Here I am again, now responding to my own thread. I just finished a week of training with Jeff Bozanic. And, one thing I learned was that I will ALWAYS carry a bail-out bottle. For a dive to 90-100 ft, the size of the bail-out MUST BE at least 19 cu.ft., to allow for some time to get oriented (if an emergency occurs), a normal ascent, and a 3 minute safety stop. So, now I change my assumptions, but the question still stands >>> Assuming I can fully use-up a pair of 19 cu.ft. cylinders, how many dive-hours can I roughly expect to get from a single pair? I must add these conditions also: no heavy work-load and average physical condition. <<< Ok, my current best answer is this: The latest rule-of-thumb I read is about 500 psi of each (O2 and dill) for an average 1 1/2 hour dive above 100 ft (keep in mind that O2 consumption has nothing to do with depth). But you must have plenty of left-over dil, because you never know when you might unexpectedly need more than usual. So I am thinking that I can dive the O2 right down to 200-300 psi, but should never let the dil get below 500 psi. What does everyone think of these as just rough ideas?