Bob DBF
Contributor
Your question was answered. In the early days, we were taught to open the valve fully, then back it off a hair (but not a quarter turn). This is what I was taught when I started diving in 1976, and was told we did this to prevent the valve from getting stuck in the open position... FYI, 1/4 turn is quite a lot, actually way too much. Tank valve do not require very many turns to open or close fully... if you feel the need to back them off, just a tiny "smidge" is enough.
I was taught to back it off a "smidge" as well, to insure it is not backseated. It was even more important when SPG's came into use, because you can see a deflection with an air check if it is only open a smidge but you will not see a deflection if opened a 1/4 turn.
As you mentioned, if anyone is mechanically challenged, all bets are off anyway.
Bob