Moerby, I tried the experiment you're talking about, but not with a CESA. I got my husband and me as far as apart as he seemed to like to dive, and then I exhaled, pretended I had no gas, and swam to him. It was harder than I thought, because of course, I was negative as well as distant, so I had to swim UP as well as across the 25 or 30 feet of space. I got to him, tapped him on the shoulder (he was, of course, head and camera in a hole), got him to turn around, signaled OOG, and got a reg from him. By the time he gave it to me, I was very glad to have it, but I hadn't reached the point of considering breathing water yet.
Going to the surface is easier, of course, because buoyancy gives you a boost, so you don't have to swim as hard as I was doing. You may not feel much urge to exhale, and again, so long as you keep your glottis open, it is not really necessary to vent a lot of gas. I remember from my CESA in my OW class that I was quite surprised that the air kept coming and coming as I went up, and how easy the drill was to do, compared with doing it across the pool. I suspect you'd be surprised that you could find some gas to let out, even if you thought your lungs were empty (which they never actually ARE, when you have finished exhaling). And as boulderjohn observes, there is nothing wrong with sucking on your regulator a bit -- you may be surprised and get quite a bit of a breath.
But, although I know I sound like a broken record here, this whole scenario of being at 100 feet and out of gas and all alone is one you only reach after so many failures that you should never, ever be there. If you plan your gas, monitor your gas, and stay with your buddy, you simply aren't going to face the "out of gas and alone" issue. I just keep feeling as though you are spending a lot of time and energy and thought on the last link in a nasty chain that needs to be broken much closer to the beginning. Please just acknowledge that you have read the references I gave you, and I'll stop bothering you about it.
Going to the surface is easier, of course, because buoyancy gives you a boost, so you don't have to swim as hard as I was doing. You may not feel much urge to exhale, and again, so long as you keep your glottis open, it is not really necessary to vent a lot of gas. I remember from my CESA in my OW class that I was quite surprised that the air kept coming and coming as I went up, and how easy the drill was to do, compared with doing it across the pool. I suspect you'd be surprised that you could find some gas to let out, even if you thought your lungs were empty (which they never actually ARE, when you have finished exhaling). And as boulderjohn observes, there is nothing wrong with sucking on your regulator a bit -- you may be surprised and get quite a bit of a breath.
But, although I know I sound like a broken record here, this whole scenario of being at 100 feet and out of gas and all alone is one you only reach after so many failures that you should never, ever be there. If you plan your gas, monitor your gas, and stay with your buddy, you simply aren't going to face the "out of gas and alone" issue. I just keep feeling as though you are spending a lot of time and energy and thought on the last link in a nasty chain that needs to be broken much closer to the beginning. Please just acknowledge that you have read the references I gave you, and I'll stop bothering you about it.