EaNX and PPO2 of 1.6

[jviehe]

A bit of all three. I can only disagree once I know for sure what you are saying. And when I am in disagreement with someone, I often find that figuring out why we disagrees leads to further enlightenment.

It does appear that what I consider to be a CNS O2 clock you say is controlled by lung/pulmonary considerations.

For example, I consider the 300 minutes max at 1.0ata ppO2, 210 minutes max at 1.2ata ppO2, and 150 minutes max at 1.4ata ppO2 to be CNS limits, not pulmonary. When I refer to CNS clock, I refer to using these sorts of limits, but expressed in %/minute terms to calculate the "% of CNS clock" that has been consumed. (For example 300 minute limit at 1.0ata turns into 0.33%/ minute, 150 minute limit at 1.4ata becoms 1/150 = 0.67%/minute).

Is the above sort of calculation what you are calling lung/pulmonary toxicity calculations? I call the above the O2 clock or CNS clock, and don't believe it has anything to do with pulmonary effects such as lung damage, edema and breathing difficulty. I've never had to calculate OTUs for any recreational dive.

-----------------------

The MOD of 1.4ata is indeed a CNS-related calculation, but it is simply a very quick and dirty way of keeping the CNS clock in limits, since it is unlikely that a non-technical open circuit diver will exceed the 150 minute per dive CNS limit for 1.4ata ppO2.

PADI calls it the oxygen exposure table, but in advanced nitrox its OTUs. Im speaking only of recreational nitrox, and im not formally trained in advanced nitrox. Using 100% of your 02 clock puts you at risk for pulminary toxicity and increases your risk of CNS toxicity. The exposure table and 1.4 limit is designed to keep rec divers withing conservative limits.

So basically, CNS is based on depth, pulminary based on time, though both are called oxtox.

 

[Charlie99]

We disagree on this.

I don't have a PADI nitrox book handy, so I googled for "PADI Oxygen Exposure Table" and found a table that, although it just says "Oxygen toxicity", is one that calculates the CNS clock. That table (the one with equivalent air depths on the backside) is NOT pulmonary calculations. That PADI/DSAT Oxygen Exposure Table is simply the NOAA CNS O2 exposure limits in a tabular form.

Erik Baker has an article "Oxygen Toxicity Calculations", which pretty clearly explains both CNS calculations and the pulmonary/OTU/UPTD calculations. Even a brief glance at that article would show you that the 150 minutes at 1.4ata ppO2 is a CNS limit, not a pulmonary limit. The pulmonary limit (for 4% loss of vital capacity) is somewhere around 5 hours at 2ata ppO2, and much longer at 1.4ata. See ftp://ftp.decompression.org/pub/Baker/Oxygen Toxicity Calculations.pdf

 

[Bismark]

Like Charlie Said.

Also, JeViehe, I make my share of typos, but you should probably start spelling pulmonary correctly (not pulminary) or one of your students is going to point that out to you......... ;-)

 

[Scubakevdm]

And when I am in disagreement with someone, I often find that figuring out why we disagrees leads to further enlightenment.

It has lead to my enlightenment! Thanks Charlie!

 

[jviehe]

We disagree on this.

I don't have a PADI nitrox book handy, so I googled for "PADI Oxygen Exposure Table" and found a table that, although it just says "Oxygen toxicity", is one that calculates the CNS clock. That table (the one with equivalent air depths on the backside) is NOT pulmonary calculations. That PADI/DSAT Oxygen Exposure Table is simply the NOAA CNS O2 exposure limits in a tabular form.

Erik Baker has an article "Oxygen Toxicity Calculations", which pretty clearly explains both CNS calculations and the pulmonary/OTU/UPTD calculations. Even a brief glance at that article would show you that the 150 minutes at 1.4ata ppO2 is a CNS limit, not a pulmonary limit. The pulmonary limit (for 4% loss of vital capacity) is somewhere around 5 hours at 2ata ppO2, and much longer at 1.4ata. See ftp://ftp.decompression.org/pub/Baker/Oxygen Toxicity Calculations.pdf

I think the only think we disagree on, or rather you disagree with PADI on, is that you can predict CNS tox or track it. PADI thinks not, and thats what I was taught and what it says in the manuals. Since they dont beleive you can track it, the oxygen exposure table (which is the NOAA math) is only used by PADI to track pulminary tox. CNS tox is simply avoided by staying below 1.4. Now, there may very well be more advanced and detailed info in other references and courses. I was simply answering the op, who is only recreationally trained, as am I.

TO quote:

Most physiologists consider attempts to predict CNS toxicity on a partial pressure/time basis unreliable. FOr this reason divers avoids CNStox by staying well within a max partial pressure.

-PADI Encylopedia

 

[jviehe]

Like Charlie Said.

Also, JeViehe, I make my share of typos, but you should probably start spelling pulmonary correctly (not pulminary) or one of your students is going to point that out to you......... ;-)

Ill see what I can do.

 

[nereas]

Like Charlie Said.

Also, JeViehe, I make my share of typos, but you should probably start spelling pulmonary correctly (not pulminary) or one of your students is going to point that out to you......... ;-)

Florida is not Canada.

In Florida they just spell stuff different!

 

[nereas]

...So basically, CNS is based on depth, pulminary based on time, though both are called oxtox.

They are actually both based on "duration of exposure" which is like what you said about "time."

CNS = central nervous system (in other words, siezures).

Pulmonary = lung tissues (in other words, injury to the tissues).

With NDL scuba, when you limit your diving depth to correspond with 1.6 ATAs ppO2 or less, you have virtually no risk of either.

1.4 is a very popular limit to teach novice nitrox divers. Although for NDL scuba this is actually way "too" conservative to really be meaningful, as was explained earlier and in the referenced posting.

When you exceed 1.6 then you need to correctly calculate to avoid risk.

When you get really high, in the ranges of 2.0 or more, then the onset of CNS is very likely.

Pulmonary injury would be very difficult to achieve with NDL scuba. You just don't spend enough time underwater to do it. Maybe yes, if you tried to set a world endurance record. Otherwise not.

 

[nereas]

Just curious and wanting to learn, but why not just go to tri-mix for a dive like this? Seems like a safer mix all around for such a deep dive.

I have noticed that a lot of divers prefer nitrox instead of trimix for certain depths and use nitrox whenever possible.

For 130 to 150 ft, EAN 25 is quite popular, although it is barely better than air.

For 100 to 130 ft, EAN 32 is quite popular, and for some the ideal mix.

For 50 ft to 100 ft, EAN 36 is a very popular mix, expecially with expert divers, who are not paranoid about a limit of 1.4 ATAs.

Nitrox is easier to get, and easier to mix, and faster to decompress from when you have only one gas. And for strictly NDL diving, nitrox gives you longer NDL times than does trimix of the same O2 levels.

Also with trimix, you need to have splendid buoyancy control. If you happen to shoot to the surface with trimix, it would likely be a fatal error. Helium is unforgiving of novice divers.

 

[jviehe]

Ok, after reading some more, I think the problem here is that we are saying the same thing, but you are saying it in more detail. PADI simply eliminates unnecesary information. According to sources other than PADI, CNS tox can be time dependant if single exposures to ppo2 exceeds whatever time. However these times are impossible to do as a recreational diver, which is why PADI discards them. For example you could be exposed to 1.4 for 150mins, which you cant do without doing deco, even at 40% nitrox. So, in effect, we are both right, but you are more right.