A pony bottle (left) attached to a larger cylinder
|Uses||Emergency Diving cylinder for supply of breathing gas|
|Related items||Bailout bottle|
A pony bottle is a small independently filled diving cylinder, often of only a few litres capacity, which forms an extended scuba set and which is fitted with its own independent regulator. 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 pony is due to the smaller size; similar terms for beer date to the 19th century (see pony glass).
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 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 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.
Options in Carrying
There are several options for the mounting of a pony bottle. The most common way pony bottle is carried is by fixing it to the side of the primary (back gas) scuba cylinder by straps or clamps, which may include a quick-release system (as in the above picture). The most common alternative is "slinging" it between two D-rings on the diver's scuba harness or buoyancy compensator. Another possibility is to mount the bottle in a small carrying bag, from which the pony bottle may be easily removed. This affords the opportunity of "handing off" the entire system to a buddy diver if that buddy needs to share air. This is a much safer procedure than the buddies having to have to be connected through use of hosed regulators. The addition of a pony bottle to the divers equipment will add an off-balanced weight to the side on which it is mounted. To compensate for this an equal balancing weight is often added to the tank band on the opposite side of the pony, or in an off-side weight pocket. Attention must also be paid to where the pony bottle second stage regulator is placed during the pre-dive buddy check.
The pony bottle is a source of redundancy by providing an alternative source of breathing gas for the diver as an effective backup to his/her primary gas supply in the event of failure of the primary system. The pony bottle is intended for use in "bail out" situations – situations in which the dive must be aborted and safe return to the surface must be facilitated. There are several alternative solutions to providing such a redundant gas supply for bail out purposes which are in common use in diving. These alternatives are listed in the following table along with a critique on how these solutions compare with pony bottle usage as a backup system:
|Alternative air source||Comparative functionality|
|Octopus||The octopus is an additional redundant second stage regulator taken off the existing first stage regulator and existing primary air supply. Because of this, a failure of the first stage regulator or the exhaustion of the gas supply from the primary tank makes this solution inferior to a totally redundant breathing gas system such as a pony bottle.|
|H and Y pillar valves||The primary tank can be fitted with a valve with two separate controllable outlets, both of which can mount its own first stage regulator. One outlet is connected to the primary second stage regulator, the other to the octopus. This eliminates the danger of failure in a single first-stage system. It does not mitigate the risk of loss of breathing gas from the single primary gas supply.|
|Doubles||Using a twin tank system with isolating manifold provides full redundancy in gas supply as well as the ability to isolate any faulty components which might be leaking away breathing gas. Doubles also provide much greater capacity than pony bottles. However, because the gas in the system is also for use during the dive itself it requires that the diver pays extra attention to gas management to ensure that an adequate amount of reserve is available for any bailout requirement at any phase of the dive. However, if the second tank only serves the purpose of bailout, the diver is burdened with considerable extra bulk and weight which is not required in a smaller pony bottle system.|
|Sidemount||Sidemount systems involve two tanks positioned alongside the diver separately, and has a similar functionality to doubles; however, no isolating manifold is used which reduces the gas available if a regulator fails. Sidemount is a preferable option for some advanced cave divers as it can penetrate smaller cave restrictions than doubles by placing the tanks in front of the diver.|
Choice of Pony Bottle Size
Given that the single function of the pony bottle is to provide a source of breathing gas for a controlled and prudent ascent to the surface in an emergency situation, it is most important that the pony bottle size is chosen to be sufficient for that purpose. Even when doing no decompression diving, the total reserves of breathing gas must still be sufficient to supply three phases of the ascent:
- enough gas to allow for a short period at depth to quickly sort out any problems, if necessary, before returning to the surface
- enough gas to make a safe gradual ascent to safety stop depth and
- enough gas to do a complete safety stop.
