Limits: what they are, why they are important and how to establish them

[Ojai Diver]

Here's a dive to 80ft with an AL80 @3,000psi

80x10= 800psi ascent pressure.
(3000-800)/2= 1100psi usable gas
3000-1100= 1900psi is now your turn pressure

I am not trying to be argumentative here, but I think I might be missing something.

Assuming that the 800 psi is adequate to get both oneself and one's air deprived buddy safely to the surface--a worthy goal--why is the entire remaining usable gas divided by 2? This seems to assume that in the event of an out of air emergency, one will continue the dive while sharing air.

If either oneself or one's buddy runs out of air during the dive, is not the dive immediately over, leaving the already budgeted 800 psi to get you both to the surface?

 

[The Chairman]

Turn pressure is the point you stop moving away from the boat/shore and turn back. In an OOA, situation, its probably best to cover as much distance back at depth as you can, since currents at the surface may take you away from your destination and can be stronger than at depth. But don't slice that pie so thin that it falls apart.