Breathing Gas
As shown in the example calculations above, the capacity of standard pony bottles make them capable as acting as redundant bail-out devices for conventional recreational diving purposes – i.e. non decompression dives at depths not to exceed 30 metres (98 ft) (for nitrox) or 35 metres (115 ft) (for air). A general rule of gas usage in this range is that the "bailout gas should match existing breathing gas". In these terms, this means that air based diving should have air in the pony, nitrox diving should be backed up by nitrox in the pony. This leads to the conditions that with respect to dive tables or dive computer, that the switch made between cylinders does not influence calculations for present or future decompression allowances. To maximise safety margins, it is quite important that pony bottles should be filled to their maximum allowable cylinder pressure to provide a maximum reserve for bailout purposes. Often in boats gas refills to these higher limits are not possible, so it is important that pony be filled prior to the dive trip excursion. Pony bottles are available with either A-clamp or DIN fitting valves so the appropriate fitting must be available should the pony need to be refilled.
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