Bailout cylinders for use with scuba Alternative air source

a pony bottle (left) attached larger cylinder

a pony bottle small independently filled diving cylinder, of few litres capacity, forms part of extended scuba set , fitted own independent regulator. in emergency, such exhaustion of diver s main air supply, can used alternate air source in place of controlled emergency swimming ascent. key attribute of pony bottle provides totally independent , redundant source of breathing gas diver. pony bottle specific configuration of bailout cylinder.

configuration — in pony bottle system back-up regulator complete diving regulator (first , second stages, , submersible pressure gauge) on separate cylinder not intended use primary breathing gas during dive. provides totally redundant emergency air supply. size of pony bottle cylinder smaller of primary scuba cylinder. should provide enough breathing gas make totally controlled return surface, including required decompression stop or safety stop planned ascent. required pony cylinder capacity depend on profile safe ascent surface required particular dive plan. pony bottle used sport diving may 6, 13 or 19 cu ft in usa, while 2 litre , 3 litre common sizes in europe. deep, technical diving or wreck diving, 30 , 40 cu ft (4 litre , 5.5 litre) cylinders used. pony bottle minimum requirement doing solo diving, has no alternative source of air in form of buddy s available tank , octopus regulator if dive depth diver not able safe free ascent. in scientific diving operations, pony bottles can standard part of tethered scuba diving operations diver solo connected surface communications equipment.

there several options mounting of pony bottle. common way pony bottle carried fixing side of primary (back gas) scuba cylinder straps or clamps, may include quick-release system (as in above picture). common alternative slinging between 2 d-rings on diver s scuba harness or buoyancy compensator.

choice of pony bottle size — function of pony bottle provide source of breathing gas controlled , prudent ascent surface in emergency situation, volume must sufficient purpose. when doing no decompression diving, total reserves of breathing gas must still sufficient supply 3 phases of ascent:

at end of time pony bottle should still hold enough gas provide sufficient pressure smooth flow regulator first stage.

the table above constructed show gas consumed in such scenario – 2 minutes @ depth bailout , preparation ascend, safe rate of ascent 5 meters followed 3-minute safety stop. calculations based on heavy breathing rate of 30 litres per minute , initial tank pressure of 150 bar (2,200 psi). in particular scenario 3  litre pony sufficient diving @ 20 meters not 30 meters. these values may vary depending on diver , other circumstances. diver selecting pony bottle can such analysis his/her own breathing rates, cylinder pressure used, , required ascent profile, or take professional advice on selection. since monitoring of remaining air key safety issue during dive, submersible pressure gauge attached pony bottle regulator should readable @ time during dive.

spare air

charging spare air cylinder larger primary scuba cylinder

the smaller cylinders have combined first , second stage regulator directly mounted cylinder neck thread known spare air after commercially supplied unit of name. these can provide limited number of breaths out-of-air emergency , suitable relatively shallow dives without decompression. smaller 1.7 cu ft spare air provides approximately 30 breaths, , larger 3.0 cu ft approximately 60 breaths @ surface pressure. number of breaths provided in practice depend on diver s lung capacity, depth, exertion , state of mind. ascent 18 metres (60 ft) @ recommended rate of 9 metres per minute (30 ft/min) , typical residual minute volume of 15 litres per minute relaxed diver use approximately 60 litres of free air (volume @ surface pressure - approximately 2.1 cu ft.) dive profiles may adequate. manufacturers recommend these cylinders use within recreational diving limits , suggest little training required in use.

the small size , weight of these units makes them relatively convenient , easy carry, , more convenient travel larger cylinders. can handed off diver in need of air in emergency more conveniently larger cylinders. spare air unit commonly carried in small pouch can attached diver s harness or buoyancy compensator. unit may attached pouch safety leash if accidentally dropped underwater not lost.

recharging done through connector supplied unit purpose, , connects main cylinder valve decant air top-up. done diver before dive. button gauge provided allow diver check pressure.

since introduction in 1980s, spare air cylinders have been subject of debate within diving community. argument against them not have sufficient capacity diver in many emergency situations surface safely, , cause divers feel false sense of safety. arguments in favour spare air cylinders both less bulky , less complicated pony bottles, on , have no hoses or pressure gauges, , air better none in emergency.