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.