Nitrox question about 1.4 and 1.6

[Bobby]

What do you mean when you say some pushed the limits and most paid with there lives? Are you talking PPO2 in excess of 2.0? 1.8? It is my understanding that there is an increased risk as you push from 1.4 to 1.6 under certain conditions including higher work loads and extended times (more than 5 minutes). Otherwise, the incidences of CNS in that range are extremely low.

The only time that 2.0 PO2 has been standard is in the military with tenders under decompression & a full helmut. When tech diving started a lot of us used the military standards and a lot toxed out. I was military trained so fortunately I lowered my PO2's when doing tech dives. There have been many cases of divers using 1.6 & 1.8 for the working part of the dive & toxing from it. Go back to the mid to late 80's data and you will find case after case. 1.4 works well for recreational diving. When you start doing tech that may require a higher work load it is common now to drop the PO2 for a higher safety margin during the working part of the dive. Hope this clears up what I meant a bit.

Bobby

 

[*Floater*]

So to get to my question which one do you use and why? Is it more of the way some others do the training or is it just a safety margin that PADI has adopted for training?

I don't do many "best mix" dives, which according to some schools of thought aren't really best to begin with, and "standard gasses" may be a better approach, but if or when I do use "best mixes", I try pick the nitrox so that at my max planned depth the ppO2 is 1.4 because this is considered a safe standard for a working portion of the dive. If I see something interesting a little deeper down, then going there - down to ppO2 of 1.6 max - for a short time is no big deal imo just as long as conditions are good (warm and effortless).

However, some people argue that even at 1.4, maybe lower depending on the length of the dive, your lungs will experience vasoconstriction from the oxygen which they say makes your lungs less efficient at off-gassing i.e. you off-gas less efficiently. So for many repetitive rec dives, or deco dives, I prefer to keep the ppO2 around 1.2 ideally. But again, if I have to go a little above that, then it's fine I just do my deco more conservatively. Also, I start my deco at ppO2 of 1.6 (assuming good conditions) in order to take max advantage (relative to risk) of the oxygen window.

However, those NOAA limits are just basic guidelines based on numerous experiments and studies that seem decent for the diving population as a whole. However, those same studies did show that individual divers vary greatly in their tolerance to O2, and furthermore, their daily tolerance varies hugely. In one study, the same diver could last over 2 hours before toxing on one day but then toxed within 7 minutes on another day. He was at ppO2 of 3.1 at rest mind you. Another exercising diver toxed at ppO2 of 1.6 at Duke after 40 minutes. Basically, the guidelines you were taught are a good place to begin, but there's a lot more to the story, and most of it is still not very well understood, so just keep on reading, and make up your own mind as to what's a good ppO2 for you in terms of risk v. benefit.

 

[MB]

Some years ago, nitrox divers were taught to plan dives based on a 1.6 PO2. As nitrox became better understood and common among recreational divers, most agencies adopted the 1.4 PO2 standard, with 1.6 as the contingency. Today, many agencies have reduced the PO2 recommendation a bit, thus the 1.2 and 1.3 recommnedations.

Planning a contingency PO2 of 1.6 requires that you have the same operational defintion of "contingency" as the science behind the recreation agencies. When folks speak about a 1.6 PO2 for contingency purposes, the assumption is that this is for a very limited time and reason, without exertion. For example, if I dropped a dive knife, and need 3-4 kicks down another 15' to pick it up, will I turn in to a convulsing pumpkin if my PO2 hits 1.6? THAT is the logic behind planning a contingency depth.

Having said that, there's nothing lost by keeping the PO2 of a dive closer to 1.2/1.4. There's still lots to be gained from a mix and profile that results in this PO2.

 

[divesmith]

There is a good article, in the question and answer section , on page 98 of the July issue of "Dive Training" regarding oxygen pp. You might want to check it out. It mentions that there is one organization that has for some time used the more conservative limit of 1.3 atm and that is the U.S. Navy.

 

[Tatakai]

WOW!!! I didn't know that I would get this kind of response on this topic. I want to let you know that my MAX is going to always be 1.4 and not go over that. From what I was told in class that at greater than the 1.6 your chance of a problem is almost certain for O2 Toxicity. I have just started on the Nitrox so I am learning from you divers. Thank you again and I will be watching for your replies to this topic.

Tatakai

 

[lamont]

WOW!!! I didn't know that I would get this kind of response on this topic. I want to let you know that my MAX is going to always be 1.4 and not go over that. From what I was told in class that at greater than the 1.6 your chance of a problem is almost certain for O2 Toxicity. I have just started on the Nitrox so I am learning from you divers. Thank you again and I will be watching for your replies to this topic.

Tatakai

actually over 1.6 your chances are not certain, but the higher the ppO2 the worse odds you have in the russian roullette game you wind up playing. at 1.6 you'll actually be reasonably unlikely to tox unless your workload is very high and 9 times out of 10 or better you'll probably be able to peak at 1.8 and live to tell about it. that can actually lead to complacency, since you can dive multiple times in a row at 1.8 and live to tell about it, until the first time you tox... and sensitivity is highly variable from exposure to exposure -- you could successfully do a bunch of 30 min 1.8 ppO2 dives and then tox after 5 mins one day...

 

[DandyDon]

1.2?! Okay, how about 1.1, or would 1.0 be safer? Are we going back to fear of the Devil Gas? jeeze

What do you mean when you say some pushed the limits and most paid with there lives? Are you talking PPO2 in excess of 2.0? 1.8? It is my understanding that there is an increased risk as you push from 1.4 to 1.6 under certain conditions including higher work loads and extended times (more than 5 minutes). Otherwise, the incidences of CNS in that range are extremely low.

"Most paid with their lives..."?? Yeah, I think Awap has a good handle on it.

I like all the benefits of Nitrox, and cutting the mix to 1.2 greatly reduces some of those with little increase in safety.

 

[lamont]

I like all the benefits of Nitrox, and cutting the mix to 1.2 greatly reduces some of those with little increase in safety.

cutting the mix to 1.2 gives you about 5 mins more deco.

 

[victor]

cutting the mix to 1.2 gives you about 5 mins more deco.

I am confused

I thought that limiting PPO2 to 1.2 just reduces the depth that you consider to be dangerous.
Thus does not affect no decompression time etc.
I am assuming rec diving without deco.

I am planning to take the class this autumn but the more I know before hand the happier I am.

 

[DandyDon]

cutting the mix to 1.2 gives you about 5 mins more deco.

For a Recreational Dive forum, I'd say that sucks.