EaNX and PPO2 of 1.6

[jim2386]

Hey guys,

I'm going to be diving the Oriskany in a few weeks with a dive group and we will be diving nitrox. I've been certified to use it for over 2 years but have never found the opportunity to use it (air has worked fine for all my ocean dives since we'd do a deep dive then a shallow dive, shallow cavern diving etc.) . My computer can handle nitrox and I'm familiar with the rules etc. but I think maybe during my classes I misunderstood my teacher and just would like some clarification.

My instructor said the limit for PPO2 is 1.6. Now is that limit tied to the same limit as your O2 clock? Let me see if I can explain what I thought he meant and then what my confusion is.

When I first took the class, my understanding was PP02 1.6 was THE limit. If you hit 1.65 you instantly black out from OxTox. This was seperate from your O2 clock where if you reached 100%+ of your O2 clock limit, you'd black out from OxTox. They were two seperate ways to croak....for lack of a better term lol.

Now that I'm thinking about it. What if it's your first dive of the day on nitrox and you go straight to the bottom and hit PPO2 1.65 for say 1 minute and come back to a level where your PP02 is 1.3 or so. Is what my instructor was trying to say was "If you go below PP02 1.6 your O2 clock is used up much much faster so you consider it "instantly" using up 100% of your O2 clock because it only takes maybe 10 minutes to use it up at PP02 1.6?"

The reason I ask is we will be diving 29% O2 and at the beginning of the dive we're planning on touching the deck at 137'. This puts me at a PP02 of 1.58. We would only stay for a minute or two and then go hang out around 110' and shallower. Now I can have my computer beep at me if I were to hit PP02 1.6, but if something goes wrong (heaven forbid) and I get distracted for a second or take a second to become neutrally bouyant and accidentlly go slightly over PP02 1.6 I'm not going to black out right?

Sorry if it's a stupid question, just like to be very prepared for things (taking my rescue diver classes this weekend, I guess it's rubbing off on me already ).

Thanks for your responses in advance!
Jim

[crpntr133]

Your probably not going to black out and die if your Po2 is 1.6. Everyones tolerance levels are a bit different. Figure it at 1.4 if you want.
Talk to some of those that have been around and you will find some that use to dive to PO2 of 2.0. Not really advised to do that now but.

[S&V]

Jim,
Just as a quick OBTW, reports from divers going to the Oriskany since the passage of Hurricane Gustav say that the previously known depth checkpoints are now about 10 ft deeper. The ship must have settled into the seabed. Anyway, the flight deck is now likely at 145 or so...
Oriskany - post Gustav report

[lamont]

When I first took the class, my understanding was PP02 1.6 was THE limit. If you hit 1.65 you instantly black out from OxTox. This was seperate from your O2 clock where if you reached 100%+ of your O2 clock limit, you'd black out from OxTox. They were two seperate ways to croak....for lack of a better term lol.

there is no "instant croak" limit.

you might be able to do a 30 minute dive on 2.3 and not have an issue. the next day at 2.1 you might tox after 5 minutes.

technical divers do 1.6 on decompression stops where they're trying to push higher ppO2s in order to decompress and they are (ideally) decompressing at rest. they also don't spend much time there.

for recreational divers the recommended limit is 1.4 since you may be at that limit for longer and may be under some exertion (swimming around, fighting current, chasing your buddy, whatever). planning for 1.4 on the bottom also gives some leeway where an accidental short excursion deeper to 1.6 isn't likely to cause you to tox.

for longer exposures and/or if you're decompressing afterwards with high ppO2 decompression gases, then you should think about backing off your ppO2 on the bottom to 1.3 or 1.2

[nereas]

Hey guys,

I'm going to be diving the Oriskany in a few weeks with a dive group and we will be diving nitrox. I've been certified to use it for over 2 years but have never found the opportunity to use it (air has worked fine for all my ocean dives since we'd do a deep dive then a shallow dive, shallow cavern diving etc.) . My computer can handle nitrox and I'm familiar with the rules etc. but I think maybe during my classes I misunderstood my teacher and just would like some clarification.

My instructor said the limit for PPO2 is 1.6. Now is that limit tied to the same limit as your O2 clock? Let me see if I can explain what I thought he meant and then what my confusion is.

When I first took the class, my understanding was PP02 1.6 was THE limit. If you hit 1.65 you instantly black out from OxTox. This was seperate from your O2 clock where if you reached 100%+ of your O2 clock limit, you'd black out from OxTox. They were two seperate ways to croak....for lack of a better term lol.

