archiebald
Contributor
There is a big difference between closing a valve on the surface (or shallow water) and running out of air at depth. Standard instruction requires us to turn diver's air off in the pool so they can get the sense of what it is like, but it isn't even close.
I can understand what you are saying but, what about "almost" closing the valve about 99% so only a trickle of air can come through. I realize that pressure will re-build in the hose between breaths but this ought to give a vastly better "feel" than closing it completely.
As for the other suggestions in this thread, the best way I have read is to carry an entirely separate rig such as a pony bottle for the test to get the most realistic simulation.