The primary emphasis on both courses is that the diver should limit their dives in duration (no-stop), ppO2 and number of dives per day (2-3), with extensive (60-90 min) surface intervals. Following those guidelines, as taught, makes CNS tracking irrelevant. It is taught, therefore, "solely as a contingency".

Planning hypothetical dives based on extended duration, use of ppO2 1.6 or multiple (3+) repetitive dives is unrealistic in respect to CNS clock - because according to the recommendations/rules of those training courses, the diver's should not be doing any of that in first place...

I was taught that it was not essential to track CNS O2 for recreation diving,...

After some number crunching to answer whether NDL will alway be reached before CNS O2 limits. I arrived at some interesting findings:

1. Using high EAN% (36 & 40) and a high ppO2 (1.6ata) limit, it is theoretically possible to hit the NOAA single dive and daily limits before hitting NDL over multiple dives.
2. It is possible to hit the NOAA single dive and/or daily limits for some profiles, e.g, 4 dives using (EAN40, 30m, 45 minutes) will hit the NOAA daily limit.

Given that planning dives with ppO2 1.6 is outside the parameters of recreational diving... and that conducting 4 (EANx) dives per day is contrary to the recommendations given in most EANx courses... the hypothetical test is substantially flawed. It would mean ignoring the protocols and procedures inherent within recreational EANx diving..

Then I realised something else, would a recreational diver have enough gas to perform the dives with a high CNS O2 loading?

Possible, given the increasing popularity of sidemount etc...

Hence, my conclusion is this: It is highly unlikely for a recreational diver to reach CNS O2 limits if ppO2 exposure is kept < 1.4 bar.

Yep... reaching the limits entails ignoring training. We should assume that training/protocols/recommendations are applied, when debating whether CNS limits are an issue. Assuming they are applied, CNS limits aren't an issue.

 

[dfx]

I am reading the Instructor Manuals - Teaching Notes from the PADI and SSI nitrox courses.

Both courses acknowledge the existence of 24-hr exposure limits, but clearly indicate that that are "not really an issue" in recreational diving. Further to that, the PADI course notes specifically indicate the surface interval credit is "unnecessarily complex" and that "you don't calculate surface interval credits".

The primary emphasis on both courses is that the diver should limit their dives in duration (no-stop), ppO2 and number of dives per day (2-3), with extensive (60-90 min) surface intervals. Following those guidelines, as taught, makes CNS tracking irrelevant. It is taught, therefore, "solely as a contingency".

Planning hypothetical dives based on extended duration, use of ppO2 1.6 or multiple (3+) repetitive dives is unrealistic in respect to CNS clock - because according to the recommendations/rules of those training courses, the diver's should not be doing any of that in first place...

Yeah, so? **** is known to happen occasionally. Saying that it normally isn't an issue doesn't equal to them not teaching it. They do teach it, but of course you can make the choice to just ignore it.

 

[DevonDiver]

Yeah, so? **** is known to happen occasionally.

Not really. It's called dive planning. Staying 'accidentally' for 180 minutes at ppO2 1.4 for a recreational diver shouldn't happen "occasionally"... it shouldn't happen ever.

Saying that it normally isn't an issue doesn't equal to them not teaching it.

It isn't an issue. Normally or abnormally. Providing divers follow the complete set of protocols and recommendations given to them for diving EANx.

If divers choose not to follow those protocols and recommendations, then all bets are off the table. Those divers, by nature, are hardly likely to be the ones to fastidiously pursue the "unnecessarily complex" process of CNS clock and surface interval credit planning anyway...

They do teach it, but of course you can make the choice to just ignore it.

They (agencies) don't teach it. Instructors teach it. I've taught more nitrox and tech courses (qualified more EANx/Advanced EANx divers) than you've done scuba dives... so please, tell me all about how it's done

 

[dfx]

Not really. It's called dive planning. Staying 'accidentally' for 180 minutes at ppO2 shouldn't happen occasionally...



It isn't an issue. Normally or abnormally. Providing divers follow the complete set of protocols and recommendations given to them for diving EANx.

If divers choose not to follow those protocols and recommendations, then all bets are off the table. Those divers, by nature, are hardly likely to be the ones to fastidiously pursue the "unnecessarily complex" process of CNS clock and surface interval credit planning anyway...



They (agencies) don't teach it. Instructors teach it. I've taught more nitrox and tech courses (qualified more EANx/Advanced EANx divers) than you've done scuba dives... so please, tell me all about how it's done

Seriously? Maybe you wanna re-read what I posted originally. All I said was that agencies do teach the 24-hour limit, meaning it's in the book and in the curriculum. Whether or not you ignore that when you teach it to your students is a totally unrelated matter, one I don't even care about.

