Please give me your pO2 opinion

[Doppler]

Take a look at the polls on my Facebook page... Interested in your approach to CNS management
techdiverTraining | Facebook (http://www.facebook.com/techdiverTraining)

[wedivebc]

Added 1.3 to the rebreather poll. I should mention that in CCR cave diving and deep dives I use 1.0 but my answer is a little wordy for the poll.

[ajduplessis]

It's easy. Stick to the prescribed limits.

[GrimSleeper]

Is it cheating to give two answers?

What ppO2 I use depends on how CNS exposure and deco time stack up against each other. On a shortish bounce, say 25-30 minutes at 55m, a back-gas ppO2 of 1.4 atm and deco switches at (or close to) 1.6 atm shortens the deco, shortens the time at high ppO2 during deco, and so reduces total CNS exposure. Win-win - lower O2 exposure and I'm out of the water sooner.

Run a much longer or much deeper profile, where exceeding 100% CNS is inevitable, and I don't want to be managing (and risking) CNS exposure at depth. The sensible solution then is to use a lower back-gas ppO2 and possibly make the first switch at 1.4 atm. That way I'm not hitting a high % CNS until I'm at 6m, maybe 9m for a really big dive, where I expect to be doing ppO2 breaks anyway, rather than having to manage shorter stops and CNS on the way up.

Which is a fairly convoluted way of saying it depends on the dive, I guess.

Oh, and whichever answer I gave on the poll counts, I don't go higher than 1.6 for in-water deco. It may be psychosomatic, but around 1.8 I can 'taste' the O2 really strongly and it unnerves me!

[PfcAJ]

I keep the bottom ppo2 around 1.2, and deco 1.4-1.6 with breaks on the lowest po2 gas I have that's comfortably breathable.

[down4fun]

I answered 1.2 for the rebreather portion of the survey. However, I will frequently boost my po2 to 1.6 at 20 feet with an O2 flush and then run it around 1.4 for the remaining deco. For large dives where CNS becomes an issue I will run 1-1.2 on the bottom and deco out on 1.2 with dil flushes for air breaks.

[fdog]

I keep the bottom ppo2 around 1.2, and deco 1.4-1.6 with breaks on the lowest po2 gas I have that's comfortably breathable.

Succinct, and I'm the same.

As an aside, my bottom ppO2 of 1.2 is driven from not having heard/read/seen any CNS incident(s) at that fraction. If there's something verifiable that were to happen at 1.2, I would promptly ratchet my bottom fraction to 1.1.

<shrugs> I'm conservative. If obligated, I want my chance of CNS to be as close to zero as possible.


All the best, James

[Dsix36]

I answered 1.2 for the rebreather portion of the survey. However, I will frequently boost my po2 to 1.6 at 20 feet with an O2 flush and then run it around 1.4 for the remaining deco. For large dives where CNS becomes an issue I will run 1-1.2 on the bottom and deco out on 1.2 with dil flushes for air breaks.

This is very close to what I do also. I will run my CNS as high as 150% without much worry though.

[JamesK]

I keep the bottom ppo2 around 1.2, and deco 1.4-1.6 with breaks on the lowest po2 gas I have that's comfortably breathable.

This.

[Doppler]

Stick to the prescribed limits.
http://www.yourfinancehelper.com/article/today/act.jpg
http://www.yourfinancehelper.com/article/today/lk.jpg

As an aside, which prescribed limits? NOAA's or the seat-of-the-pants adaptations commonly practiced and seemingly endorsed by many of the major agencies?

[PfcAJ]

I've always found the CNS% thing to be BS. You can tox well below it, and not tox well past the "100%" value. So uh.... wat do?

[fdog]

I agree with the angst over the extreme variability with CNS% and ppO2.

Still, in all the data (what little there is, anyway) there has to emerge a TCLo and TDLo.*

I would think the prudent diver would stay under these values, where you'd have the least chance of an event; but on reflection, where one places one's self in relation to TCLo is a matter of personal philosophy.


All the best, James



*: TCLo = Toxic Concentration Lo, the lowest concentration of a material known to cause a toxic reaction. Even just getting a headache would qualify as a toxic reaction. TDLo = Toxic Dose Lo (http://en.wikipedia.org/wiki/Lowest_published_toxic_dose). TCLo would best equate to ppO2, and TDLo for CNS%.

[Randy43068]

Done.