Pony bottle
Encyclopedia
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 exhaustion of the diver's main air supply, it can be used as an alternate air source
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.
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 harnessthat specifically supports a back mounted pony bottle exclusively for use by a qualified scuba diver in shallow water diving or for boat maintenance purposes.
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.
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.
The table above is constructed to show gas consumed in just 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 and an initial tank pressure of 150 bar (2,175.6 psi). In this particular scenario the 3 litre pony is just sufficient for diving at 20 meters but not 30 meters. It is important that anyone selecting a pony bottle should do such an analysis for his/her own breathing rates, cylinder pressure to be used, and required ascent profile, or take advice in the selection. Also it is important to note that since monitoring of remaining air is a key issue in ascent, a submersible pressure gauge readable at all times also needs to be attached to any pony bottle system.
purposes – i.e. non decompression dives at depths not to exceed 30 metres (98.4 ft) (for nitrox) or 35 metres (114.8 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.
Diving cylinder
A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....
, often of only a few litres capacity, which forms an extended scuba set
Scuba set
A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving....
and which is fitted with its own independent regulator
Diving regulator
A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases...
. In an emergency, such as exhaustion of the diver's main air supply, it can be used as an alternate air source
Alternate air source
In underwater diving, an alternative air source, or more generally alternative breathing gas source, is a secondary supply of air or other breathing gas used by the diver in an emergency...
in place of a controlled emergency swimming ascent
Controlled Emergency Swimming Ascent
Controlled Emergency Swimming Ascent is a technique used by scuba divers as a safety measure when a diver has run out of breathing gas in shallow water and must return to the surface....
. The key attribute of a pony bottle is that it provides a totally independent and redundant source of breathing gas for the diver.
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 divingWreck diving
Wreck diving is a type of recreational diving where shipwrecks are explored. Although most wreck dive sites are at shipwrecks, there is an increasing trend to scuttle retired ships to create artificial reef sites...
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
Solo diving
Solo diving is the practice of scuba diving alone without a "dive buddy". Solo diving, once discouraged, is now beginning to gain acceptance among experienced divers who have skills in self-sufficiency and redundant backup equipment....
, 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 harnessthat 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.
Alternative solutions
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. |
| Bailout bottle Bailout bottle The term Bailout bottle or Bailout cylinder refers to a scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure. A Bailout cylinder may be carried by a scuba diver in addition to the primary scuba set, or by a... – Commercially available examples: Spare Air or H2Oddessy Extra Air |
The bailout bottle is quite similar to the pony bottle in that it is a totally redundant air supply, with separate first stage and second stage regulators and a separate gas cylinder. In the bailout bottle however the regulators are both integrated into the cylinder itself, so that it provides a totally hoseless system. The unit is small, typically of no greater capacity than 0.5 litres. In an emergency the diver removes the bailout bottle from where it is stowed and breathes from it directly as he/she makes an ascent to the surface. The shortcoming of the bailout bottle with respect to the pony bottle is in its very limited storage capacity. |
| 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. |
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 (49.2 ft) | Max depth 20 metres (65.6 ft) | Max depth 30 metres (98.4 ft) | Max depth 40 metres (131.2 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 (10.6 cu ft) |
| Ascent from max depth to 5 metres (16.4 ft): ascending at 9 m/min | 67 litres (2.4 cu ft) | 113 litres (4 cu ft) | 229 litres (8.1 cu ft) | 379 litres (13.4 cu ft) |
| Safety stop: 3 minutes at 5 metres (16.4 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 (2,175.6 psi) | ||||
| Pony bottle 3 litre | 450 litres (15.9 cu ft) | 450 litres (15.9 cu ft) | 450 litres (15.9 cu ft) | 450 litres (15.9 cu ft) |
| Pony bottle 6 litre | 900 litres (31.8 cu ft) | 900 litres (31.8 cu ft) | 900 litres (31.8 cu ft) | 900 litres (31.8 cu ft) |
The table above is constructed to show gas consumed in just 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 and an initial tank pressure of 150 bar (2,175.6 psi). In this particular scenario the 3 litre pony is just sufficient for diving at 20 meters but not 30 meters. It is important that anyone selecting a pony bottle should do such an analysis for his/her own breathing rates, cylinder pressure to be used, and required ascent profile, or take advice in the selection. Also it is important to note that since monitoring of remaining air is a key issue in ascent, a submersible pressure gauge readable at all times also needs to be attached to any pony bottle system.
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 divingRecreational diving
Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels...
purposes – i.e. non decompression dives at depths not to exceed 30 metres (98.4 ft) (for nitrox) or 35 metres (114.8 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.