How conservative is the PPO2 Limit

[flots am]

I've noticed browsing through forum threads on the recommended Nitrox limits, the question of how close you can sail to the MOD limits or time exposure limits based on PPO2 (partial pressure of oxygen) limits of 1.4 or 1.6 b often surfaces (pun intended). How conservative are the limits?

I've only heard of a couple of cases, of people toxing at 1.4, ever, so I don't exceed 1.3.

It's not especially scientific, it's more like "see where other people got hurt and don't go there." I don't know of anybody toxing at 1.3, but if it happens, I'll bump back to 1.2.

flots.

 

[mathauck0814]

I worry less about the dosage and more about the exposure. I'm not above elevating the PPO2 on my unit to get out of a suddenly and unexpected unpleasant decompression stay - for a short while.

 

[Foxfish]

I'm simply stating here that the PPO2 limits stated and the discussion relates to recreational Nitrox use. I understand that different limits may be set for hyperoxic exposure in different circumstances.

Or maybe to put it a slightly different way, the real risk is not so much the signs of oxygen toxicity like having a seizure, it is what happens as a result of the seizure. If you are sitting in a barometric chamber and have a seizure it is unlikely to be life threatening. If you are a commercial diver wearing a diving helmet with a communications link to topside and you have a seizure, while there are some risks, they are considerably less than if you are a recreational scuba diver with only a normal mask. If you have a seizure while scuba diving, you will almost inevitable lose your regulator and drown. Even with someone nearby, the examples above demonstrate that getting the regulator back into a persons mouth while they are fitting is not always possible.

Diving conservatively means avoiding situations that can lead to injury or death. It means choosing a PPO2 and MOD with that in mind.

I've read a few articles and watched a few videos on diving the blue hole in Dahab. Apparently it is a beautiful dive. The fly in the ointment is the growing number of gravestones near the shore of people who have died on the dive. There are a lot of weird and wonderful theories about why these divers have died. Ironically for some this adds to the mystique and attraction of the location. Apparently the water is very clear and the current minimal. Experienced divers in commenting on the deaths point out the explanation is relatively simple. A common scenario resulting in death would be a diver descending to the arch at 50 or 60 m, near the MOD for air, being overcome by nitrogen narcosis and then descending further to a point where oxygen toxicity results in a seizure.

One of the well know diving deaths videoed on Youtube happened at Dahab <http://www.youtube.com/watch?v=eejQPUyeNiY>. In this case, the most sensible explanation was that the diver intentionally raced down to 92 m alone, took a video of his dive computer to show off how deep he *dived*, had a seizure and died. You can hear his regulator free flowing at the end. Incidentally the partial pressure of air at 92 m (301 m) is 2.2 b.

To me the idea of descending to depths well beyond the recommended limits and endangering your life to earn 'bragging rights' is simply stupid. The gravestones at Dahab and the sad video stand are very tangible reminders of the risks involved.

 

[Dr. Lecter]

Earlier you got huffy when I pointed out that there's no real answer to your original question, and said this thread (I won't abuse the term "discussion" by using it here) was about recreational nitrox limits. Now you're pontificating about the wisdom of diving deep air.

Again, what's your point? That you think everyone should dive what you consider a conservative pO2? Ok, cool. Thank you for sharing that bit of insight.

 

[mathauck0814]

To me the idea of descending to depths well beyond the recommended limits and endangering your life to earn 'bragging rights' is simply stupid.

Whoa - there's the axe he's grinding.

... descending to depths well beyond the recommended limits

What depths? I thought we were talking about PPO2? Now I'm confused whether the limits are on depth or PPO2...

... and endangering your life

You might consider not diving. Every time you strap on a tank you're potentially endangering your life. Hell, every time you drive to work, or just about anything else (are you presently wearing your helmet?)

... to earn "bragging rights"

Do people really brag to you that they did a dive with a PPO2 of 2.0+ Seriously?

 

[Foxfish]

Earlier you got huffy when I pointed out that there's no real answer to your original question, and said this thread (I won't abuse the term "discussion" by using it here) was about recreational nitrox limits. Now you're pontificating about the wisdom of diving deep air.

Again, what's your point? That you think everyone should dive what you consider a conservative pO2? Ok, cool. Thank you for sharing that bit of insight.

The point is - there is good reason to respect the PPO2 limits set by the various diving agencies. Anecdotal evidence has been provided to support that claim.

Air is just Nitrox at 21 %. How is diving beyond the MOD on air different to diving beyond the MOD using Nitrox in the context of recreational diving?

 

[Rhone Man]

The point is - there is good reason to respect the PPO2 limits set by the various diving agencies. Anecdotal evidence has been provided to support that claim.

Air is just Nitrox at 21 %. How is diving beyond the MOD on air different to diving beyond the MOD using Nitrox in the context of recreational diving?

Although nobody has ever conclusively proven it, there is certainly widely held anecdotal belief that a higher ppN2 retards the onset of oxygen toxicity, and that accordingly you are less likely to tox breathing air at a certain ppO2 than you would be at the same ppO2 breathing an oxygen enriched mix. Incidents of divers toxing on air at high ppO2s seem to be virtually nonexistent. But like so much relating to oxygen toxicity though, we cannot know for sure.

The reknown technical diver John Chatterton posted quite eloquently on that point on Scubaboard a couple of years back. If I was slightly less lazy I would try and find the post.

