One of the things I am most asked by my friends who want to buy a dolphin/atlantis rebreather on Ebay is how to tell the difference.

The Atlantis was the earlier version,  and most of them seen on Ebay have been at least partially upgraded to the Dolphin.

The differences are subtle, but often important.  It is also important  to know that the two are not completly compatible.

The Atlantis, for example, had a scrubber leak problem.  The lid was too flexible, and the gasket not adequate to stand up to real world use. To be fair, I know of at least one atlantis scrubber that has not yet leaked, but the owner is very careful not to warp it.  The lids of the dolphin and atlantis scrubbers look similar, but they are not interchangable.

The entire scrubber may be changed out, however, and this is a good idea.

How to recognize an Atlantis scrubber: both P connectors are black, and screw in to the scrubber body.  On the Dolphin, one connector is red, although this is not always the case if it has been retrofitted with an aftermarket attachment. In this case, look at the base of the P connector shaft for a shiny screw that is used to affix it to the scrubber.  If neither of these is readily visible in the photo, the original Atlantis scrubber had german writing engraved all over it.

Counterlungs:  The old counterlungs are clear, and have chrome hose connectors.  There is nothing really wrong with the old style, except that the hose fittings sometimes cut the breathing bags.   The newer Dolphin counterlungs are black with color coded (black and red) hose connections. you can see these on the top of the case even if you can't see the counterlungs themselves.

The dosage units have slightly different hose connections, hard to describe, but the Dolphin ones are incrementally better.  The whole dosage unit is connected to the counterlung with  a P connector, though, so it can be swapped out. The individual dosage units can also be swapped out, if you want to use the same bypass.

There are some differences in the various buoyancy jackets that came with these, but it hardly matters, since almost everyone eventually throws them away and gets wings instead.

A well kept Atlantis may remain divable indefinitely, as will most any dive gear, but in general, the more Dolphin parts you get, the better it will be.

Both of these units have a major flaw for the serious rebreather diver, in that they are both semi-closed rebreathers (SCRs.)

Rebreathers were hyped as a cure all of the open circuit problems:

Big, heavy bottles,   bubble noise, limited duration, buoyancy problems, and using excessive gas consumption at depth.

The semi-closed rebreathers are a half-step in the right direction. The basic principle of a SCR is to blow enough gas through the breathing loop to flush out excess nitrogen, and supply oxygen in a range that will support life within a fairly wide range of depth and exertion.

Because they were invented in a time when oxygen monitoring was expensive and not very good, they were designed to fly blind.

The result is that you still make noise, you still make bubbles, you still need a fairly large tank, you still use more air at depth, and you are constantly hassling with the auto-inflation/burping cycle of the counterlungs, which is the opposite of good buoyancy.

On top of this, you have the added expense of nitrox fills and sodasorb.  Your dive plan is limited in advance by your choice of orifice and gas mix.  It is always a good idea to plan the dive, and dive the plan, but with an SCR, it is mandatory.  There is no changing your dive profile underwater, so it limits your ability to respond to unforseen opportunities once you are down.

It is true that for the most part, you can get a longer dive, and also that under most circumstances,  you will use less gas than on open circuit.  Its also a fairly inexpensive stepping stone to closed circuit rebreathers.  The dragers are generally made of quality components, and many people take advantage of  more modern electronics and methods to extend the capabilities of the Dragers.

This is often called hacking the rig, and although I do not condemn any attempt to improve one's dive gear, I also never under any circumstances would be caught advising anyone to do so.  These rebreathers currently represent a very large segment of the total rebreathers in use today, and when maintained and dived according to original specs, is among the very safest available. When you step outside that envelope, all bets are off.