whats the main difference betweeen a small tank with a alternate reg on it and a spair air
i know the amount of air is a huge factor but why dont 6 CF tanks or even 20 CF tanks have the nozzle like on "spair air" systems?
Spare air is a proprietary system that can work decent for people staying shallower than 50-60feet, but realistically only carries a few extra breathes of air for an OOA emergency, and if you are somehow entangled, just prolongs the inevitable a few more moments.
A pony bottle is universal (as far as fittings) with standard 1st/2nd stages and fill stations (without any special adapter). A pony also has the ability to carry much more air (depending on the size) than any spare air.
The reason that larger pony bottles don't have a breathing nozzle like a spare air is that they would be hard to manage while trying breath from them and holding the bottle.
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I personally use one of my al40 tanks for a pony (if I carry one at all) since that is what I have. I will have plenty of air for an emergency and it just so happens to be one of my deco bottles as well, so it isn't money spent on an extra bottle just as a pony bottle. I clip it to my left side and there it stays until it is needed.
Hope this helps.