Number 1 was a 22 minute shore dive to a maximum depth of 11m during which I blasted through 160 BAR of air. Why so bad? It was a group outing where a number of tanks were brought for the entire group - you grabbed one (the vans were parked about 20-30m from the shore up a slight slope), dived it and returned it to the used pile, grabbed another for the next dive etc. My problem was that after my first dive that day I grabbed another tank without checking it (it had an adaptor to allow yoke regs) so I grabbed a hex and tried to remove it to find that it was bound tight. Took the tank back, grabbed another and connected my regs, turned it on and heard air escape. Tried redoing it, getting someone else to do it and an instructor but eventually found that someone had bashed the valve out of shape so it would never seal. Took that tank back and got another one that did actually work. Only issue was by this point I was stressed out and tired from lugging so many tanks about so it took me about 10 minutes in the water to relax by which time I had blasted through most of my available gas.