DaleC
Contributor
Not touching your buddy's isolator valve unless specifically asked to do so is extremely important, as DevonDiver said. A doubles diver should be proficient enough with performing valve drills, so that even if some loony on the boat or whatever turns it one way (thinking he/she's being helpful...it DOES happen), the doubles diver can easily deal with it underwater, usually while simultaneously questioning the parentage and personal hygiene habits of whoever messed with their gear through their regulator. You don't want to be the cause of all those expletives, do you?!
Hey, I resemble that remark!
The first time I ever dove with a doubles diver he asked me to turn his air on for him at the rail. I mistakenly turned the left post off (it was already on). At 100' or so he discovered this mistake when he went OOA on the right cylinder (hey, his SPG still said he was full). He immediately knew what was wrong and opened the valve. Complacency and inexperience....
Lessons learned:
1. Familiarize yourself with the correct way to turn all valves on and off from various orientations.
2. As Lynn said, even if they think everything is correct, the doubles diver must verify that they have proper flow through all valves. It may take an extra second or two but it is a short cut not worth making.