At the end of this time the pony bottle should still hold enough gas to provide sufficient pressure for smooth flow from the regulator first stage.
|Stage of ascent||Max depth 15 metres (49 ft)||Max depth 20 metres (66 ft)||Max depth 30 metres (98 ft)||Max depth 40 metres (130 ft)|
|Sort out problem: 2 minutes at max depth||150 litres (5.3 cu ft)||180 litres (6.4 cu ft)||240 litres (8.5 cu ft)||300 litres (11 cu ft)|
|Ascent from max depth to 5 metres (16 ft): ascending at 9 metres per minute (30 ft/min)||67 litres (2.4 cu ft)||113 litres (4.0 cu ft)||229 litres (8.1 cu ft)||379 litres (13.4 cu ft)|
|Safety stop: 3 minutes at 5 metres (16 ft)||135 litres (4.8 cu ft)||135 litres (4.8 cu ft)||135 litres (4.8 cu ft)||135 litres (4.8 cu ft)|
|Total||352 litres (12.4 cu ft)||428 litres (15.1 cu ft)||604 litres (21.3 cu ft)||814 litres (28.7 cu ft)|
|Air available at 150 bars (2,200 psi)|
|Pony bottle 3 litre||450 litres (16 cu ft)||450 litres (16 cu ft)||450 litres (16 cu ft)||450 litres (16 cu ft)|
|Pony bottle 6 litre||900 litres (32 cu ft)||900 litres (32 cu ft)||900 litres (32 cu ft)||900 litres (32 cu ft)|
The table above is constructed to show gas consumed in such a scenario: 2 minutes at depth for "sort-out"; a safe rate of ascent to 5 meters; followed by a 3 minute safety stop. Calculations are based on a heavy breathing rate of 30 L/min (1.06 cu ft/min) and an initial tank pressure of 150 bar (2,200 psi). In this particular scenario the 3 litre pony is just sufficient for diving at 20 meters but not 30 meters. A diver selecting a pony bottle would do a similar analysis for his/her own breathing rates, cylinder pressure to be used, and required ascent profile, or take advice in the selection. A submersible pressure gauge is required on the pony bottle regulator so that the pressure can be monitored during use to ensure that the diver surfaces before the gas runs out.
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 in open water. A general rule of gas usage in this range is that the "bailout gas should match existing breathing gas" so that the switch made between cylinders does not influence calculations for present or future decompression allowances. To maximise safety margins, 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 in these cases the pony can 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.
It is very important that the diver includes testing of pony bottle functioning as part of the pre-dive tests to insure that it is fully ready for use. (Some divers take pony bottles mounted in a way that the cylinder valve is easily accessed, and deliberately dive without first opening up this cylinder valve -a highly dubious and dangerous approach, especially in recreational diving). Because of the disparity between pony bottle capacity and main tank capacity, it is vital that the regulators and the associated SPG for these different units be easily and noticeably different, as mixing up these regulators or gauges can quickly lead to a false ”out of air” emergency. It is also important to never use the pony bottle as part of the normal air supply for a dive or to extend an enjoyable dive by means of pony bottle gas – even if it is within decompression limits. The pony is an emergency device, and needs to be treated as such.
Pony bottles in Technical Diving
In technical diving, where larger volumes of breathing gas need to be supplied the usefulness of pony bottles greatly diminishes. This type of diving is the province of twinsets, rebreathers or even more complex assemblages. The diver however still needs to consider bailout – an interruption of the planned dive by breathing gas problems that requires a safe exit to the surface. In certain technical diving equipment configurations the use of larger pony bottles can again come to the fore, particular if the diving is not excessively deep and decompression requirements are modest. An example can be in the case of rebreathers, where the diluent gas supply is also used as a bailout bottle for rebreather failure. The capacity of the diluent bottle is very often insufficient for this bailout purpose, and an appropriate larger sized pony can serve as a "backup to the bailout".
- Von Mayer, Robert (2001). Solo Diving - The Art of Underwater Self-Sufficiency (2 ed.). Aqua Quest Press. pp. 71–75. ISBN 978-1-881652-28-1.
- Somers, Lee H (1986). "A compact and portable diving system for scientists.". In: Mitchell, CT (eds.) Diving for Science 86. Proceedings of the American Academy of Underwater Sciences Sixth Annual Scientific Diving Symposium. Held October 31 - November 3, 1986 in Tallahassee, Florida, USA. (American Academy of Underwater Sciences). Retrieved 2011-07-30.
- For example: "Xs Scuba PonyPac Harness". Retrieved 3 November 2011.
- Busuttili, Mike; Trevor Davies; Peter Edmead; et al. (1959). Sport Diving. BSAC. pp. 72,130. ISBN 0-09-186429-1.
- Gurr, Kevin, Technical Diving from the Bottom Up, 2002, Periscope Publishing, p52-54
-  Liddiard,John; "Bailout", Diver Magazine and jlunderwater.co.uk website,May 2008