Having said all that about doubles, there are still times when it makes more sense to dive "lite". Out of the water, double tanks are very heavy and cumbersome, which makes them practical only for boat entries, where you simply have to stagger across the deck and fall overboard. At other times, and in less deep situations, you can maintain a safe redundant air supply using a much lighter "pony bottle" - a small tank of 20-40 cubic feet, weighing only about 10-20 lbs, with its own regulator that is used only in an emergency. Some of the situations where this configuration is useful to include medium-depth dives, 40-80 ft, training and practice dives ( especially in the quarry, where it is possible to reach quite deep water from the shore ), and excursions to the tropics.
Larger pony bottles ( 40-50 cf ) are also useful as "swing bottles" for carrying special decompression mixes, while the smaller ones are small enough to actually take with you when you travel. Most airlines will allow it in checked baggage if you remove the valve. This is very useful since many tropical destinations are not equipped to provide the kind of safety equipment that we take for granted here. The smallest pony bottles are really not big enough to get you out of trouble on a deep Jersey dive, however.
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