In the 130 or so years since the first rebreathers were developed, four main design philosophies have come to dominate rebreather construction:

Oxygen Rebreather
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. For example, under normal use EAN 50 will commonly drop to under 40% and could drop below 10%.

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.

Early Electronic Closed Circuit Rebreather

The Halcyon RB80
The Halcyon semi-closed circuit rebreather operating system's capacity for reliable, safe, and intuitive operation has been proven by an array of world-record diving accomplishment. No other rebreather in the world is used as consistently and effectively for leading edge exploration as Halcyon, with divers regularly exploring deep wrecks and caves that are in excess of six hours from the surface at depths of 300feet.

Halcyon RB80 Rebreather

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 Bucchalay 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.