Trouble understanding nitrox po2

[Scubadives]

First off hi, I am new to the site! I recently went to a naui nitrox class, and while there thought I understood everything, even made an 88% on the test. Once I got home, and thought about how to plan for my upcoming dive trip im stumped.

I understand that max po2 of 1.4 is 150 min max in a single dive, and 180 min in a 24 hour period.

I also understand that unless you have a computer, you calculate your dive profile as being square using your max depth(po2) to figure your tables.

Its easy as pie to plan your first dive, just follow the chart, and it will let you know what you can do as an "A" diver.

My problem is we are doing 3 dives per trip, and planning out the next dives is causing me headaches.

I know how to find out my standard residual nitrogen, and can find that and also the times that would limit our next dive based off of that.

I cant seem to find a way to figure out limiting po2 for the next dive, knowing that po2 adds up like nitrogen, do you add it to the next dive group to get a new letter like you do with residual nitrogen for new letter groups?

There is a "po2 clock", but how does it work for multiple dives planned in a different po2 "range", do you average it out, use the max po2 for that 24 hour period, does it add together?

Example
Using ean32 say for the first dive you went to 90fsw which is a calculated po2 depth of
1.2 for 30min this puts you as a "G" diver.

You take 2 hour SI and plan for your next dive at 60fsw(po2 .90) the table says I can stay for 71 minutes and have a residual nitrogen of 29 minutes, and you plan to that whole 71 minutes unless po2 will limit me......?

At this point how do I calculate my max 24 hour total, or limiting po2 for this dive considering that it adds, or averages up? Is it 1.2 from the first dive, and .90 and you add the minutes which would be a 2.1!!po2 and 101 minutes used on "po2 clock", or is it averaged. Ending up as a 1.05 and 101 minutes for the day?



Or do you get a credit for SI with po2, like you do with residual nitrogen? ie is that why after my 2 hr SI I get moved from a "G" diver to a "D" diver, or is the reduction in diver group only related to nitrogen offgassing during SI?

My instructor, and classmates might have confused me when I asked this they said NO, it adds up in a 24 hour peroid, and you cannot subtract any oxygen exposure minutes within that...

 

[UCFKnightDiver]

I wouldn't worry about the CNS clock or exposure limit personally, it's very hard to exceed as a recreational diver, and I don't think the limit is very valid at all. *shrugs*

 

[Rhone Man]

There is no proven scientific answer, but from memory, I think the NOAA tables use a "half time" on oxygen expsure of 45 minutes. So if you had a cumulative oxygen exposure of 30%, and then you did a 90 minutes surface interval, that exposure would decay by three quarters (half and then half again, for 2 x 45 minutes), so you would be down to 7.5%.

But Rox is right; it is going to be pretty hard for you to hit it doing purely recreational diving, even with 3 (or even more!) dives a day.

 

[Thalassamania]

Take a look here, it should help.

 

[Scubadives]

I did some reading, and have narrowed it down to a few terms that I am looking to learn to simplify the massive wall of text that my first post was . How to calculate CNS exposure time, or Oxygen exposure.

I understand that it would be hard to be P02 limited doing rec diving, I am just not the kind of person that can live comfortably with an answer like "it would be hard to hit that limit". I really want to learn to calculate exposure time for multiple dives on nitrox.

If I were to convulse and drown when my wife and daughter eventually meet me in heaven I would get a swift kick in the balls from both of them for just taking the word about something that can kill you instead of learning and checking it myself you know

 

[UCFKnightDiver]

Post deleted

 

[Rhone Man]

Take a look here, it should help.

"90-minute half-time decay of CNS loading"

My mistake - the half time is 90 minutes (not 45, as indicated above). God, it is amazing that I am still alive sometimes. What would I do without a computer to keep track for me? :shocked2:

 

[gcbryan]

I did some reading, and have narrowed it down to a few terms that I am looking to learn to simplify the massive wall of text that my first post was . How to calculate CNS exposure time, or Oxygen exposure.

I understand that it would be hard to be P02 limited doing rec diving, I am just not the kind of person that can live comfortably with an answer like "it would be hard to hit that limit". I really want to learn to calculate exposure time for multiple dives on nitrox.

If I were to convulse and drown when my wife and daughter eventually meet me in heaven I would get a swift kick in the balls from both of them for just taking the word about something that can kill you instead of learning and checking it myself you know

If you go back and look at the book from your Nitrox course there are usually tables that show you limits and how to calculate them for the CNS clock.

