The "500 psi belongs to your buddy" is a bit of shorthand. in fact, you should not be in a position that you and your buddy need to air share from depth with only 501 psi in the tank. Under normal circumstances, you need to begin your ascent with sufficient pressure to ARRIVE at the surface with 500 psi left. That way, if you need to air share, you and your buddy dip into the 500 psi, but do not exhaust it. As you and other have noted, there are lots of good reasons to keep a reserve in the tank. SPG accuracy, regulator performance, water intrusion are good examples.
For normal recreational dives, with normal size tanks, normal breathing rates, etc the "500 psi" rule seems to work (IMHO). Every certified diver is responsible for their own dive planning, and circumstances may dictate that a larger reserve may be indicated. Exceptional depths, higher than normal air usage, potential for getting lost or any of a hundred factors may cause a buddy team to select a higher turn pressure and reserve. I don't think anybody (certainly not me) is going to criticize a diver or team that elects to use a higher minimum reserve.
Thank being said, the training agencies need to pick some sort of guidline for training. 500 psi seems to work in most cases and is easy to remember. As with any "rule" it is up to the instructor to explain the thinking behind the rule so that the divers can make their own informed choices.
For normal recreational dives, with normal size tanks, normal breathing rates, etc the "500 psi" rule seems to work (IMHO). Every certified diver is responsible for their own dive planning, and circumstances may dictate that a larger reserve may be indicated. Exceptional depths, higher than normal air usage, potential for getting lost or any of a hundred factors may cause a buddy team to select a higher turn pressure and reserve. I don't think anybody (certainly not me) is going to criticize a diver or team that elects to use a higher minimum reserve.
Thank being said, the training agencies need to pick some sort of guidline for training. 500 psi seems to work in most cases and is easy to remember. As with any "rule" it is up to the instructor to explain the thinking behind the rule so that the divers can make their own informed choices.
