pickens_46929:
This is what I was taught in my recent TDI Adv. Nitrox / Deco procedures class.
Still provides wiggle room if there is some trouble... Imaging planning for a max. PO2 of 1.6 during the working portion of the dive and something happens....
You make a good point and it is very good practice. (And I agree in large part because I am also TDI trained and like their approach)
But on the other hand the US Navy did research with extreme PPO2 exposures and developed time limits to cover those exposures.
For example the vast majority of people will not have an O2 hit even with a 2.0 PPO2 for a minute or two. So dropping 10 feet below your MOD to retrieved your dropped dive light is never going to be recommended or sanctioned by a training agency, instructor, etc, but it is also
probably not going to result in an oxtox incident for most divers.
The lower PPO2 limits are set to establish larger safety margins, but it is catastrophising the issue to say that dropping a few feet below the depth at which a 1.6 PPO2 occurs for a few seconds or even a minute or two is going to get you killed. The stress and panic that ensues when an over indoctrinated diver finds himself or herself a few feet below the MOD and thinks he or she is about to die is far more dangerous.
Lets say for example that I am doing a dive to 150 feet. I lose all the back gas in my redundant doubles (very unlikely), my buddy dissapears and I only have my deco bottle of 50% (with an MOD of 70 ft). In this case doing a free ascent to 100-110 feet and then switching to the 50% for the miniute or so that it takes to continue to ascend to my first deco stop at 70' is most likely not going to kill me despite the 2.18 peak PPO2 due to the very brief time I am in excess of a 1.6 PPO2. It's not written in stone that it won't kill me but it's probably a better option than drowning if I need to breathe something.
Other examples of this are the now (mostly archaic) deep air records where divers went to 300-400 plus feet on air for short periods of time (2.76 PPO2 @ 400'). The majority of these record setters and attempters survived. And Hal Watts, one of the survivors, still offers deep air training and certification to 240 ft (a 1.74 PPO2 on air).
In short, it's never a good idea to exceed a PPO2 of 1.6 but it is also probably not going to result in an oxtox if you, for example,
briefly drop 10' or 20' below the MOD to assist your in distress dive buddy and get him or her back up to the MOD. You have to do a quick cost benefit analysis, but it helps to really understand the magnitude of the risks rather than to only know the admonition against ever doing it that is promoted by training agencies.
With regard to long term O2 exposure, you have to dive your buns off to exceed the NOAA CNS clock limits even at a 1.6 PPO2 and it's all but impossible to do it at a PPO2 of 1.4. And this is using a very conservative 24 hour clock method that does not give you credit for reduction that occurs during surface intervals.
