Configuration
In a pony bottle system the back-up regulator is a complete diving regulator (first and second stages, and often a submersible pressure gauge) on a separate cylinder which is not intended for use as primary breathing gas during the dive. It provides a totally redundant emergency air supply. The size of the pony bottle cylinder is usually smaller than that of the primary scuba cylinder. It should however provide enough breathing gas to make a totally controlled return to the surface, including any required decompression stop or safety stop required along the way. The required pony cylinder capacity will depend on the profile for safe ascent to the surface required for a particular dive plan. A pony bottle used for sport diving may be 6, 13 or 19 cu ft in the USA, while 2 litre and 3 litre are common sizes in Europe. For deep or deep technical diving or wreck diving 30 and 40 cu ft (5 litre and 7 litre) cylinders are often used. The pony bottle is a minimum requirement for someone doing solo diving, who has no alternative source of air in the form of a buddy's available tank and octopus regulator if the dive is to a depth where the diver is not able to do a safe free ascent. In scientific diving operations, pony bottles can a standard part of tethered scuba diving operations where the diver is often solo but connected to the surface by communications equipment. Several scuba manufacturers also have produced a minimalist backpack harness that specifically supports a back mounted pony bottle exclusively for use by a qualified scuba diver in shallow water diving or for boat maintenance purposes.
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