 

[Foxfish]

Although nobody has ever conclusively proven it, there is certainly widely held anecdotal belief that a higher ppN2 retards the onset of oxygen toxicity, and that accordingly you are less likely to tox breathing air at a certain ppO2 than you would be at the same ppO2 breathing an oxygen enriched mix. Incidents of divers toxing on air at high ppO2s seem to be virtually nonexistent. But like so much relating to oxygen toxicity though, we cannot know for sure.

The reknown technical diver John Chatterton posted quite eloquently on that point on Scubaboard a couple of years back. If I was slightly less lazy I would try and find the post.

I've provided several examples of divers dying from oxygen toxicity at depth. I'll accept that in some cases it wasn't explicitly confirmed. Did you watch the video of the diver dying at the Blue Hole? It seems pretty clear to me that the person died from a seizure. Are you suggesting otherwise.

 

[Kevrumbo]

Or maybe to put it a slightly different way, the real risk is not so much the signs of oxygen toxicity like having a seizure, it is what happens as a result of the seizure. If you are sitting in a barometric chamber and have a seizure it is unlikely to be life threatening. If you are a commercial diver wearing a diving helmet with a communications link to topside and you have a seizure, while there are some risks, they are considerably less than if you are a recreational scuba diver with only a normal mask. If you have a seizure while scuba diving, you will almost inevitable lose your regulator and drown. Even with someone nearby, the examples above demonstrate that getting the regulator back into a persons mouth while they are fitting is not always possible.
Diving conservatively means avoiding situations that can lead to injury or death. It means choosing a PPO2 and MOD with that in mind.
I've read a few articles and watched a few videos on diving the blue hole in Dahab. Apparently it is a beautiful dive. The fly in the ointment is the growing number of gravestones near the shore of people who have died on the dive. There are a lot of weird and wonderful theories about why these divers have died. Ironically for some this adds to the mystique and attraction of the location. Apparently the water is very clear and the current minimal. Experienced divers in commenting on the deaths point out the explanation is relatively simple. A common scenario resulting in death would be a diver descending to the arch at 50 or 60 m, near the MOD for air, being overcome by nitrogen narcosis and then descending further to a point where oxygen toxicity results in a seizure.
One of the well know diving deaths videoed on Youtube happened at Dahab <http://www.youtube.com/watch?v=eejQPUyeNiY>. In this case, the most sensible explanation was that the diver intentionally raced down to 92 m alone, took a video of his dive computer to show off how deep he died, had a seizure and died. You can hear his regulator free flowing at the end. Incidentally the partial pressure of air at 92 m (301 m) is 2.2 b.
To me the idea of descending to depths well beyond the recommended limits and endangering your life to earn 'bragging rights' is simply stupid. The gravestones at Dahab and the sad video stand are very tangible reminders of the risks involved.

The Blue Hole is notorious for the number of diving fatalities which have occurred there earning it the sobriquet, "World's Most Dangerous Dive Site" and the nickname "Diver's Cemetery". The site is signposted by a sign that says "Blue hole: Easy entry". Accidents are frequently caused when divers attempt to find the tunnel through the reef (known as "The Arch") connecting the Blue Hole and open water at about 52 m depth. This is beyond the PADI maximum recreational diving limit of 40m and the effect of nitrogen narcosis will be significant at this depth. Divers who miss the tunnel sometimes continue descending hoping to find the tunnel farther down and become increasingly narced.
The "Arch" is reportedly extremely deceptive in several ways:

It is difficult to detect because of the odd angle between the arch, open water, and the hole itself.
Because of the dim lighting and the fact that most light enters from outside, it appears shorter than it really is. Divers report that the Arch appears less than 10 m long but measurements have shown it is 26 m from one end to the other.
There is frequently a current flowing inward through the arch towards the Blue Hole, increasing the time it takes to swim through.
The arch continues downward to the seabed which is beyond view and there is therefore no "reference" from below.

Divers who resist the temptation of the Arch and remain within their training and limitations are in no more danger than on any other Red Sea dive site. However, the Arch has proved irresistible for many and thus the divesite is considered unsuitable for beginners and a potential trap for even experienced divers.

http://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea))

I think the more likely cause rather than high PO2 exposure & Ox-tox seizure, is the extreme narcosis, loss of buoyancy and tragically running out of air by novice recreational divers mistakenly trying to attempt this dive on a single 11L (AL80) tank. . .

 

[Rhone Man]

I've provided several examples of divers dying from oxygen toxicity at depth. I'll accept that in some cases it wasn't explicitly confirmed. Did you watch the video of the diver dying at the Blue Hole? It seems pretty clear to me that the person died from a seizure. Are you suggesting otherwise.

You may have misread my post. I said that incidents of divers toxing on air are virtually non-existent. There are plenty of divers who have expired at depth on air, although most of the cases I am familiar with it was believed to CO2 buildup caused by breathing resistance rather than O2 toxicity. The Steve Feldman case was a classic example of this. If I was to take an educated guess, I would assume the diving death you refer to at Dahab was much more likely to be caused by chronic and massive narcosis rather than toxing in such a short space of time. But of course I cannot know for certain.

Anyhow, I hope you have obtained enough salient information in this thread to make your own informed decisions on relative toxicity risk. Good luck and safe diving.