RB80 in use off of Grand Cayman

Halcyon's design of the RB80 rebreather surpasses all previous standards for safety, gas efficiency, and ease of operation in self contained, mixed gas recirculating systems. Not only does the Halcyon RB80 reduce the likelihood of the most common hazards associated with existing rebreather systems, but it also integrates well with open circuit diving skills while increasing gas efficiency by approximately eight times the gas carried. An 80ft3 bottle would, on average, provide the equivalent of a 640ft3 useable breathing supply with the RB80.

Halcyon's operating system is free of single point addition failures. Potential breathing supply failures are signaled by intuitive alarms which use physical indicators to make it immediately apparent if a diver forgets to turn on his supply bottle or runs out of breathing gas. These alarms can only be used with Passive Addition systems, like the Halcyon, that force a diver to breathe off the bottom of the counterlung because part of his previous exhalation has been dumped overboard. If the actual addition mechanism fails, each successive inhalation is shorter which gives the diver a less abrupt version of the kind of warning afforded by open circuit SCUBA. Intuitive alarms make the Halcyon safer than Active Addition (i.e. Constant Flow) units such as the Atlantis, which would continue to recirculate the same breath while the FO2 continues to drop into a potentially dangerous zone.

Halcyon Design

Gas Circulation During Inhalation
The diver's inhalation through the mouthpiece forces the exhalation non-return valve closed while negative pressure opens the inhalation non-return valve. The contents of the outer bellows are drawn up through the scrubber bed, through the hose and into the mouthpiece to the diver's lungs. As the outer bellows collapses from the inhalation's negative loop pressure, the inner bellow non-return valve closes. Further collapse causes the contents of the inner bellows to be discharged through a pressure discharge valve. The volume of discharged gas is replaced through the gas addition inlet with fresh supply gas by the rebreather's addition regulators.

The volume of replaced gas is keyed to the diver's inhalation. Toward the end of the inhalation, the outer bellows bottoms out and triggers the addition regulators until inhalation ends. Thus, the Halcyon replaces only the volume of gas removed and maintains a stable fraction of oxygen in the breathing loop. Should a failure prevent one regulator from providing the required gas volume, the other addition regulator automatically makes up for the shortfall. Under normal operation both regulators function for gas addition. Both regulators equalize the loop during descents while loop pressure generated by ascents is released through the overpressure relief valve.

Gas Circulation During Exhalation
The gas begins to travel through the circuit as the diver exhales through the mouthpiece forcing the inhalation non-return valve closed and directing breath through the exhalation non-return valve. Gas then travels through the flex hose. Gas enters the discharge bellows during exhalation, it is not discharged however until inhalation.

Water Removal
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 accumulated water into the discharge bellows where it is automatically released into the ambient water. Hence, the diver doesn't need to concern himself 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.

Halcyon Rebreather Alarms and Warnings

Alarms And Task Overload
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.

The alarms themselves have four basic attributes:
Intrusion Quotient
Intrusion is a function of how immediately and unequivocally the alarm makes its activation known to the diver and others. The more serious the problem, the more intrusive the alarm should be. For example, alarms for hyperoxic or hypoxic conditions should be highly intrusive because both conditions can cause unconsciousness with little or no warning.

Failure Susceptibility Quotient
This quotient is a function of whether or not the alarm itself is subject to failure due to another malfunctioning component, such as a dead battery, broken wire, or loss of diving pressure (such as a gas driven sonic alarm that has lost its supply source).

Information Content
Quotient Information content is a function of how much information is imparted to the diver and/or others once the alarm has been recognized as such. For instance, does the alarm identify the specific problem or merely identify the type of problem? If computer controlled, does it suggest remedial action?

Verification Quotient
Verification is a function of how much corroborating information can be gleaned from other sources such as pressure gauges, secondary displays, breathing characteristics, operating sounds or bubble emissions. The higher the verification quotient, the higher the confidence level in the alarm.

Both the alarms and the breathing systems they monitor should be optimized to increase the alarms' effectiveness in all four categories, but every design is a compromise that has to emphasize some attributes over others. The intended use for the rebreather, the type of individual diving with it and the amount of operational support all dictate where the tradeoffs will occur.

Some alarms are intrinsic parts of the rebreather itself. For example, a gurgling noise in the breathing loop clearly indicates the presence of water where it shouldn't be. Whether the water was inadvertently introduced into the loop by the diver's ill-management of the interface or through an actual leak can be verified by pressurizing the loop and looking above for bubbles. The diver might then watch for signs of hypercapnia in the event of reduced scrubber efficiency. In addition, swimming or working positions could be changed to reduce the possibility of "caustic cocktail" inhalation or ingestion.

The more intensive the primary task, the more chances the alarm will not be immediately heeded by a diver. Even heads up displays on electronic units are sometimes ignored. This is a good reason to select a rebreather that is no more complicated than absolutely necessary for the job. For instance, there's less chance of developing hypoxia if there's no separate source for pure oxygen. Hence, a closed circuit unit should not be selected if the duration of the dive doesn't require it.

The ability to make an alarm so intrusive that it can't be ignored is limited by the fact that the alarm itself must not become a safety liability. Examples would include things like shutting off the breathing loop so the diver can't breathe or electronically opaquing the mask lens so the diver can't see. Moreover, electrically driven alarms are more suspect because of the possibility of battery failure, and the intrusion quotient of an alarm is considerably higher if the alarm doesn't have to be monitored.

Passive addition semi-closed systems with bellows counterlungs are easier to design intuitive breathing alarms for than other types of operating systems. Passive systems reduce the potential for hypoxia by making a full correction with every breath, thus reducing the large oxygen partial pressure variations common to active semi-closed systems. With the Halcyon system, both over addition and under addition conditions result in unmistakable breathing variations that are readily identifiable and difficult to ignore.