The Halcyon rebreather is configured in much the same way as a set of doubles for open circuit double tank diving. Divers must carry ample bailout to complete the dive on open circuit, should the rebreather fail. Therefore, a Halcyon diver would wear back mounted doubles with an isolation manifold. The Halcyon RB80 mounts unobtrusively between a diver's double cylinders on a modified backplate. Shallow open water diving could allow a small single tank to be mounted to the side of the RB80 in the same manner that one would mount an drysuit inflation or bail-out bottle on a single cylinder. The appropriate tank size varies with dive activity and environment. In most open water settings aluminum 40cf bottles (or smaller) would allow ample gas supply while longer-range diving or cave penetrations would commonly call for 80cf or larger cylinders.
Supply gas manifolds
The Halcyon rebreather is offered with a dual inlet gas manifold that allows divers the ability to plug various gasses into the system, and change them during the dive, as conditions and/or depth vary. Divers may use any mix that would be appropriate for open circuit diving, gaining the benefit of the rebreather by greatly extending the mileage a diver would get out of the supply.
The RB80 is a powerful gas management tool. This system increases gas efficiency an average of eight times more than conventional open circuit consumption. The Halcyon also provides notably greater longevity than popular semi-closed rebreathers, which provide for equivalent efficiency or allow for the use of various bottle sizes.
The Halcyon operating system minimizes the extreme nature of oxygen variation common with other semi-closed rebreather systems. While using the RB80, the deeper one dives the less of a gap exists between supply oxygen fraction and breathable gas supply. However, this oxygen drop can be dangerous if the diver is not respectful of the potential oxygen variation. The extent of the oxygen drop varies according to supply gas and the