Rebreather Design History
In the 130 or so years since the first rebreathers were developed, four main design philosophies have come to dominate rebreather construction:
A pure oxygen rebreather usually has one supply bottle containing oxygen that is used to replenish what has been metabolized by the diver. Since the loop contains pure oxygen, this type of unit is depth limited and a pre-dive breathe and purge routine is necessary for diver safety. These rebreathers remain in regular use by the military, but the hazards are high and the training protocols intense. This type of system is unlikely to find much application outside of covert military operations.
Semi Closed Active Addition
Active addition rebreathers are also commonly referred to as constant mass rebreathers. These units add gas to the rebreather at a constant mass flow rate until the supply gas is exhausted or they are turned off. Active addition has an insidious risk: should the addition mechanism fail, become partly blocked with debris, or should the supply gas be depleted, a diver could easily experience hypoxia without warning. In these cases, the volume of gas would remain almost constant since inert gas would not be depleted, and the scrubber would continue to remove CO2. The diver would continue to rebreathe his own breath, while metabolizing all available oxygen until hypoxic symptoms occured. These units require that divers predetermine their exertion rate, and choose a flow rate to match. Should the diver need to exert more than anticipated, the breathable oxygen content could decrease substantially -- in some cases to dangerous levels.
Semi Closed Passive Addition
A rebreather is considered passive if gas addition is tied to the diver's Respiratory Minute Volume (RMV) and depth. In other words, a passive addition rebreather only supplies gas as a diver's breathing causes a negative pressure gradient. This type of rebreather is less likely to cause hypoxia than the active addition units because, should the addition fail for any reason, the diver would be warned by shorter and shorter inhalations. Since failures on these units result in an immediate and easily recognizable change in breathing characteristics, the diver is not required to monitor anything.
Fully Closed System
Closed circuit rebreathers replace the metabolized oxygen in the breathing loop but rely on a control system to maintain the amount of oxygen at a pre-determined level. Since Closed Circuit Rebreathers (CCR's) mix the diver's gas on the fly they typically have two supply bottles. One bottle contains pure oxygen and the other contains a dilutent. Since gas does not escape except during ascent, CCR's only need to supply enough oxygen to maintain the desired PO2 by replacing oxygen the diver metabolizes. CCR's are more efficient than semi-closed circuit rebreathers but rely on electronics and oxygen sensors to mix gas dynamically. This requires divers to be conversant with a far more complex unit, and rely on electronic controls, which in turn require intricate measurements.
The RB80's Approach to SCCR Design
While the advantages of the Halcyon system have already been demonstrated in advanced diving contexts, its simple design and operation are equally as useful for less technically demanding diving. Divers who are unaccustomed to the preparation required for extreme diving will greatly benefit from this rebreather's reliability and ease of use. Halcyon's designers have both substantially reduced the overall size of the rebreather and come up with a system that incorporates cutting edge technical knowledge derived from some of the world's most aggressive diving activity. Starting with patented technology, dedicated Halcyon divers like Reinhard Buchaly worked with the design team to incorporate features, such as the innovative resistive alarm and an automatic water collection and removal system, into a smaller, more convenient unit. The resulting system is fully capable of both safely supporting the average diver and enabling the most extreme dive exploration. Before Halcyon's arrival, two general operating systems were available in various configurations: fully closed circuit rebreathers (CCR) and active addition semi-closed circuit rebreathers (SCCR). CCR devices use complex electronics to blend mixtures of oxygen with other gasses, such as nitrogen or helium, and have historically been used 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 SCCR design produces significant variations in breathing supply oxygen levels, which increases the risk of hypoxia. In these units, variations in oxygen content can be significant enough to nearly eliminate the decompression advantages associated with breathing gases such as Nitrox. The Halcyon RB80 supplies breathing gas as required by the diver's respiratory rate, providing less oxygen variation and better predictability. The significant reduction in oxygen variation over all other designs maximizes the advantages of advanced breathing gases. This "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 has proven to have significantly reduced oxygen content variations over a wide range of diver activity levels when compared to typical SCR units (constant mass flow injection systems). Halcyon's patented respiratory-coupled "resistive alarm" gives the system a more significant margin of safety. The diver's inhalation directly replenishes gas discharge because addition valves are triggered by the movement of the rebreather counterlung. Other SCR (CMFI) units can fail without effectively signalling danger to the diver. In contrast, Halcyon's system provides an obvious indication of a spent breathing supply. A diver whose breathing supply is depleted receives a continually smaller dose of fresh breathing gas until the supply is entirely consumed. This feature prompts the diver to switch to the open circuit backup system which is integrated into the rebreather mouthpiece.