As recreational divers, we are instructed in the proper use of an alternate air source, your dive buddies octopus. The independent alternate air source is a standard in many types of technical diving. It is also a practice that some recreational divers have adapted as well. The leading recreational certification agencies might mention an independent alternate air source in some of their course material, however, their primary recommendation is still using a dive buddies octopus. That is all well and good but over 30% of the deaths while diving happens during times of buddy separation (excluding events such as heart attacks). Another point is that many of the dives that results in multiple deaths, were triggered by one divers out of air situation.
Consider this critical flaw that can happen while diving with a buddy using a typical shared air in case of emergency mind set. Your dive plan takes you down to about 120 feet, you plan on staying there and being back at the ascent line with two minutes NDL on your dive computer or 70 bar. As you get ready to head to the surface your dive buddy’s reg starts to free flow. Now you have just 70 bar to share. At first it may seem enough to ascent to 15 feet and do your safety stop and then head to the surface. Two divers sharing 70 bar may seem like enough but realistically it is doubtful. Studies have shown that most dive teams use four times the air while sharing not just doubled. On a dive that deep, protocols normally would say that there is a hang tank. I have seen many dive operators not hanging one.
If you are solo diving, then you have no dive buddy to rely on and having a independent air source is required. The amount of air you need for emergencies will depend on many factors, however, the depth that you are diving to is one of the main considerations.
One of the earliest independent alternate air source is Spare Air. Larry Williamson, the patent holder for spare air, designed the device after he ran out of air on a deep dive. His buddy was not near by and he started to the surface with the air in his lungs. He almost made it but blacked out near the surface. The dive boat recovered him and the crew saved his life. He says that he felt all he needed was one or two more breaths. That prompted him to develop Spare Air, which hit the market in 1979. A Helicopter Emergency Egress Device (HEED) using the same processes was developed for the US Navy in 1984. As the practicality of the device became known other specialized versions became available. The standard model used by divers is Spare Air Model 170 with a maximum capacity 1.7 cu ft / 48 liters. A larger version, Spare Air Model 300, has a maximum capacity of 3.0 cu ft / 85 liters. The model 300 also has a Nitrox version.
These bottles are relatively small, only 8.75″ / 22.23 cm long for the model 170 and 13.4″ / 34 cm long for the model 300s. Each have a diameter of only 2.25″ / 5.71 cm. This makes them small enough to be attached to the BCD. They are also easy to pass to another diver. The larger model gives around 57 breaths at surface pressure. The manufacturer says that for most divers, this will allow a diver to surface from 100 feet/30 meters. Many divers believe 20 meters is a better maximum depth.
The regulator is easily removed from the tank once it has been purged making it easy to carry on an airplane. When reassembled for use, the diver can refill it from a scuba cylinder in about 2 minutes.
There are many critics of Spare Air, mostly concerning their ability to have enough air to safety return the diver to the surface. Divers needing a greater capacity have turn to pony bottles. While the Spare Air has a single stage regulator, most of the pony bottles have a standard two stage scuba regulator. The pony bottles are in fact scuba tanks that are just smaller than the primary tank. A pony bottle used for recreational diving are commonly found as 6, 13 or 19 cu ft in the USA, or 2 liter and 3 liter in metric sizes. Larger 30 and 40 cu ft (5 liter and 7 liter) cylinders are sometimes used for wreck diving, deep or deep technical diving. One key aspect is that the pony bottle is not a stage bottle, it is only for out of air situations. Therefore it is not considered while planning your dive. The size of the bottle you need will depend on your dive plan. The 13 cubic foot cylinder should be more than adequate for any dive at recreational depths not requiring a decompression stop and not in an overhead environment. The smaller sizes can be attached to your primary tank with the hose running under your arm and being secured under your chin. They can also be used on a sling as are the larger ones.
Having a pony bottle does have its disadvantages compared to Spare Air. Being larger, the pony bottles can be more troublesome while traveling and like the Spare Air the regulator must be removed. So if you bring one on the plane, it will need to be filled before you dive .The pony bottles are filled the same way as a primary scuba cylinder. That will require prior coordination. Unless they are attached to the primary tank they can be awkward to carry on the dive boat and cumbersome underwater. This may cause some divers to evaluate if it is necessary on the current dive. The Spare Air can just clip on to the BCD and forgot until needed. There have been cases where divers have forgotten they have a Spare Air clipped to their BCD since they are so small.
Both the Spare Air and the different pony bottle set ups have their places. Which one would be best for you is something only you can answer.