Diver0001:In fact, the pressure was so low that I could open the tank valve and blow air back into the tank with my mouth.
As, not once, you remove the valve, if the tank was in a vacuum as you apparently indicate, water would fill the tank nearly completely before you ever got your mouth on the neck. I don't want to call you a liar, I'm sure you performed some sort of experiment, but somethings fishy, and it might just be the way I interpreted your words, so please forgive the intrusion. But based on this one quoted sentence, I'd like to ask some additional questions Diver0001 about your test:
1. At what depth and type of water was the experiement performed.
2. What brand, model, and year of 1st stage
3. What brand, model, and year of 2nd stage.
4. What brand and model of cylinder (click here for a list) was used.