first let me appologise if i am repeating anything already said. i am new here and did not actually read all posts in this thread. that being said.....i too recently started toying with the idea of getting some type of back up air supply. i had some extra cash that i had to promise to spend on myself so i started researching different options. one of the more popular options that shows up online is the "spare air 3cf". it seemed to be an easy option. easy to carry, transport, service, and fill. and i have seen a few people who use them. but after looking into it further i found that for a bit more money i could get a decent regulator, small pony bottle, filler hose, and a system to attach it to my main tank.
i am not sure if posting links to what i bought is allowed so i won't. but i will say i got a sherwood brut reg, a bright yellow 6cf pony bottle w/valve, a tank to tank filler hose, a reef scuba X bracket, and yellow hose wrap for under 400 bucks.
i tried it out just last month in mexico. at 30 feet i got 40 rather deep breaths. which is just about what i expected. so 40 breaths at 2 atm (30 feet) means i should get about 20 breaths at 4 atm (90 feet) if i remember my theory correctly.
now the most comments i hear from people about a small pony bottle system is that they are just that......small. maybe too small. and a lot of advanced divers suggest a lot bigger tanks like 19 to 40cf. i do not disagree with the reasoning for having a larger tank. but i think we need to keep in mind why we want it. is it to be completely independent and not have to rely on a buddies air if we need it ? or is it to have a reliable, redundant air supply that will give us not only some added piece of mind, but also enough air to easily make it to our buddy for asistance or even make it to the surface on our own in an emergency if we absolutely had to from a reasonable depth.
the small 6cf is not a true back up air supply. it is a "bail out". if you want a true back up system, dive twins. manifold them with split regs, take a solo diver course and be on your way. but as far as i am concerned, for the average recreational diver. a small system like the one a have now is fine. just remember to plan a dive as if you do not have it. and always practice good buddy skills no matter what type of system you decide to get.
the only draw back i find is the extra weight i have to pack for travel. but i am glad i have it. is it necessary ? no. but if you feel more comfortable with it. then do it.
just my 2cents
