sst:
I always lose a few bar when I laugh underwater!
My hubby and I when we were first learning how to maximise our air consumption were always checking.
At one stage he was 20 bar higher than me, so I gave him the age old \/ signal which caused him to burst out laughing. By the time he had stopped laughing and got his breathing back under control we were equal.
(Oh, and before you think we were reckless, this was at 4m on the way up from and after our safety stop, with just over half a tank left each).