Hi I GO DOWN,
Here's a rough draft of a piece you may see published soon. Hope you find it informative.
"Cylinders providing extra air for emergency situations come in three forms: The small bailout bottle like Spare Air, larger bailout bottles such as the pony, and independent twin tanks.
Small bailout bottles, which typically are attached to the BC, come in sizes from a tiny 1.2CF to Spare Air's 6CF "600-Twin." Pony sizes, which attach to the main cylinder by bands of various sorts, range from 6CF up to 50CF, although with the latter it would be safer & more cost-effective to set up standard size independent twins.
Which set up is best for you depends on your dive plans. For example, for dive plans in shallower water, say above 60 feet, the largest small bailout bottle may suffice. With deeper recreational diving, a pony bottle would be better. When technical diving is planned, such as depths exceeding the recreational, overhead environments, and where there are known entanglement hazards, independent twins are often wisest.
Spare Air advertises 30 surface breaths for the 1.7CF unit, 57 for the 3CF & 144 for the twin 3CF set up. More useful, however, are estimates of emergency gas needs while actually diving. Depending on breathing rate, to ascend from 60' at a brisk 60FPM without a safety stop requires anywhere from about 2 to 7CF air. From 100', it's from about 3CF to over 12CF. For a slower ascent and 15' safety stop for 3", you'll need proportionately more gas. It of course would be prudent to base your calculations on the increased consumption rates expected in an emergency situation.
As you can see, 3CF of air won't get you far. For this reason, it is important that a diver carrying a small bail out bottle not be lulled into a false sense of security. Once in trouble, he will have scant no time to solve problems or even make himself positively buoyant. At best, the diver will have enough gas to make an emergency ascent and reduce the risk of AGE because he's breathing off of something.
Other things you'll want to consider are the logistics of transporting & rigging various systems. Larger capacity cylinders typically mean greater size & weight. While small bail out bottles are connected to the BC via Velcro straps, holsters or lanyards, and ponies are attached by bands, doubles require a special BC. Finally, while small bail out bottles are minimally affect buoyancy, ponies & doubles do impact buoyancy, trim & drag, and compensation needs to be made. It is also important to consider inconvenience & nuisance factors as whatever your system, if you don't take it diving it can't assist you.
Cost is always a consideration, and none these systems are cheap. Since you asked about Spare Air, they offer 3 rigs. The MSRP for the 3CF is around $300, 3CF nitrox is about $330 & the twin 3CF closely approaches $500. Pony and independent doubles set ups can run in this same range depending quality of components, but can easily be assembled for well less than the price of the Spare Air twin 3CF unit. Be aware that whatever set up you decide on, the cylinders, regs & and associated pieces need the customary inspections & servicing. If you go with the Spare Air, I'd suggest ordering the optional dial pressure gauge, or supplying your own bolt gauge, to replace the standard pop-up stick model.
Regarding fills, bail out bottles usually are filled from the diver's main cylinder, although they can be compressor filled. Most divers carrying ponies, except for the smallest sizes, typically prefer to have these compressor filled rather than off their standard tank.
As final note, remember that the systems discussed here are nothing more than emergency gas sources, and are not to be thought of as ways of extending dive times. The diver still is required to properly monitor his main air supply, and follow whatever buddy plans have been established prior to the dive."
Best regards.
DocVikingo