 

[runez]

Given that planning dives with ppO2 1.6 is outside the parameters of recreational diving... and that conducting 4 (EANx) dives per day is contrary to the recommendations given in most EANx courses... the hypothetical test is substantially flawed. It would mean ignoring the protocols and procedures inherent within recreational EANx diving..

When I did a SDI Computer Nitrox course, 1.6 was taught to be the default max ppO2 (and 1.4 for challenging dives). But then again, I've been told that SDI is supposedly known for pushing the boundaries. I don't think it's substantially flawed, but you could also see the hypothetical test of 1.6 as a "what-if" scenario, and an attempt to put some perspective into - "why not 1.6?".

Personally, I use a max ppO2 of 1.4 when diving, because I don't see the benefit of 'pushing' the ppO2 to 1.6 being worth the increased risk.

Possible, given the increasing popularity of sidemount etc...

I'm assuming that recreation diving consists of using only single tanks (whether BM or SM..). But you're right, in the event that a diver carries enough gas, it is probably for CNS O2 loading to become a problem, e.g. using EAN40, at 20m (ppO2 =1.2), NDL of 98 minutes with of a single dive limit of 210 minutes or 24-hr limit of 240mins.

Yep... reaching the limits entails ignoring training. We should assume that training/protocols/recommendations are applied, when debating whether CNS limits are an issue. Assuming they are applied, CNS limits aren't an issue.

I agree, it's just that from personal experience, not all training protocols are aligned. I thought this thread will be useful for those taught by other agencies with differing standards from the norm, and allow divers to make more informed decisions.

Cheers =)

 

[DevonDiver]

Seriously? Maybe you wanna re-read what I posted originally. All I said was that agencies do teach the 24-hour limit, meaning it's in the book and in the curriculum. Whether or not you ignore that when you teach it to your students is a totally unrelated matter, one I don't even care about.

They teach to AVOID the 24-hour limit. It's a contingency.

They also teach to avoid toxicity. That's not the same as teaching someone to tox.

It's only pertinent to tables-based calculations - where the diver is planning a square profile dive, and that profile might be based on 1.4ppO2. Nonetheless, PADI's recommendation for max 2 nitrox dives per day, covers that anyway. Try planning 2 tables dives (square profile) with <40%...and see what sort of CNS% you can feasibly get...

The trend in tuition (certainly via PADI) is not to teach tables any more. Thus, CNS clock is pretty much defunct from recreational diving considerations. I'm guessing you've not seen the new PADI Nitrox course materials...

 

[Dr. Lecter]

That is why a large number of serious tech divers like the Shearwater computers so much. It is the benchmark IMO for dive instruments. I see they now also sell the NERD and I would love to hear what the CCR guys think based on actual experience.

I agree entirely -- which is why I dive two Petrels now and plan to get Shearwater controllers when I move to CCR. When I finally do, I think one of my Petrels may become a very expensive DPV instrument Still, it would be nice if the SW manual explained what the CNS credit calaculation is and on what source(s) it's based--shockingly, the SP manual quoted above does a slightly better job than the SW manual in that regard.

The NERD is interesting but I've heard some concerns about its viability as anything more than a fancy, $2500 HUD because of the inability to use it should you have to go off the loop to bail out. Hopefully all the bugs will be worked out and the DiveCAN version available when I have to pick an electronics package.

 

[scubadada]

This might just be a (significant) omission in oceanic computers.

Yes, that is my conclusion. My Oceanic computers track O2 exposure for a 24 hour period without any surface interval credit and do not take the 90 min half life of elimination into account. By this method, it is possible for me to exceed the NOAA 24 hour exposure, see my post #7. In reality, my exposure is reduced by the duration of all of my surface intervals. Otherwise, I really like my Oceanic computers. I hope they fix this.

Unless your recreational computer locks you out when it thinks you've hit your CNS limit, it really doesn't make any difference

Fortunately, my computers do not lock me out. The warnings/alarms are annoying but I can always get the information I need, sometimes by accessing alternate screens.

 

[shoredivr]

Somebody diving nitrox on a recreational liveaboard trip where 4 or 5 dives a day are sheduled over 4 or 5 days can exceed the 24 hour clock.

 

[scubadada]

Still, it would be nice if the SW manual explained what the CNS credit calaculation is and on what source(s) it's based--shockingly, the SP manual quoted above does a slightly better job than the SW manual in that regard.

It was dazzling how little information was in the manuals on this topic when I was trying to research it. I missed the short statement in the SW manual and did not look at SP. As Dr Lecter points out, neither describes elimination in any detail and and the half life used. The manuals could stand a little improvement. Personally, I like knowing these things