Pony Bottle vs Spare Air: Which Redundant Air Source, and What Size?

Octopus, pony bottle or Spare Air: what is the difference?

An octopus is a second regulator on your main tank. A pony bottle and a Spare Air each carry their own gas. That one difference decides what each of them protects you from.

BSAC's safe diving guide asks every diver to carry an alternative supply on every dive: an octopus, which is an extra second stage on your main regulator's first stage, or an independent supply such as a pony cylinder with its own regulator. The octopus covers the common case, an out-of-gas buddy next to a diver who still has plenty. But as Jill Heinerth puts it in DAN's Alert Diver, it won't do any good if you have depleted the air in your own tank, because both regulators share the same gas.

A pony bottle is a small cylinder with its own valve, regulator and gauge, slung at your side or clamped to the main tank. A Spare Air is a much smaller cylinder with the regulator built into the top, carried in a holster on the BC. Its maker, Submersible Systems, has sold it as a redundant air supply since 1979. Both still work when your main tank doesn't. The difference is gas volume, and our tank duration calculator shows how quickly depth spends it.

Spare Air models and common pony bottle sizes

Spare Air's diving and watersports units come in three sizes, and even a 13 cu ft pony (AL13) holds more than twice the gas of the largest one. Spare Air rates its units in surface breaths of 1.5 L each. Pony cylinders are sold by water capacity in litres, or by rated cubic feet in the US.

Sources disagree on the AL19. Luxfer lists 2.9 L of water capacity and Catalina lists 2.7 L for its S19, so check the stamp on the cylinder you buy. Small steel ponies exist too: Faber's catalogue lists 3 L steel cylinders at 200, 232 and 300 bar, and the 200 bar version weighs 3.6 kg.

Prices vary by country and shop. House of Scuba, a US dealer, lists the Spare Air 300 package (holster, leash and refill adapter) at $299.99 and the 600 at $389.99. It lists a bare Catalina 19 cu ft cylinder with a K-valve at $199.99, and an XS Scuba 13 cu ft pony set with regulator, gauge and bracket at $574.95, so the cylinder is the smaller part of a complete pony.

How long does each one last at depth? A worked example

Much less than the surface numbers suggest, because gas use rises with absolute pressure. The 57 surface breaths of a Spare Air 300 become about 20 at 18 m (60 ft) and 14 at 30 m (100 ft). Heinerth makes the same point in DAN's article: at 100 feet a small bailout bottle lasts a quarter of its surface time, and if it is underfilled or the diver is panicking, the gas may last only a minute or two.

Here are the inputs. Take a relaxed SAC of 15 L/min and double it to 30 L/min for an emergency, the same two-times multiplier our gas planner uses for a stressed rate. Keep no reserve, because a bailout bottle is breathed down. The Spare Air 300 counts as its rated 85 L, and the 3 L steel pony at 200 bar holds 3 × 200 = 600 L. The formula is the tank duration calculator's: minutes = usable gas ÷ (SAC × (depth ÷ 10 + 1)), rounded down to a whole minute.

The calculator's tank box starts at 1 L, so a Spare Air can't be entered there; we ran the same formula with 85 L. Read the aluminium rows as ceilings: multiplying litres by bar gives an AL13 393 L, against the 374 L Luxfer rates it at.

• Gas use at 30 m: 30 × (30 ÷ 10 + 1) = 120 L/min.
• 3 L pony: 600 ÷ 120 = 5 minutes.
• Spare Air 300: 85 ÷ 120 = 0.71 minutes, about 42 seconds.
• At 18 m the rate is 30 × 2.8 = 84 L/min: 7 minutes on the pony, about 1 minute on the Spare Air.

Is it enough to reach the surface?

From 30 m, a 3 L pony at 200 bar is. A Spare Air is not, once you count a controlled ascent with a safety stop. Minutes at depth are the wrong test, because nobody plans to stay down on a bailout bottle. What matters is whether it covers the trip up.

We sized that with the rock bottom model from our gas planning calculator: one minute at depth to sort out the problem, an ascent at 9 m/min (30 ft/min) to 5 m counted at its average depth, a 3-minute safety stop, then the last 5 m to the surface. The planner gives that gas for two divers sharing one tank. A bailout bottle has to get only you up, so the figure is half: 333 L from 18 m and 505 L from 30 m at 30 L/min. Our rock bottom walkthrough builds the two-diver number phase by phase.

The Spare Air falls short even in shallow water: the 3-minute stop alone takes 135 L at 30 L/min (3 minutes × 1.5 bar × 30). It can cover a straight ascent at 9 m/min with no stop and no time at depth, from about 14.7 m at 30 L/min or 23.5 m at a calm 15 L/min. That second figure matches the maker's claim of enough air for a safe ascent from 75 feet (23 m), for a calm diver going straight up.

Other planners add more margin. SDI's Steve Lewis allows three minutes at depth and a five-minute stop at 6 m, and gets 1,090 L for one diver from 40 m (rounded up to 1,200 L), where our model gives 685 L. His pick for a single-tank diver is a 6 L (40 cu ft) aluminium pony. Treat the 3 L pony as the smallest bottle that works at 30 m: it reaches the surface with 95 L left, about 32 bar.

