Nitrox - 1.40 or 1.60 PO2?

[JahJahwarrior]

Some of this has already been said.

The other side of the coin is the CNS clock.

According to the IANTD, at a Po2 of 1.4, you accumulate .65% of your CNS clock per minute. This gives you approximately 154 minutes until you accumulate 100% of your CNS clock, via my understanding of math (divide 100 by .65 to get total minutes before you read 100). A Po2 of 1.6 accumulates 2.22% of your CNS clock per minute. This gives you approximately 45 minutes until you accumulate 100% of your CNS clock.

If you mix your gas to have a po2 of 1.6 at your max depth, and you stay near your max depth for most of the dive, in other words, dive with a po2 of 1.6....you severely limit your bottom time for that dive and subsequent dives, and increase the needed surface intervals.


Of course, when it comes to setting alarms on computers, that's personal preference. And when it comes to momentarily pushing a Po2, many times you'll probably survive pushing it to 1.6. Of course no gaurantees you'll survive the dive on 1.4 I would not dive at 1.6, and prefer to deco at less than 1.6 even due to the increased CNS loading. To each their own.

 

[ucfdiver]

To each his own no matter how much they talk about it no one has ever suffered a hit at 1.6 or below and even with a number above it they had to be in that environment for 30 minutes or more and be working really heard!

That statement is incorrect.
Divers4Life | Liz & Tim's Adventures

Please folks, when you're discussing things that may effect people's decision making on safety protocol, be sure that you've researched it. I think a more accurate statement would be "there have been very few o2 hits within the standard 1.4-1.6 max ppo2 range". It's important to not give divers a sense that there's zero chance of a hit if they follow standards.

I know people who deco on a ppo2 of 2.0 and have done so for several years. Not suggesting it, but with people toxing at 1.4, and some people always deco'ing at 2.0, individual susceptibility is certainly a factor.

 

[Gene_Hobbs]

Preface: I'm not picking on you but you, Papa_Bear, are talking in absolutes so it makes quoting you easier than some of the others... In fact I agree with your acceptable level of risk and reasons why. It amazes me how many miss the time component in this.

To each his own no matter how much they talk about it no one has ever suffered a hit at 1.6 or below and even with a number above it they had to be in that environment for 30 minutes or more and be working really heard!

At least we all keep evaluating our position and checking all the available information. Oh wait, someone didn't download the Vann presentation. There were 5 cases of symptoms and/or fatalities 1.6 or below reviewed (2 of these at 1.4, 1 at 1.3, ). Drugs may have been a factor in some but we know a significant potion of the recreational diving community does take some type of medication. (RRR ID: 7684)

Until I see hard core evidence that it is a risk I will set my computer to 1.6 hard bottom and dive that MOD profile! I think this is a good topic so that just maybe people diving VooDoo Gas will do their own home work and not just take someones word for it!

Maybe we should all do more reading/ watching to make sure we know what is actually in the literature.

More reading that includes large numbers of dives and symptoms below 1.6 ATA (reviewed by Vann in the presentation BTW):

Leitch, DR. A study of unusual incidents in a well-documented series of dives. Aviat Space Environ Med. 1981 Oct;52(10):618-24. PubMed ID: 7295250

Arieli R, Arieli Y, Daskalovic Y, Eynan M, Abramovich A. CNS oxygen toxicity in closed-circuit diving: signs and symptoms before loss of consciousness. Aviat Space Environ Med. 2006 Nov;77(11):1153-7. PubMed ID: 17086769

For the record, the USN has a reason they use 1.3 as a rebreather set point...

 

[The Kraken]

JJ,

For "MOST" recreational Nitrox divers, the CNS clock never comes into play. Now that doesn't mean it won't.

Basically what this thread is saying is that you "can't have your cake and eat it too."

If you reduce your PPO2 to someting like 1.2, then you essentially negate the reason for diving nitrox, in my humble opinion.

I generally set my PPO2 for a 1.4. Will I dive a 1.5, yes. A 1.6, yes, but only for a very short time.

This is sort of like "the mask on the forehead is a sign of a diver in distress". Although it "MAY" be an indicator, it is not a matter of fact.

But anyway.

I hope all who read this thread will go back to their texts and do some more reading.

As always . . .

Safe dives . . . . . . .
. . . safer ascents !!!

the K

 

[nereas]

...
For the record, the USN has a reason they use 1.3 as a rebreather set point...

Now you have mixed apples with oranges, or more specifically CCRs with O/C.

 

[nereas]

... There were 5 cases of symptoms and/or fatalities 1.6 or below reviewed (2 of these at 1.4, 1 at 1.3, ). Drugs may have been a factor in some but we know a significant potion of the recreational diving community does take some type of medication. (RRR ID: 7684)

...

Yet another mistake of mixing apples with oranges, or more specifically drugs and scuba, this time.

 

[nereas]

Ever since the ancient Greeks first invented logic, in the 7th Century B.C., it has fallen into the wrong hands, and been misused, at times, it seems.

Next time I am in the city of Athens, I will bring this problem to the attention of the Elders there. We seem to have opened Pandora's box.

 

[Thalassamania]

The gear that is supplying the mix really does not matter, it is solely a question of how long you are exposed to a given partial pressure. Granted with O/C you can lessen the partial pressure by ascending, but its not like you get much or a warning, regardless of the gear.

 

[nereas]

The gear that is supplying the mix really does not matter, it is solely a question of how long you are exposed to a given partial pressure. Granted with O/C you can lessen the partial pressure by ascending, but its not like you get much or a warning, regardless of the gear.

The gear that is keeping you at a constant ppO2 is called a CCR.

And the gear that allows you to vary your ppO2 is called O/C.

And as East is east, and West is west, n'ary the twain shall meet.

 

[ucfdiver]

Ever since the ancient Greeks first invented logic, in the 7th Century B.C., it has fallen into the wrong hands, and been misused, at times, it seems.

Next time I am in the city of Athens, I will bring this problem to the attention of the Elders there. We seem to have opened Pandora's box.

This is 4 posts in this thread without giving ANY new information, and the 4th tongue inc cheek comment towards another member. Can a moderator please put a stop to this trolling?

JJ,

For "MOST" recreational Nitrox divers, the CNS clock never comes into play. Now that doesn't mean it won't.

Basically what this thread is saying is that you "can't have your cake and eat it too."

If you reduce your PPO2 to someting like 1.2, then you essentially negate the reason for diving nitrox, in my humble opinion.

I generally set my PPO2 for a 1.4. Will I dive a 1.5, yes. A 1.6, yes, but only for a very short time.

This is sort of like "the mask on the forehead is a sign of a diver in distress". Although it "MAY" be an indicator, it is not a matter of fact.

But anyway.

I hope all who read this thread will go back to their texts and do some more reading.

As always . . .

Safe dives . . . . . . .
. . . safer ascents !!!

the K

Completely agree. I was just stating that in order to assess risk, we have to realize that we're not dealing with certainties here, we're dealing with low probability.