ams511
Contributor
A second stage will generally fail open, so another one attached to the same tank is of little use.
This is incorrect. Each second stage is independent of the other so if one fails the other will not be affected. In the event of a first stage IP creep there is a higher probability of both regulators failing but one could still be usable because of differences in tuning and type. In the event of a total first stage failure (high pressure air or turret detaches) then both second stages will be affected.
---------- Post added June 2nd, 2015 at 05:57 PM ----------
As long as there is a second, second stage, then that's redundancy.
I wasn't thinking of second stage failure, as in freeflow, as much as losing a mouthpiece, or having an exhaust diaphragm get folded or caught so the reg would breathe extremely wet. I've also been in the water with someone whose second stage came loose from the hose.
The question is what is to be done in case of a regulator failure? I would assume PADI and SSI would advocate calling the dive and switching to the pony rather than trying to fix the malfunctioning second stage under the water. In that case an octopus on the primary first stage would not add anything over the pony. So the octopus would be a redundant failure point.