Turning the tank valve back half-a-turn often confuses divers and can inadvertently make breathing difficult at depth. This diver kept a cool head and ascended on her buddy's alternate air source.
While diving with my dive buddy in Florida, I noticed that upon each inhalation the needle of my submersible pressure gauge (SPG) fluctuated. The needle dipped down with each breath before returning to the correct psi reading for my tank. I continued diving while keeping a close eye on the gauge and, upon reaching a depth of approximately 55 feet, (16 meters) it suddenly became very difficult for me to breathe. I looked at my SPG mid-breath and saw the needle drop down to zero psi, and it did not readily move back up. I felt like there was no more air available to me even though I knew there was at least 1200 psi (80 bar) in my tank. I signaled "out of air" to my buddy, and used her alternate regulator. We made a controlled ascent to the surface, and I was not injured. Upon inspecting my gear, I realized that instead of turning my tank all the way on and half a turn back, I had turned it all the way off and half a turn on. Upon reaching a depth below 33 feet (10 meters), I had experienced inadequate air pressure delivery from my tank to my regulator because the tank was barely on and could not continue to deliver the same volume of air at the increased pressure.
It must have been disconcerting to suddenly be unable to breath at 55 feet (16 meters) depth. This diver kept a cool head, signaled "out-of-air" to her buddy, secured the alternate air source and made a controlled ascent. This is exactly what is taught in entry-level diver courses worldwide.
The days when we needed to turn our valve back half-a-turn are long gone, but still some instructors teach divers to do this. Why? The safest way to ensure you have adequate gas for any dive is to open your tank valve all the way, then look at your SPG while taking a couple of breaths.
If your needle does not move then your valve must be open and if your needle goes down towards ‘Zero' then your valve must be closed. If you are in the habit of turning it back half-a-turn then, like this diver discovered, you might have enough gas to breath normally at the surface but you could find yourself short of breath at depth. Remember, the safest way is to turn your tank valve all the way open or all the way closed.