Thistlegorm tank turn off!

[Quero]

I teach a quarter turn back, and for practical reasons, not as a knee-jerk nod to tradition, but just to make sure the valve spring isn't overly-compressed in the open position. Where I work and dive it is unusual for divers to supply their own tanks. Thus, our tanks get heavy usage with lots of cranking the valves on and off as they are filled multiple times a day, every day: for a single dive, a valve is opened when first rigged in order to check pressure, closed until it's time to gear up, opened again for the dive, closed again so the tank can be re-filled, opened when the filling whip is attached, closed when the fill is complete. If we crank the valve open and don't turn back a quarter of a turn, occasionally a valve will stick open after the dive and will be difficult to close. Turning a quarter turn saves wear and tear on the valve assembly. Yes, with a perfectly-maintained valve assembly, the handwheel should turn easily and we should be able to leave the valve fully open, but as a way to prolong the life of the valve assembly and to protect against people over-tightening the valve when opening it, I find the quarter turn back technique to be practical.

 

[Marek K]

I don't turn the valve back a full or even quarter turn when I open it, but I do back it off just a touch. Not sure why... I think it's the way I was tought in 1985, and I've just internalized it.

I'm not sure how turning the valve full-on would help. In that position, it would be essentially be jammed or "stuck" a bit, requiring some effort to "pop" it so it turns. Before it does so, it would feel just like it were fully closed.

When it's backed off just a hair, and you (or a well-intending divemaster) turn it counter-clockwise toward On, it will turn easily for just a fraction of a turn before hitting the stop -- thereby immediately telling you it's open.

That's if it's turned counter-clockwise.