Halcyon Dive Systems
Understanding Rebreathers

Understanding Rebreathers

What is a rebreather?

Divers considering use of the Halcyon RB80 should have a basic understanding of general rebreather function. Since rebreathers inherently increase the risk of any dive, individuals should first consider whether their activity justifies the use of a rebreather at all. Divers should then familiarize themselves with the general risk of rebreathers, and the specific risks of the system, under consideration. The following discussion outlines very broad principles of rebreathers, along with Halcyon RB80 specifics. A rebreather is a device that captures and recirculates at least part of a diver's exhalation, thereby allowing the diver to rebreathe part of his previous breath. Since at least some of the exhalation is recirculated and rebreathed, 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. Divers should choose the simplest possible rebreather that meets safety and dive objective requirements. Increased complexity comes at the price of higher costs, more training requirements, and more components that are subject to failure. Only after a risk/benefit analysis has been made, in favor of the rebreather, should it be used. Possible benefits include a genuine operational need for exhaust bubble reduction, extended duration requirements that cannot be met with open circuit equipment, increased thermal retention, and reduced dehydration.Early Halcyon Rebreather Testing


  • Additional time
  • Gas management/extension of available gas
  • Hydration
  • Heat
  • Lack of bubbles
  • Decompression efficiency


  • Elevated risk
  • Additional complications
  • Additional effort

Common components of a rebreather (and how they function)

All rebreathers have a breathing loop or circuit whose characteristics 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 or circuit, 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). Regular dive gear is a prime example of "open circuit" scuba. Regardless of the circuit classification, all modern rebreathers share a few common features such as:

A mouthpiece connected by two hoses for inhalation and exhalation A carbon dioxide scrubber A means of adding supply gas A counterlung

1. Breathing Loop or Circuit Classifications

As the terms imply, a "breathing loop or circuit" is a pathway that allows at least some portion of a diver's exhalation to travel back around to the diver's mouth to be inhaled again. As seen in the simplified drawing, the loop consists of a mouthpiece, two hoses, and a counter lung. One hose is for inhalation, and the other for exhalation. One-way non-return valves are located on each side of the mouthpiece ensuring that gas only travels in one direction, and hence cannot bypass the carbon dioxide scrubber.

2. Carbon Dioxide (CO2) Scrubber

A rebreather eliminates CO2 build-up by directing exhaled gas through the loop to a scrubber, where it is neutralized. The scrubbed exhalation gas travels through the loop toward the mouthpiece, where supply gas is introduced. The method of supplying fresh gas to the rebreather is another means of classification and may be active or passive.

3. Active or Passive Gas Addition

Because a semi-closed circuit rebreather allows part of the exhalation to be discharged, supply gas must be added to make up the missing volume of gas that was discharged or metabolized by the diver. The most common method for adding supply gas is active addition; the second method is passive addition. While passive addition rebreathers, such as the Halcyon RB80, have many benefits over active addition rebreathers, they remain in the minority. If gas is continuously added to the loop, regardless of the diver's breathing rate, then the rebreather uses active addition. If, instead, supply gas is only added in relation to what is required by the diver's Respiratory Minute Volume (RMV) then it uses passive addition. Further description of passive addition systems will be covered in the specific description of the function of the RB80.

4. Counterlung

All rebreathers must have a counterlung of some type. The counterlung normally consists of two bags or bellows, one for discharge and one for inhalation. Of course shape, location, and design all have an impact on breathing resistance, just as counterlung placement impacts performance as the diver moves through the range of horizontal and vertical positions during a dive.