zielit - 9/17/2013 8:27 AM
From ks54: The pony bottle is not used for breathing. (AS GAS PLANNING GOES) It is a replaement for a failed primary gas supply.

Amen to that. So many divers don’t get the difference between stage tank and pony bottle.

There is also another alternative or maybe addition (very cheap and handy) - SpareAir bottle. Main draw back it’s only 1 c.f. but since I bought it I never dove without it (wreck, no wreck). Very good for your "instant buddys" when diving on vacations so often found so far away, or for the photographers too busy to check the air while waiting for this one great shot or just simply for added redundancy and will at least get you to either you buddy or in most cases to the surface.