Picture a gun with 1000 chambers pointed at your head with a single bullet. Wouldn't you prefer to reduce your chances at death if you could? It seems to me that the decompression benefits of raising a PPO2 to 2.0 or even 2.5 are not worth the risks. Clear some stuff off of your schedule if you can't wait the extra 5 minutes.
I regularly do an O2 flush at 20 feet (a 1.6 PPO2) and fly my unit at a 1.3 or 1.4, but I'm not willing to get more aggressive than that. The fact is that divers have died from drowning due to CNS toxicity. Oxygen tolerance is different for everyone and can change in an individual because of CO2 levels, hydration, exertion, the amount of sleep you had the night before, drugs, etc.
Yes, chambers regularly operate at 2.8 PPO2, but it isn't lethal when people tox in the chamber. Even chambers generally use 5 minute air breaks for every 20 minutes on O2. Anyone here taking air breaks during their dives? As far as the military goes, I think that they are willing to accept higher risk for their divers than I am willing to take, especially when I'm diving a rig that gives me the option of using a lower PPO2 (ie, not an oxygen rebreather). I also don't have to weigh my risks of CNS tox against people shooting at me.
There are quite a few people advocating using very high PPO2s, but I can't see the benefit. Take the following dives for example (I'm sorry but I only have my VR3 with me right now to run tables, but you will get the idea):
- 140 feet for 45 minutes using a 1.3 on the bottom and starting O2 at 40 feet (a 2.19). You will have a TTS of 33 minutes with the following stops:
70ft for 1 min
60ft for 2 min
50ft for 2 min
40ft for 3 min
30ft for 5 min
20ft for 7 min
10ft for 11 min
- 140 feet for 45 minutes using a 1.3 for the entire dive. You will have a TTS of 39 minutes with the following stops:
70ft for 1 min
60ft for 2 min
50ft for 3 min
40ft for 5 min
30ft for 6 min
20ft for 9 min
10ft for 11 min
It hardly seems worth the risk to save 6 minutes deco. You would be better to shorten your bottom time by a minute or two.
Now do the same dive using a 1.6 for the entire dive (except at your 10 foot stop). You will have a TTS of 27 minutes with the following stops:
60ft for 1 min
50ft for 1 min
40ft for 2 min
30ft for 4 min
20ft for 7 min
10ft for 9 min
Congratulations, you saved yourself 12 minutes. Now you can get back to work sooner.
I realize that longer and deeper dives create more of a difference, however, they also create more of a risk of a CNS hit. Even on a 3 hour trimix dive, you would only reduce your decompression by about 20% if you were to run a 1.6 instead of a 1.3. Again, shorten your bottom time if you don't want to do the deco.
I guess if I were in a real hurry to shorten deco times, I'd bump up my PPO2, but I'd use a full face mask and I'd try to find a really good dive buddy.
Aloha,
Charlie