h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Re Spare air's, they are crap, do not bother. They will only provide you with a very few breaths from deep down, and from shallower, you can do a free ascent easily. Even if your tank runs dry at depth, as you come up and the pressure drops, some air will become free again.

If you want redundancy, a pony bottle of a twin set is a far superior choice. A pony in the UK normally means a 3 litre tank (I'm guessing at 20 CuFt here), which at 200 bar pressure gives you about 600 L of air. A single regulator on the first stage and a button pressure gauge screwed into the first stage is all that is required.

As your main tank will be filled for every dive and your pony tank only occasionally, they will almost certainly have been filled from different compressors, so in the case of a compresser leaking a tiny bit of oil which burns some lovely carbon dioxide into your tank, if you get the sudden headaches and other symptoms, you can switch to your pony and bail out, knowing you won't be breathing more of the same.

Each to their own, but I have my Pony Reg on a 2 M hose that doubles up and down the tank twice under some elastic shock cord and then comes in under the right arm. Plenty of reach so you don't get tangled up with your buddy when you donate, and perfectly tidy when you don't need it.

The ONLY use for spare air's I have seen that makes sense if for aircrew on helicopters, who need a few breaths to escape from the helicopter at shallow depth and do not have the space for full scuba gear, a pocket on their flight jacket/vest takes the spare air, but apart from that, they are toys only.

Dive safe...

Yeah, I'm not a fan. I view them as unnecessary weight and something else to get tangled up. As someone who did a lot of beach entries down some steep and rocky paths in full kit, I could definitely do without. I've got my octopus, and, failing that, my buddy's octopus.