Re: Why run out of air?
I used to be able to last a 'normal' dive on a 12l cylinder. Then all of a sudden I started guzzling air. and even on a 15l tank couldn't last long enough to last a normal 'holiday' dive and often had to resurface on my own, I didnt realise that the paraflu I'd had had eaten away at my lungs to the stage they were barely working. I had kids around then so had to stop diving and it wasn't until another complaint led to some x-rays that revealed the damage the bug had done.
I was diving assuming I'd use air at around the rate I always had and was surprised to find sometimes after only 15min of a 40m dive I'd be down to 75bar and assumed I'd been hyperventilating or something as normal life didnt seem impinged. Fortunately I'd not gone deep enough to need decompression which could have led to a few problems