Pony bottle

Pony bottle

A pony bottle (left) attached to a larger cylinder
Uses	Scuba diving cylinder for supply of emergency breathing gas
Related items	Bailout bottle

A pony bottle is a small diving cylinder which is fitted with an independent regulator, and carried by a scuba diver as an extension to the scuba set. In an emergency, such as depletion of the diver's main air supply, it can be used as an alternate air source or bailout bottle to allow a normal ascent in place of a controlled emergency swimming ascent. The key attribute of a pony bottle is that it provides a totally independent and redundant source of breathing gas for the diver. The name is due to the smaller size, often of only a few litres capacity.

Pony bottles are used by divers who understand that no matter their preparation and planning, accidents may happen, and cannot, or do not choose to depend on another diver for emergency breathing gas. They are carried by the diver in one of several alternative configurations, and the capacity and contents should be sufficient to allow a safe ascent from any point in the planned dive profile.

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 pony bottle is usually smaller than the primary cylinder, but it should provide enough breathing gas to make a totally controlled return to the surface, including any required decompression stop or safety stop along the way. The pony cylinder capacity will depend on the profile for safe ascent to the surface required for a particular dive plan. Popular sizes for a pony bottle include 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 basic requirement for low risk solo diving if the dive is too deep for a safe free ascent, as there in no source of emergency air from a buddy. In scientific diving operations, pony bottles can be standard equipment in tethered scuba diving operations where the diver is often solo but connected to the surface by communications equipment, and an emergency gas supply is mandated. Several scuba manufacturers produce a minimalist backpack harness that supports a back mounted pony sized bottle exclusively for use in shallow water diving or for boat maintenance purposes.