 

[Bubbletrubble]

I understand that it would be hard to be P02 limited doing rec diving, I am just not the kind of person that can live comfortably with an answer like "it would be hard to hit that limit". I really want to learn to calculate exposure time for multiple dives on nitrox.

It's good that you are insistent about understanding how to calculate this. This certainly should have been covered in your class.

I'm not sure how NAUI teaches its nitrox class. I'm familiar with the way PADI teaches it.
PADI uses the DSAT oxygen exposure tables to make the calculation. Exposures are simply additive, and students are taught to not exceed the 100% maximum in a "rolling" 24-hour period. If a diver reaches the 100% maximum exposure limit, PADI advises to wait 12 hours until the next dive (washout period). Based on the EAN mix being used, duration of the dive, and the maximum depth, each dive will have a % oxygen exposure associated with it.
Here's an example of how you would calculate it:
Monday 8am 32% EAN 80 fsw for 36 min. -- O2pp of 1.1 = 15%
Monday 12am 32% EAN 60 fsw for 36 min. -- O2pp of 0.9 = 10%
Monday 4pm 32% EAN 60 fsw for 54 min. -- O2pp of 0.9 = 15%
Cumulative O2 exposure on Monday night is 15% + 10% + 15% = 40%
If the diver planned to do another dive at 10am on Tuesday, at the beginning of that morning's dive, his cumulative O2 exposure would be only 10% + 15% = 25% since the Monday 8am dive O2 exposure would have "expired" in the intervening 24-hour period.

If I were to convulse and drown when my wife and daughter eventually meet me in heaven I would get a swift kick in the balls from both of them for just taking the word about something that can kill you instead of learning and checking it myself you know

As long as you adhere to the maximum operating depth (MOD) of your EAN mix, you should have a low probability of experiencing convulsions. Toxicity caused by cumulative exposure to high partial pressures of oxygen probably manifests differently (at least for the recreational diver).

Although your nitrox class materials probably cover this, I'll try to review the material succinctly. It is thought that UW convulsions as a manifestation of oxygen toxicity are due to instantaneous effects on the central nervous system (CNS). Several agencies advocate planning for a maximum instantaneous oxygen exposure of 1.4 ata. It's thought that the probability of oxygen toxicity substantially increases at oxygen partial pressures exceeding 1.6 ata.

Prolonged exposure to high oxygen partial pressures results in the Lorraine-Smith effect, which describes a direct time-dose effect of oxygen on the lungs. Symptoms associated with this phenomenon include: burning sensation in the chest, problems breathing, lung irritation, coughing, and shortness of breath.

So, to recap:
instantaneous ox-tox = effect on CNS --> UW convulsions
cumulative ox-tox = effect on lungs --> coughing, burning, shortness of breath, breathing problems

This is how PADI teaches it. It is probably an "OK" way of doing things for a recreational diver.

NOAA teaches things differently. Of course, if you want to do advanced deco with high O2 mixes, please get the proper training first. The way PADI teaches it in its basic nitrox class would definitely not be applicable.

As with all things that we don't fully understand, it's advisable to dive conservatively with respect to oxygen exposure. PADI's simplification of the matter is probably not 100% correct. For instance, it's certainly possible that prolonged exposure to high partial pressures of oxygen can do damage to CNS neurons via free radical formation and other mechanisms. It's unknown to what extent this might occur in the context of scuba diving.

 

[Van Isle]

One method:

For each dive calculate the percent of 24 hour limit used up by that one dive. Assume square profile and base it on highest ppo2.

i.e. 20 minutes at 1.4ppo2 uses 11% of your total CNS clock, as per TDI tables based on NOAA limits.

If you did 7 of those in a day you add all the %totals, i.e. you'd have used 77% of your daily total limit.

On 32% you'd have to be hovering around 111fsw for 20 minutes to get that, and it's just not likely that you'd do that 7 times in 24 hours, hence some comments that you shouldn't worry about it. Plus there is such a huge rounding error inherent in these tables that render them even safer. "Recovering" CNS clock time is, I've heard, an "advanced nitrox" kinda thing.

PS. don't learn how to dive on the innet. I'm not trained or certified in SCUBA, nitrox, or instruction; I don't even have a driver's license.