camerone

At the risk of missing a few, I'll start with what I remember.

I had an Atlantis that I "upgraded" to a Dolphin.

The Atlantis canister is vastly different, and should be avoided at all costs. They never quite sealed properly, as the plastic was too flexible, and they were very prone to problems. The Dolphin scrubber is a lot better - if you can believe that...

The counterlung bags are of a different material, and the Dolphin included a flowmeter, which made orifice checking easy. The Dolphin ones are a lot more rugged.

The flow jets are identical in flow, but slightly different in construction. The new Dolphin ones seal with a second O-ring on the outside of the jets and a larger/longer cap. They're cross-compatible, however, between units, as are the hoses.

I've heard that there are two different diameters for some of the hose pieces, but I've found them to be compatible across the two (breathing loop, P-port connections.) The fittings (nuts, clamps) are a little more color coded on the Dolphin than the Atlantis, but if you don't care, it doesn't matter. The bags come with nuts, IIRC, so they should be usable as a set, although, again, I'd avoid the Atlantis breathing bags.

The BC has undergone several revisions between the Atlantis and various Dolphin vintages.

The Atlantis and earlier Dolphins used a different first stage regulator (piston, I believe) than the newer Dolphins, which have switched to a diaphram setup, IIRC.

That's about it for differences, that I can remember. Wacky hose threading, the basic shell, the dosage unit mechanical design, etc, is all the same, or it was for me, anyway.

WRT spares -

A spare flow-block ("dosage unit") would also be a good thing, as you could either swap it in, or just cannibalize orifices from it when you get the inevitable clog on a liveaboard week... this would be my #1 choice of spare parts...

Another breathing loop hose and a spare mouthpiece would be a good thing to have, although the Drager parts seem to have a really bad habit of drying out if they're not used, which causes the check valves to warp, necessitating replacement. Also, the Drager mouthpieces are anything but service-friendly with their clamps/zip tie setup...holding on to a spare one (assembled) may not save you anything if you find it doesn't work when you need it. When I used to dive a Drager, I was always fuming about the second rate engineering that went into the mouthpiece design...

A spare first stage and hose set could be useful; I would even suggest getting a TecMe replacement for the nut to convert it to standard DIN, so you could use either a special Drager valve, or a standard DIN cylinder, which might make travel a lot easier...much safer solution than an inline fitting adapter.

Hope this helps somewhat...