DevonDiver
N/A
You can plan a dive without something as convoluted as Rock Bottom. It's so dirt simple. Take out the 500psi your supposed to come back with add a little for you safety stop (say 300psi)
So, at what pressure do I need to leave the bottom to ensure that I arrive at my safety stop with 300psi and a 500psi reserve?
PADI students are taught to plan dives by accounting for time and depth. Consideration of air consumption is woefully neglected.
I'd estimate that most of the recreational divers I encounter will do nothing more than 'guesstimate' their gas use on a dive - that guestimation becomes better with experience. Novice divers don't have much of a clue. More experienced divers will have a clue, but can be rapidly thrown into error if anything unforeseen occurs (causing accelerated respiration) - again, because they have no conceptual understanding of gas consumption, they often fail to appreciate how cause-effect are linked with regards their air time.
When asked, many divers won't even have considered their gas usage in respect of dive planning. Their gas state is a purely reactive issue - monitoring their gauges and if they notice they are getting low, they'll make the decision to ascend. Because no strict limit is pre-planned, that means it's much easier for them to deliberately or negligently over-stay beyond a safe minimum.
All of which increase the likelihood of low/out-of-air emergencies.
and then divide [the remainder] by 2; to get your turn around..
Did I edit that right?
Maintain a reserve of 500psi.
Allot 300psi for the safety stop.
Divide the remaining (consumable) gas by half to establish a mid-point in the bottom-time?
3000 - 500 - 300 (= 2200psi) / 2 = 1100 psi until the midpoint.
Turn point is at (1100 + 500 + 300) 1900 psi (on a 3000psi tank).
Although those calculations still neglect any air consumption that occurs between leaving the bottom and reaching the safety stop.