Spare Air vs a Pony Bottle

You may have seen it before, a small yellow canister hanging from a divers BCD or used by snorkelers who want to stay under a few seconds longer. Is Spare Air what its cracked up to be? Let's take a look and see.

What is Spare Air?

Spare is is a small tank ranging from 3 to 6 cubic feet that can be filled by the user using a pump or from a scuba tank using an adapter. It has a built in regulator, a button pressure gauge and is rated at 3000 psi/207 bar. The smallest tank, 3 cubic feet, gives 57 breaths on the surface. The largest tank, 6 cubic feet, gives 114 breaths at the surface. These tanks do not require visual inspections or hydrostatic tests and can not be filled at your local dive shop.

Spare air was created by Larry Williamson after a personal out of air experience, in his account it seems he was solo diving. He intended it to be used as an alternative to a buddy sharing air. The navy has been using spare air since 1987, their pilots having one as standard gear in case of water landing.

A Pony bottle

Ponies come in several sizes ranging from 6 cubic feet to 40 cubic feet. They are rated at 3000 psi and require visual inspections as well as hydrostatic tests. You can fill them at a filling station or by using a whip from another scuba tank (will not fill to 3000psi). They can be purchased with or without a valve in either din or yoke and require a 1st and 2nd stage as well as a spg. Using spare air consumption rate as a reference, at the surface you get this many breaths from a tank:

Spare Air 6 cu - 115 breaths
AL40 cu - 760 breaths
AL30 cu - 575 breaths
AL19 - 364 breaths
AL13 cu - 247 breaths
AL8 cu- 153 breaths.

Air consumption and depth

At rest, the average person breaths 12 to 16 breaths per minute, in stressful conditions that can elevate to 40 - 60 breaths per minute or more. We know from our scuba training air compresses as we go deeper. At 33 feet/10 meters air is twice as dense, at 66 feet/20 meters air is 3 times as dense and at 99 feet/30 meters 4 times as dense. What does that mean? It means 115 surface breaths is equal to 57 breaths at 33 feet, 38 at 66 feet and 29 breaths at 99 feet.

Actual usage

I dive for the NY Aquarium and 3 cu Spare Air is standard equipment when we hookah dive (tank on the surface connected to a long hose). The exhibits that we use them in are no deeper than 12 feet meaning we get about 42 breaths from a 3000 psi fill. I have yet to see a Spare Air that has a 3000 psi fill, especially when hand pumping. Its usually more between 1800 and 2200. ***Note they use button gauges so the reading is not as accurate, it could be more or less.

Costs

A 3 cubic foot Spare air will run you about 300 dollars. a 6 cubic foot about 360 dollars. For roughly the same price as a 6 cu Spare Air, you can get a 13 cu pony with first and second stage, hose and button spg

Benefits

The benefits of a Spare Air are it's much lighter and easer to travel with. They tend to be neutrally buoyant and you can fill them yourself with no need to assemble or disassemble anything.

The benefits of a pony are more air for your buck and reliability. They can be filled at any filling station where you get your scuba tanks filled and if any part fails, you can replace it with standard scuba equipment They can also be rented at most dive shops.

Drawbacks

With Spare Air it can literally take hours to pump up a tank to 3000 psi. You can use your scuba tank to fill it sure,but that defeats the purpose of having extra air. Due to low capacity, in an emergency or at depth Spare Air will last much less than advertised, and that's assuming you can get it to 3000 psi. I have seen people blow through 500 psi, 13 cu of air in an AL80 in under a minute. 6 cu of air is not much. When filling by pump, unlike a scuba tank, the air is not filtered. Any unintentional gas that's harmless at the surface, such as light smoke from a BBQ, can be deadly at depth.

Pony tanks require a lot more maintenance. They also require special equipment to mount and add extra buoyancy characteristics. Assembly and disassembly is required and if you should decide to travel with it, the valve must be removed meaning a visual inspection must be done before use.

Final thoughts

Spare Air may seem like a great idea but it gives a false sense of security. The capacity is so low, it can barely get you to the surface from 60 feet in a stressful situation. For snorkelers its adds risk as well. If you breathe compressed gas at depth, you must ascend at 1 foot per second. Doing otherwise can result in a lung over expansion injury.

If you are truly worried about running out of air, a pony is a much safer way to go. But of course monitoring your air in the first place works too.

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