kensuf, I don't think you'll see any instructor argue that PADI has the best literature out of any agency out there, granted the quality of information that is in some of that literature can be questionable, but for the most part it is all very very good....
Vegan, I second the oxyhackers, but you have to think like an engineer to be able to get the most out of it. I.e.
They may not explain drysuit inflators and wing inflators, but they function essentially identically.
They may not fully explain drysuit dump valves, but they work just like a wings dump valve with adjustable OPV pressure which is the turning action on the valve.
Piston regulators are piston regulators, diaphragm regulators are diaphragm regulators. This goes back to the original designs in the 40's and 50's, they all work the same now as they did back then. A double hose regulator works on the same principles as a one hose regulator as far as the user creates a slight vacuum which raises a diaphragm and presses a lever to release the air from the first stage. It has check valve and sends it through a long hose to the mouth, but the operating principles are the same.
Tank valves haven't changed in 50+years, they did away with the J valve when SPG's came into use, but the valve itself hasn't really changed.
Most of this information is in the manuals, they have troubleshooting guides, parts explosion diagrams, some have animations of gas flow etc, Poseidon does this because the servo assisted regulator design is a bit complex compared to normal regulators which just open an orifice.
http://www.frogkick.dk/manuals/
You will find most of the manuals in there, no animations, but there will be plenty of diagrams and diagnostics. You'll find after a few flips through them that most of them are basically the same thing.
Notable exceptions
Poseidon MK3 first stage, uses a ruby sphere as a sealing surface instead of a normal HP seat, positively brilliant, only one on the market
Poseidon Jetstream/Xstream, these are downstage second stage designs meaning that instead of more pressure causing them to open, they need a certain amount of pressure to seal. They are also servo assisted so the only similarity to a normal regulator they have is that they reduce IP down to ambient, have a mouthpiece, and that's about it. I love mine
Oceanic Omegas, servo assist, but they are downstream designs, these are funky little regulators
Most every first stage is either based off of an Apeks diaphragm design, or a Scubapro Piston design. Some are blatant copies, others variations on a theme, most second stages are all about the same. Some have pnuematic balancing, scubapro, some have manual adjustments, but everything basically functions the same as the original Poseidon Cyklon did in the late 50's when it was released, and looks the same as whatever the first non-side exhaust regulator was.