Now that I'm thinking about it. What if it's your first dive of the day on nitrox and you go straight to the bottom and hit PPO2 1.65 for say 1 minute and come back to a level where your PP02 is 1.3 or so. Is what my instructor was trying to say was "If you go below PP02 1.6 your O2 clock is used up much much faster so you consider it "instantly" using up 100% of your O2 clock because it only takes maybe 10 minutes to use it up at PP02 1.6?"

The reason I ask is we will be diving 29% O2 and at the beginning of the dive we're planning on touching the deck at 137'. This puts me at a PP02 of 1.58. We would only stay for a minute or two and then go hang out around 110' and shallower. Now I can have my computer beep at me if I were to hit PP02 1.6, but if something goes wrong (heaven forbid) and I get distracted for a second or take a second to become neutrally bouyant and accidentlly go slightly over PP02 1.6 I'm not going to black out right?

Sorry if it's a stupid question, just like to be very prepared for things (taking my rescue diver classes this weekend, I guess it's rubbing off on me already ).

Thanks for your responses in advance!
Jim

Thal (Thallassamania from Hawai'i) gave a great explanation of that in a thread recently here:

Reasons NOT to use Enriched Air?

Ox Tox hits normally occur well beyond !.6 ATAs of ppO2. However, the retail scuba training agencies all agree on 1.4 ATAs as the limit to ensure that Ox Tox seizure never occurs. So that is what they teach to novices. Then you are on your own to figure out exactly what the limits mean.

At a ppO2 of 1.6 ATAs your maximum safe exposure time is 45 minutes. However your NDL limit with basic nitrox is going to be a lot less than this, therefore even at 1.6 you are going to be quite safe from Ox Tox seizures, simply because NDL limits are a lot less.

There are all sorts of inexperienced novices that think 1.4 is magic, and that anything more is UN-safe, and that at 1.4 you are absolutely safe.

In reality, at 1.4 ATAs ppO2 your maximum safe exposure time is 150 mins, which is just a larger safety margin for NDL scuba, that's all.

If you should ever progress beyond simple NDL diving, on to CCR, SCR, or deco diving, you will learn that 1.2 ATAs is the limit that you should consider, due to the long underwater times, sometimes which may also be at the 1.6 ATA limit point. However for your current NDL diving, this is not at issue.

[jim2386]

Thanks guys, makes me feel a lot better. Like I said, the dive leaders said the longest we'd stay is to touch the deck and head back up. Thanks for not flaming me, just trying to be safe and have a really good experience at the same time! I figured the dive instructors here who've dove it many many many times knew what they were talking about, but for my own knowledge it makes me feel much more comfortable!

[Charlie99]

NOAA's best guess at the relative risks can be found in the table at the very bottom of this article: DAN Divers Alert Network : OXTOX: If You Dive Nitrox You Should Know About OXTOX That table implies that the CNS clock runs 4 times faster at 2.0ppO2 than at 1.6ppO2.

When I first took the class, my understanding was PP02 1.6 was THE limit. If you hit 1.65 you instantly black out from OxTox. This was seperate from your O2 clock where if you reached 100%+ of your O2 clock limit, you'd black out from OxTox. They were two seperate ways to croak....for lack of a better term lol.

This seems to be a pretty common misconception, but with many instructors leaving their student thinking that they will black out as soon as they pass 1.4ppO2. It is definitely just a time-dose sort of thing. The clock runs faster at higher ppO2. CNS is a highly variable, not only person-to-person but also from day-to-day with the same person. So the limits have a pretty large margin of safety, very appropriately since CNS convulsions is likely to lead to drowning.

[CoralSeaMan]

If you are a recreational diver, you should not exceed your training even if an instructor is along. These things are not very predictable but if you are going to the 145 foot range your are exceeding every limit you have been trained for.

[Gene_Hobbs]

Reasons NOT to use Enriched Air?

Dick's write up there is a great refresher... Some other resources worth checking out if you have the time are:
*wikipedia nitrox article -needs some work but not bad, check out reference #3 as well.
*DAN Tech Conference talk on O2 tox
and if you are really having fun (and still awake)...
*Rubicon Research Repository suggested reading list - oxygen toxicity

Have fun on the trip!

[ucfdiver]

Isn't your partial pressure of nitrogen a bit too high at 140-150ft as well? I would be VERY concerned about my reaction time if a buddy needed me.