I was doing a training dive where we had a person acting as a dive safety officer. His briefing included telling everyone to arrive back on the surface with at least 500psi in their tank. I asked him: For people who are diving bigger tanks, do they still need to be back with 500 psi? For example, another diver had double 120s. If he got back to the surface with 250psi, he would still have more air in reserve than the guy with an AL80 at 500psi. The DSO said "no, everyone needs to be back on top with 500."
500psi in an AL80 seems to be pretty standard as the "get back with" figure. If you do a safety stop at 10m, that means 2 atm (i.e. just under 30psi) of air inside the tank will have you OOA. In other words, at 10m, 500psi left means ~470psi usable, which means, actually, 12 cu-ft of usable air at the end of your final stop.
To get to the end of a safety stop at 10m with 12 cu-ft of usable air in an HP120 would mean having (if I've done the arithmetic correctly) 380 psi (again where ~30psi of that is "unusable at 10m depth", right?).
And double HP120s would mean 205 psi to have 12 cu-ft left.
Is it pretty universal and standard to expect everyone to "get back on the boat with 500psi" even if they are diving bigger tanks or doubles? Or would it actually be considered correct to plan for a specific volume of usable gas on exit, rather than a specific psi that is used without regard to the capacity of the tank being used?
500psi in an AL80 seems to be pretty standard as the "get back with" figure. If you do a safety stop at 10m, that means 2 atm (i.e. just under 30psi) of air inside the tank will have you OOA. In other words, at 10m, 500psi left means ~470psi usable, which means, actually, 12 cu-ft of usable air at the end of your final stop.
To get to the end of a safety stop at 10m with 12 cu-ft of usable air in an HP120 would mean having (if I've done the arithmetic correctly) 380 psi (again where ~30psi of that is "unusable at 10m depth", right?).
And double HP120s would mean 205 psi to have 12 cu-ft left.
Is it pretty universal and standard to expect everyone to "get back on the boat with 500psi" even if they are diving bigger tanks or doubles? Or would it actually be considered correct to plan for a specific volume of usable gas on exit, rather than a specific psi that is used without regard to the capacity of the tank being used?