There are two schools of thought on this issue. One says you should turn the tank valve back approximately ¼ turn once it’s opened all the way, the other says turn it all the way till open and leave it there. The first line of thinking, turn it back a 1/4 turn, will tell you that it's to protect the valve from getting stuck. The other line of thinking will tell you to turn it all the way open and leave it open to reduce the risk of turning the valve the wrong way so that its off instead of on. So who’s right and does it matter? Turns out that there have been no reported mechanical malfunctions with a tank valve that has been turned all the way on without turning back a ¼ turn; further more, there have been no reported incidents due to tank valve failure for any reason. As a Tech Diver in training, I have been taught to leave the tank valve all the way open to reduce the possibility of turning the valve off, mistakenly thinking to turn it on during a very deep and long dive. Life and death can lie in the balance if that tank valve is off vs. on. I’ve also asked various engineers for their opinion on this issue and most have admitted that leaving the valve open is safe. It’s highly unlikely, and it would be extremely unusual, that you would have an incident from a faulty tank valve.
So all the way on or a ¼ turn back, you decide. But from the evidence I've gathered, it seems that it really doesn’t matter which way you go. I’ll stick to my tech training thus far and leave mine open all the way. I’ll continue to teach my students and instructor candidates to do the same.
