Quote: (Originally Posted by nigelh)
We are not seeing divers dropping like flies on 1.3 bar ppO2s. There may be a case for reducing it on longer dives or in repetative dive situations but as a general rule it has a good track record with no reported problems.
Nigel, your posts are generally very good, but on this one I think you may have overlooked something.
Divers do not have to die like flies for there to be a problem. Divers are dying from O2 convulsions on rebreathers, with a surprisingly high figures given the number of RBs in circulation. Just look over the accident reports. I must confess to agree with Cedric on this one.
With DCS, divers take a risk and when the 1 in 10,000/hour risk bites them, they get treatment.
With CNS, divers take a risk, and when the 1 in 100,000/hour risk bites them, they are dead.
It is a simple matter: look at the CNS probability curve, decide how many sigmas you want to keep safe, and dive that.
To save 10 or 20 minutes of deco, I for one reduce my max PPO2 to 1.1, though saying that, I use it throughout the dive.
What some do is say "Well, for medical treatment of the bends we whack in a PPO2 of 2.3 and they don't convulse, so it must be OK to go with 1.4", or even 1.6 I have seen a couple of times.
Again, just look at the accident figures. Divers are dying from O2 convulsions on rebreathers. Very experienced divers.
As regards agencies, I think they are wrong on this one and should be recommmending 1.1, or even 1.0.
Alex