Oxygen Toxicity risk with Nitrox?

[BoltSnap]

Not for every agency. Mine is at 60m.

For recreational diving???

What agency is that please?

 

[tursiops]

For recreational diving???

What agency is that please?

He might mean CMAS, which has a "Recreational Trimix Specialty" to 50m. However, from the CMAS standards, we find this:

2.1.62 “Sport diving” shall mean diving to a depth no greater than 40 meters, using only compressed air or Nitrox (with no more than 40% oxygen) as a breathing gas, never requiring a mandatory decompression stop and having direct vertical access to the surface from which an emergency swimming ascent is possible during an emergency. A synonym for sport diving is “recreational diving”.​

 

[dmaziuk]

ANMP "level 3". BSAC Dive Leaders go to 50 and I think Brits may have a 55 somewhere in there too.

Whether you want to consider that "recreational" or not is, of course, entirely up to you.

 

[Seaweed Doc]

Interesting to see people saying that time is critical to ox tox. I guess that surprised me. My understanding, likely flawed or overly-simplified, is that pulmonary ox tox does depend a lot on time at depth. But CNS ox tox may involve relatively short time at depth.

Am I mistaken?

That said, I would agree with the consensus on this:

1. It's a bad idea to not dive your plan.
2. In a lot of training curricula, 1.4 is MOD and 1.6 is for contingency/emergency purposes. As an instructor, for liability reasons and because I don't have more expertise than the curriculum on this matter, I can't advise anything different.
3. Don't trust experienced divers. There's an old saying that brand new divers die of lung overexpansion injuries. Experienced divers die of the bends. (The first is panic-related. The second is from pushing limits until something breaks.) Follow your training.

 

[Stoo]

My maximum depth was 42 meters, and I went below 36.6 on 5 dives.
This happened to me because of diving in the blue (lack of orientation) and seeing some nice hammerheads below, plus noticing other divers were doing the same.

O2 toxicity is the result of a buildup of O2 over a period of time as well as a number of other factors including your level of exertion. Of all the things that are poorly understood in diving, O2 toxicity is right up there.

As others have pointed out, the MOD you weren't supposed to exceed was based on a ppO2 of 1.4. That's a safe, and somewhat conservative value, assuming you're not working hard etc. A brief incursion below your MOD to a depth where your ppO2 is 1.6. Most of us that use pure O2 for decompression "plan" on a ppO2 of 1.6 while we're at 20'... but we're resting and likely holding an anchor line. So you're good... unless you're doing something like chasing Hammerheads and exerting yourself.

The bigger issue that I see is your failure to keep track of your death and following divers who are exceeding the dive plan. Discipline is a real thing in diving. There are plenty of times where a diver may want to modify a plan and that's ok. What's not ok, is drifting beyond your planned depth. Had you not stopped, and just gone a "little" deeper where your ppO2 is 1.8... well NOW you're starting to push it.

 

[Jacob BV]

You were approaching a PP02 of 1.6. This used to be considered safe when I first got certified for Nitrox but today a maximum of 1.4 or even 1.3 is recommended for the active portion of the dive. Helium mixes are less forgiving as nitrogen does have some effect on reducing toxicity.

This is the first time I've heard this. Do you have any sources or threads about this? The nitrogen reducing O2 tox that is.

 

[KenGordon]

nteresting to see people saying that time is critical to ox tox. I guess that surprised me. My understanding, likely flawed or overly-simplified, is that pulmonary ox tox does depend a lot on time at depth. But CNS ox tox may involve relatively short time at depth.

Am I mistaken?

It depends what you mean by “relatively short time”. If you read the incidents which are put down to ox tox it often takes a surprisingly long time (10s of minutes) at outrageous ppO2.

If you take a look at the CNS tables you will see that the allowed time decreases sharply with ppO2. 3 hours at 1.3, only 45 minutes at 1.6. Time and depth are involved, but not in a simple linear way.

 

[S****N]

For recreational diving???

What agency is that please?

We used to have no limit as an instructor. The Nelos (Flemish division of Befos which is the Belgian division of CMAS) changed this a few years ago to 60m.

 

[S****N]

He might mean CMAS, which has a "Recreational Trimix Specialty" to 50m. However, from the CMAS standards, we find this:

2.1.62 “Sport diving” shall mean diving to a depth no greater than 40 meters, using only compressed air or Nitrox (with no more than 40% oxygen) as a breathing gas, never requiring a mandatory decompression stop and having direct vertical access to the surface from which an emergency swimming ascent is possible during an emergency. A synonym for sport diving is “recreational diving”.​

Decompression dives are just part of rec diving or sport diving for us.
Furthermore it doesn’t make sense to look things up in CMAS documents because CMAS is really just an amalgam of local federations with their own set of rules which are more or less compatible with the CMAS rules but are not limited to those rules.

 

[Doc Harry]

Lriemann, didn't you learn the answers to your questions when you became Nitrox certified?