Prior to Halcyon's arrival, two general rebreather operating systems were marketed in various configurations: fully closed circuit rebreathers (CCR) and active addition semi-closed circuit rebreathers (SCR). CCR devices utilize complex electronics to blend mixtures of oxygen with other gasses, such as Nitrogen or Helium. CCR units are more appropriate for high-risk military applications, where elaborate support teams, constant training, and expensive maintenance reduced the operational risk. SCR devices extend the capacity of a pre-mixed breathing gas, by stripping out exhaled carbon dioxide, and reusing the remaining exhalation. The most popular SCR design can create significant variations in breathing supply oxygen levels, introducing a notable risk of hypoxia. In these units, variations in oxygen content essentially eliminate decompression advantages associated with breathing gases, such as Nitrox. The Halcyon rebreather supplies breathing gas as required by the diver’s respiratory rate, providing less oxygen variation, and better predictability. Gas replenishment is mechanically triggered by the diver’s breathing cycle. Full advantage of advanced breathing gases is maximized by the significant reduction in oxygen variation over all other designs. The Halcyon "on demand" gas delivery system avoids the inherent limitations of CCR, while providing significant performance advantages over all other SCR platforms. The patented Halcyon technology significantly reduces oxygen content variations over a wide range of diver activity levels, when compared to typical SCR units (constant mass flow injection systems). Moreover, this on demand feature is coupled with a naturally intuitive "alarm" feature, that provides the diver with an immediate and obvious indication that supply gasses are depleted, or that rebreather function is somehow compromised.
Halcyon's patented respiratory-coupled "resistive alarm" gives the Halcyon system a true margin of safety. Gas discharge is directly replenished when the diver inhales, as addition valves are actuated by the movement of the rebreather counterlung. Other SCR (CMFI) units can fail without offering any sense of danger to the diver. By contrast, Halcyon's operation creates an intuitive and obvious indication of a spent breathing supply. In the event that a diver’s breathing supply is depleted, the individual receives a continually smaller dose of fresh breathing gas until the breathing supply is entirely consumed. This "alarm" feature signals the diver to simply switch to the open circuit backup system, which is integrated into the rebreather mouthpiece.