When do training agencies require a redundant air source?

Solo diving courses require one, and BSAC strongly recommends one below 30 m. For a buddy dive shallower than that, BSAC accepts an octopus as the alternative supply.

• PADI Self-Reliant Diver: you must be an Advanced Open Water Diver, at least 18, with 100 logged dives. PADI lists a redundant gas source among the required gear, along with a DSMB, a redundant dive computer and a redundant surface signalling device. The training dives include switching to a redundant air supply in simulated emergencies.
• SDI Solo Diver: minimum age 21, SDI Advanced Diver or equivalent, and proof of 100 logged dives. The skills include an emergency change-over to a redundant air supply at no more than 30 m (100 ft), and you plan dive limits on your own air consumption rate.
• BSAC: an alternative supply on every dive, which can be an octopus. A pony used as that supply should hold at least 3 L, and auxiliary cylinders under 3 L are not considered adequate. Below 30 m, BSAC strongly recommends an independent supply such as a pony.

How to set up a pony bottle: slung or back-mounted

Sling it at your side, or clamp it to the main tank with a bracket. In both cases you need to reach the valve and the second stage yourself, without help from a buddy.

Heinerth's sling setup clips the top of the bottle to a chest D-ring and the bottom to a hip ring, with a bungee pulling the tank neck toward the armpit. Without the bungee, she writes, the bottle tends to ride perpendicular to the body, which hurts streamlining and risks entanglement and regulator damage. She practises unclipping and replacing it in her normal dive gloves, so she can pass it to a diver in need or to the boat. A back-mounted pony rides in a bracket such as XS Scuba's pony mount, which fits under the BC's tank strap. Lewis prefers a side-mounted bottle because it is out of the way but accessible.

• Fit a regulator you trust. Lewis's advice is to spend money on it, because it has to perform when you are under stress.
• Fit a pressure gauge and read it before every dive. Some small bailout bottles only show full or empty, Heinerth notes, which won't reveal an underfill.
• Clip the second stage to a chest D-ring, as Heinerth does, so it is where your hand expects it.
• Have a pony filled at a fill station like any cylinder. A Spare Air refills from a full scuba tank through an adapter, and Heinerth warns that hand-pumping one to 207 bar (3,000 psi) is nearly impossible.
• Label the gas. If the pony holds nitrox, Lewis says to mark the contents and check the maximum operating depth before every dive.
• Fly with it empty. The rules are in the questions at the end of this page.

Drills that make a pony bottle worth carrying

A bailout bottle you have never breathed from underwater is a guess. Spare Air tells owners to practise pulling the unit from its holster, to breathe one dry above water and count the breaths, and to do a practice ascent on a normal dive. It warns against breathing the unit completely dry underwater, which can flood it. Lewis's rule for a pony is a complete ascent breathing from it at least a couple of times each season, and drilling the switch until it is fluid.

Brief your buddy as well. BSAC suggests a dry run of the out-of-gas response before the dive, so your buddy knows which regulator you will hand over and where the pony's valve sits.

The downsides: task loading, false confidence and upkeep

A pony helps only if you reach for it. In a case DAN published in 2023, a group diving at around 80 feet (24 m) all carried aluminium 80s with pony bottles. One diver ran out of air, panicked and grabbed his buddy's full-face mask instead of using his own full pony bottle. DAN's lesson was to practise air sharing and redundant gas systems until they are second nature.

Extra kit is extra work. Heinerth describes one of her own close calls as brought on by distraction and task loading. A second cylinder also adds drag: in a 1996 study of swimming with scuba, adding extra air tanks or a drysuit raised the energy cost of swimming by 25%, and more energy means more gas breathed. Our guide to lowering your SAC rate covers the other things that push it up.

False confidence is the quieter cost. A bailout bottle covers a failure of your main gas, not a plan that ran the main tank too low. Lewis says to ignore the pony's gas in the dive plan, so plan the main cylinder with rock bottom as if the pony were not there. For the Spare Air, the maker's own FAQ says the model is designed for recreational dives shallower than 100 feet (30 m).

Then there is upkeep. Heinerth notes that a small bailout bottle needs the same care as a full-size tank, including hydrostatic testing and a visual inspection sticker. Spare Air's FAQ asks for a yearly service and a hydrostatic test every five years. A pony adds a second regulator to your service bill.

Pony bottle or Spare Air: a decision table by dive type

Size the bottle for the deepest point of the dive, then round up. The table uses the one-diver ascent figures above at 30 L/min. On those figures an AL13 covers the full ascent to about 22 m and a 3 L pony at 200 bar to about 35 m. If your own stressed rate is higher, so is the bottle you need.

To check your own numbers, put your stressed SAC into the planner's rock bottom tab, halve the litres for a one-diver ascent and divide by the pony's water capacity. If you are choosing the main cylinder at the same time, our tank sizes guide compares the full-size options.