Kendall Raine
Contributor
This raises a philosophical question: when do gas management rules of thumb (thirds, half plus 200, off the bottom at 700, back by 500, etc.) substitute for understanding the underlying concepts (e.g. rock bottom) behind them? Leaving the bottom with 700 psi is a not a bad rule per se, but under what circumstances does it fail (either because it's too liberal or too conservative)? How is it derived? In any event it does not directly address turn pressure (factoring for example OOG/current/stress event), and so requires many simplifying/limiting assumptions. Divers certified without the knowledge/ability to truly plan a dive (including gas under various contingencies-@rjack321's point in post #8) are cheated of this most fundamental skill-the admonition to "plan the dive and dive the plan" then becomes a joke if students aren't taught skills sufficient to actually plan. Once gas planning is mastered rules of thumb-easy to remember under stress-can be derived for the specific dive. I think that's what @wetb4igetinthewater was getting at.How do you distill all that into some very simple, easy-to remember-under-stress rules? The beauty of "be back on the boat with 500 psi" is how simple it is; but it does not tell you how to do that. "Leave the bottom at 700 psi" tells you how to do that, but does not allow for a buddy.
What are some simple rules you can distill from your planning document?