What is a rebreather?

A rebreather is a device that captures and re-circulates at least part of a diver’s exhalation, thereby allowing divers to re-breathe part of their previous breath. Since at least some of the exhalation is re-circulated and re-breathed, gas duration is extended and overboard discharge is reduced. Therefore, a rebreather may be thought of as a gas extension device that simultaneously reduces bubble output.

All rebreathers have a breathing loop with characteristics that help classify the type of unit. The extent to which the exhalation is contained in the circuit determines whether the rebreather has a fully closed circuit, semi-closed circuit, or open circuit. If the entire exhalation is retained in the loop, the rebreather is referred to as a Fully Closed Circuit Rebreather (CCR). If a portion of the exhalation is vented from the loop or circuit, the rebreather is referred to as Semi-Closed Circuit Rebreather (SCCR). Finally, if the entire exhalation is discharged into the environment and dumped overboard with each breath—as with scuba—the device is referred to as Open Circuit (OC).

The Halcyon rebreather supplies breathing gas as required by the diver’s respiratory rate, providing less oxygen variation and better predictability than other semi-closed rebreathers. 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 SCCR platforms.

Are rebreathers dangerous?

All rebreathers have an inherent risk greater than that found in open-circuit scuba. Dive instruction should work to instill respect for the potentially fatal problems associated with rebreather diving, and active rebreather divers must take care not to become complacent. Two of the most obvious risks in rebreather diving include oxygen supply problems and CO2 accumulation problems. However, a wide range of factors could affect a diver’s safety while using a rebreather. Divers should evaluate whether the benefit of rebreather diving outweighs the risk inherent in their use.

What is a resistive alarm?

Because there have been many cases where rebreather divers ignored perfectly functioning alarm systems and/or failed to make the proper adjustments, it's important to effectively calculate what is required to get the diver's attention, how much information should be imparted, and how involved the remedy should be.

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 counter-lung. 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.

How does the RB80 remove water from the loop?

All rebreathers accumulate water during operation, both from poor orifice management (such as at the mouthpiece) and from the condensation of water within the loop because of heat generated during CO2 scrubber reactions. The Halcyon RB80 funnels accumulate water into the discharge bellows where it is automatically released into the ambient water. Hence, divers don't need to concern themselves either with water collecting in the unit or with efforts to remove it. Instead, these operations are coupled to the respiratory cycle and repeat themselves with each breath.

Why is the Halcyon RB80 usually attached to large cylinders?

Many rebreather divers develop a false sense of security because gas consumption is very low when compared to open-circuit diving. However, rebreather failures require that divers return to open-circuit supplies; this is known as bailout gas. In truth, the bailout gas required varies little between differing designs, as a failure requires all divers to return to open circuit. The volume of gas required to safely reach the surface is the factor that determines the quantity of bailout required.

The Halcyon rebreather is usually configured in much the same way as a set of doubles for open-circuit, double-tank diving. In this case, a Halcyon diver would wear back-mounted doubles with the Halcyon RB80 mounted unobtrusively between a diver's double cylinders. However, dives in shallow open water could allow a small single tank to be mounted to the side of the RB80 in the same manner that one would mount a dry suit inflation or bailout bottle on a single cylinder.

The appropriate tank size varies with the 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.

What gas can I use with my Halcyon RB80?

Gas selection relates partly to the diving logistics and partly to a diver’s level of training. The Halcyon rebreather is offered with a dual-inlet gas manifold that allows divers the ability to plug various gasses into the system and to 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.