You certainly don't want that last one you posted! That thing has a crappy bite valve on it and is WAY overpriced! I would also avoid the first one as it has extra parts that aren't needed- that little extra hose.
Mine is only 2 cuft. I have never had to use it and ONLY carry it with me for freediving. I have played around in a pool and got about 20 breaths off of it in 12' of water.
As far as pony bottles go, anything less than 30 cuft. and your just kidding yourself. 40cuft. shoud be the gold standard and they will work as a stage bottle if/when you get into technical diving. I would never strap one onto my main tank as they get in the way and it limits their use. If you sling one, under your left arm, it floats nicely and you don't know its there. If you need it you have it and you can even ditch all of your other gear, in case of an entangelment, and escape. If your buddy has a problem you can take it off and hand it to them while you help them ot of what ever jam they are in. The reason you need 30-40cuft. is so you can pull your safety/deco stops on your way up.
Having said all of this, if there is ever a time where I think I mightneed a pony I go straight to doubles with an isolation manifold.
A "stick" is a blending stick made out of PVC and has baffles in it. It allows you to safely add 02 into the air intake of your compressor so that you never have more than 40% 02 going into your air compressor at any time- you need to stand next to it and watch your 02 meter while filling to insure that it doesn't go above this. Most compressors can't handle more than 40% 02, unless it's somthing special like a RIX, and some compressors can't even handle that- it depnds on how clean it is and what kind of filters it's running. We've been pumping mixes up to 40% fthis way for a long time without any problems. On dive trips to the islands I see dive operators mixing this way on a large scale without any problems.
Partial Pressure mixing is where you mix your gas in the tank itself. I use this method for deco mixes (50%) and trimix- varies depending upon the depth. The man problem with this method is that you must 02 clean the tanks and valves and should use hyperfilter air, although I've read that 85% of the compressors out there can be used without any problems in this area. To get the most out of a tank, as Riverdiver mentioned, you need to have at least 3 bottles of any one gas to rotate through to get the most gas for your money.
The only people I know who have ever had an 02 fire where using this method- partial pressure blending. It turns ot that they had a 90 degree bend in their piping system and that created enough of a heat build up that they had a flash- everyone survived and they have since rebuilt their system. This event also happend 15 years ago, before there were any course to teach you how to do this stuff, and they have since gotten a lot better at it.
The rule of thumb when PP blending is to go slow. The slower the better to prevent heat bulid up and fires- as well as to get a more accurate mix when your done.
Jon
