John,Two years ago I did a scuba refresher in the pool with a couple who had not been diving for a couple years. When we suited up, I had them do a pre-dive safety check, of course. I told them that when they checked their air, they should look at the gauge while taking 3-4 breaths, and if the needle did not move, it meant their air was turned on. They did that, and the wife confirmed he understanding that if the needle was not moving, it was OK. I worded my response carefully--"It means your air is turned on."
We got into the pool and worked on skills for a while before she suddenly bolted toward the surface in panic. On the surface, I tried to inflate her BCD and discovered her tank was empty. "How much air was in your tank when you checked it," I asked.
"It was down in the red," she said, "but the needle wasn't moving so I figured it was OK."
Last week in my OWSI class, I went to demonstrate this in the pool (on myself of course, showing my students). In order to make the needle move, I had to have someone almost close my valve completely. I want to do the same in open water at depth, but not with my class.