Nitrox - What do you dive PO2 - 1.4, 1.5 or 1.6?

[RJP]

I was reading an article that talks about PO2..

Instruction: CNS Oxygen Toxicity | Scuba Diving
This point stuck out at me -
"Please note: There is no more danger with a 50 percent exposure to oxygen at 1.6 than there is with a 50 percent exposure at 1.4 or 1.3. It's the total dose, not the PO2 that determines your risk factor."

I called a local LDS and inquired about one of the dives for this summer - he said to the sand is 130ft so you would be diving 28% Nitrox - which is 1.38 PO2... Would I be allowed to dive 30% at 1.5 or is that pushing the limits?

The crew on the boat would take a dim view of a passenger diving a "hot mix" for the destination wreck. 30% wouldn't get you left at the dock for a wreck with enough relief that you could stay shallower - say the Stolt or the Algol - but we'd want to understand your dive plan and your determination to stick with it. Say you dropped a light over the side of the deck of the Algol at 100ft. Do you have the restraint to watch it go bye-bye... or are you going to chase it?

 

[Basking Ridge Diver]

Since I am currently diving air - I am chasing it...

Now that I am learning to understand the predicament I would stay safe at 130 to the sand with 28% and 1.4 PO2...

 

[MB]

Hey Basking... A lot of chat here about 1.6 for deco, but are you doing deco dives? If you are a regular kinda cat doing regular kinda dives, its a good idea to set a limit for 1.4 for the max portion of your dive, and skip the drama regarding 1.5 and 1.6. Its OK to dive 1.2 or 1.3 - you still get a lot of benefit from diving EAN with those mixes!

And, do you always set your mix for the sand? That's a very reasonable thing to do, but if you plan to explore the deck of a wreck, and not venture over the side to the sand... AND if you have the self discipline to avoid dropping down to recover that Rolex that is 50' below your mix set at 1.4, then many divers don't use the sand as their depth for planning a 1.4 mix. Its safer to plan that way, but not as "required" as many internet divers proclaim.

 

[Basking Ridge Diver]

MB - no I do not do any deco dives at this point. I was more interested in making sure I understand the Nitrox portion prior to taking the class. So this was more of a question from starting to learn the formulas and doing some research.

Understood both points about going to the sand vs having the discipline to follow the plan of the PO2... Which is further than I was a couple weeks ago...
Thanks

 

[NAM001]

I set my mix for sand at 1.5 The majority of the dive will be at 1.4 or less. The 1.4 aspect allows you to make an excursion to 1.6 or better if I need to. buddy save ect.
I did some spreadsheet work years ago running a formula dealing with some aspect of cns% per minute. I found that when I plotted it at about 1.6 things became logrythmic and no longer linear. so that was the purpose of the 1.6 limit I determined. Not all physically respond the same so that explained the 1.5, and the 1.4 provided the buffer for the 1.5 crew. Of course there are some I suppose that will have problems at 1.3 or so but statistically odds are very much n the divers favor. I firmly believe that going to 1.7 or so is not a bad thing if you have to but one should not do so as not to temp fate. NOw I did on one occasion hit 1.78 on 32% because I did not watch the depth gage while on the Oriskani. Once i saw i was past i beet feet up to ppo2 1.0 for a while. That distraction was what the 1.4-1.6 buffer was for. Others I am sure MIGHT say that explains a lot about me. Hmmmmm

 

[FinnMom]

Have you heard the bit about better to be on the boat wishing you were in the water than in the water wishing you were on the boat?

So much better to sit on the boat and think "Gee, maybe I could have used a higher PPO" than to peek over the edge of a cloud from heaven and think "Gee, if only I'd used a lower PPO".

 

[hroark2112]

I let myself get distracted on a dive once and when I looked at my computer, I was sitting at 1.9...not good.

I usually try to maintain 1.1 until I'm on deco, then it's 1.6. If I get distracted, I might drop to 1.0 or get up to 1.2, but I'm normally pretty good at watching where I am.

There are exceptions, but those are not really for this discussion.

 

[Iralub]

I've been using 1.3 (doing strictly recreational diving, no deco).
Given that cns tox survival rates are uncomfortably close to zero, I want the probability of it happening to be comfortably close to zero.

 

[SillyQuestions]

The old limit in the 1950s for ppO2 was 2 bar, with very few recorded incidents...

I'd be interested to hear from any habitat divers getting on the O2 at 30ft what sort of experience/stories they have of oxygen seizures at that depth!


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[KenGordon]

The old limit in the 1950s for ppO2 was 2 bar, with very few recorded incidents...

I'd be interested to hear from any habitat divers getting on the O2 at 30ft what sort of experience/stories they have of oxygen seizures at that depth!


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In a chamber they will use 2.8 bar. Of course when someone fits there is a nurse and the risk of drowning in your cup of tea is lower than in the water. I seem to recall that being immersed has an impact on the likelihood of a fit, so I don't think habitats are representative of finning about in the working part of a dive. Also time is a factor, being at 1.9 for a couple of minutes and being ok doesn't mean being at 1.5 finning hard for 20 minutes after x minutes at 1.4 will be ok.

I think high ppo2 is a game of diminishing returns in any case. Once you are no longer breathing air you gain a lot in deco terms, but the difference between 30% and 32% to make sure you MOD reaches the bottom will not be a big deal. Where I dive there isn't such a difference between the top and bottom that you'd not plan for the bottom. I understand that there are huge intact reef'ed ships where that doesn't apply, for those pay attention!