Pony bottles are just another piece of expensive gear the industry is trying to tell you that you need. make the last 1000PSI of your primary tank part of your "gas plan", and you'll be fine. Do you carry a spare gas cannister in your car? I used to, when I was a teenager, because I thought everyone did. I never ever used it, because I always got gas in time before I ran out. Same with diving. Monitor your air consumption, know what you are doing, and don't rely on some gadget to save your butt. I have no use, nor do I understand the purpose of a pony bottle. It's just meant to make you look more important. I went diving yesterday at one of our local "holes" - the amount of expensive gear present could pay the national dept of a small country! Ease up, people! Use your naugin instead of your wallet. I get a kick out of playing stupid and asking some of these heavy-hung divers "What's that for?" I usually get a startled look and a "Well, it's for in case I run out of air, of course!" My "Why would you run out of air? Don't you have gages?" gets the defense mechanism going, and before you know it, we're knee deep in an argument over how to dive safely. I used to teach boating courses, and one of the rules we taught was the rule of thirds: 1/3 of the fuel is for the way out, 1/3 for the way back, and 1/3 for what you didn't expect. I apply the same to diving. I always have 1000psi by the time I get to a shallow, safe environment. Then I just hang out and do either my deco obligation, safety stop, or just surface with still some air to spare. Who says you have to suck your tank dry? Just because you have a pony bottle hanging off your waist doesn't mean you can plan your dive to the absolute limit of your air capacity. If that's your philosophy, then invest in twins, or high pressure steel tanks - or both. Remember the old J-valve? It had an internal pressure valve which would constrict air flow at (I think) 800psi, and then you could flip the lever down and get the extra 800 "reserve". It was supposed to work like a "built in" pony bottle. Problem was, people were likely to rely on that reserve, and incorporated it into their dive plan. This could have disastrous effects when some realized that the'd forgotten to flip the lever up before the dive, and when the air flow restricted, they were actually out of air, not at their reserve stage. I once had a J-valve tank, but I never used it as such.