This has been an interesting thread, and an informative one too. Seems like there have been good arguments for both sides of the issue. For what it's worth, here's what I do and why...
- For recreational diving I back off 1/4 turn. I do this because the valve is effectively open all the way and yet there is no strain on the internal parts. The last thing I do before donning my BC is turn on my valve - I leave it off till this point (well, after initial check). As it is only backed off 1/4 turn, if I feel the DM cranking on my valve as I'm about to giant stride then I know that he's busy shutting it off (which has happened). I have never gone in negative via giant stride and therefore I do checks with my buddy on the surface (as well as during the first 15' of the descent). Generally speaking, if you ever go in negative then it'll be a backroll-style entry where there is no DM messing with your valve. I find this to be true because if you have to go in negative it means that all divers need to get in and down together, for whatever reason. Anyway - I teach my students this 1/4 turn method, as well as to check their SPG while breathing the reg - but with the caveat that unless the valve is barely open this check won't reveal a partially closed valve at the surface. It does, however, help to keep them thinking about the life support system strapped to their back. I also teach them to breath while checking the SPG at depth for the same reason. (Though with the advent of sampling SPGs - i.e. digital ones, there is the potential for not seeing a dip in pressure)
- For tech diving that way I was taught wass that you are responsible for your own valves. No one touches them except for you. With doubles the tank valves are open to the stops so that during a shut-down there is no second guessing which way you have to turn them. I was also taught that the isolator should only be opened two turns to facilitate quick isolations - though no doubt there is debate on this. Ironically, I have had a valve jam in the open position on me while on doubles (it was fully open) at depth. Luckily, it was only during drills and even my buddy couldn't budge it.
- As far as roll-offs are concerned, for twins the valves turn in opposite directions and (for me) the primary reg is installed on the right post - meaning it will roll open, not off, during contact with an overhead object. I don't know this for certain, but I would think that roll-offs are somewhat less likely on singles due to the location of the valve.
John
