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The '''Halcyon RB80''' is a non-depth-compensated passive addition semi-closed circuit [[rebreather]] of similar external dimenrions to a standard AL80 [[scuba cylinder]] (11&nbsp;litre, 207&nbsp;bar aluminium cylinder, 185mm diameter and about 660mm long).

About 1/8 of the respired volume of breathing gas in the circuit is discharged during each breathing cylcle by a concentric bellows counterlung system, which reduces the loop volume and is replenished by internal valves, triggered by low loop volume, similar to the function of the demand valve of a scuba regulator.

The Halcyon RB80 was introduced as a replacement for the much bulkier and more mechanically complex [[Halcyon PVR-BASC|PVR-BASC]], which was depth compensated and used a ballasted bellows counterlung.

==Specifications==

*5.6&nbsp;lbs (2.54&nbsp;kg) scrubber absorbent load provides approximately eight hours of scrubber endurance
*8:1 counterlung volume ratio provides substantial gas extension, with a loop oxygen fraction slightly lower than the feed gas composition
*The inner bellows automatically vents water from loop
*Semi-closed operation eliminates the risk of unit-induced oxygen toxicity (max PPO2 is the fresh gas connected)
*Full access to on-board gas supply in either semi-closed circuit or open circuit bailout
*Cylinder volume to user's choice. Large cylinders can be carried.<ref name="Halcyon RB80" >Staff, Halcyon official website, ''RB80 Rebreather'', http://www.halcyon.net/RB80 accessdate 6 May 2013</ref>

==Breathing cycle==
[[File:Passive addition SCR loop schematic.png|thumb|Schematic diagram of the breathing gas circuit of a passive addition semi-closed circuit rebreather.{{ubl
|1 Dive/surface valve with loop non-return valves
|2 Exhalation hose
|3 Counterlung fore-chamber
|4 Non-return valve to discharge bellows
|5 Discharge bellows
|6 Overpressure valve
|7 Main counterlung bellows
|8 Addition valve
|9 Scrubber (axial flow)
|10 Inhalation hose
|11 Breathing gas storage cylinder
|12 Cylinder valve
|13 Regulator first stage
|14 Submersible pressure gauge
|15 Bailout demand valve
}}]]

Inhalation closes the exhalation non-return valve and draws gas through the inhalation non-return valve. The pressure drop draws the contents of the outer bellows through the scrubber, inhalation hose, non-return valve and mouthpiece to the diver. As the concentric bellows contract, the pressure in the inner bellows increases and first closes the internal non-return valve, then pushes its contents through the outer non-return valve, discharging to the surroundings.
When the bellows is fully contracted, the bottom cover triggers the gas addition valves which inject gas until the diver stops inhaling and the bellows no longer presses against the trigger.
If the volume in the breathing circuit has been compressed by an increase in depth, the gas volume will be even less and the addition valves will be triggered for longer, bringing the volume back to the appropriate level. The RB80 uses two addition valves in parallel, so that if one fails, the other will provide the required gas.<ref name="RB80 design" />

Exhalation closes the non-return valve in the inhalation hose and pushes gas through the exhalation hose to the bellows counterlungs which will expand to accommodate the exhaled volume. If there is an excess beyond the capacity of the fully expanded bellows, as will occur if the gas expands due to a decrease in ambient pressure during ascent, the excess gas will simply pass out through the inner counterlung s non-return valves to the surroundings. The design of internal ducting leads water in the exhalation side of the loop into the inner counterlyng bellows, and from there it is discharged to the surroundings along with the gas during the inhalation stage of the cycle.<ref name="RB80 design" >Staff, Halcyon official website, ''Design specifics'', http://www.halcyon.net/RB80/design accessdate 6 May 2013</ref>

==Safety==
If the breathing gas supply is depleted, the dose of fresh breathing gas decreases until the breathing supply is used up. The diver will notice the decreasing volume of gas available which signals the need to switch to an independent open-circuit bailout system, which is integrated into the rebreather mouthpiece as a bailout valve.
Water which leaks into, or accumulates in the breathing loop drains into the inner bellows from which it is automatically discharged into the environment along with the discharged air when the inner bellows is emptied during each inhalation.<ref name="RB80 FAQ" />

==Configuration==
The RB80 is usually carried between a set of back mounted isolation manifolded double cylinders, supported on a backplate and wing harness. Dives in shallow open water could be done using a small single tank mounted to one side of the RB80.<ref name="RB80 FAQ" />
It can also be side-mounted for tight restrictions. Occasionally extreme dive profiles require the use of a rebreather for bailout, and the RB80 can be carried as a back mounted pair, as one back mount and one side mount, or both side mounted for those occasions.

The appropriate tank size depends on the dive activity and environment. In most cases the volume of gas should be sufficient that at any time during the dive the gas remaining in the cylinders is sufficient to reach the surface on open circuit, after completing all required decompression.

Gas selection is basically to use the gas that would be appropriate for an open circuit dive of the same profile.

The RB80 has a dual-inlet gas manifold that allows divers to change gas mixtures during the dive to suit the depth or for decompression.
<ref name="RB80 FAQ" >Staff, Halcyon official website, ''RB80 FAQ'', http://www.halcyon.net/RB80/FAQ accessdate 6 May 2013</ref>

=== Loop gas calculation ===
{{further|Rebreather diving#Passive addition}}
Oxygen partial pressure in a passive addition system is controlled by the breathing rate of the diver. Feed gas is added by a valve which is equivalent to an open circuit demand valve in function, which opens to supply gas when the counterlung is empty - the moving top plate of the counterlung works like the diaphragm of a demand valve to operate the lever opening the valve when the counterlung volume is low.<ref name=IEEE2001>{{cite journal |author=Nuckols, ML; Finlayson, WS; Newville, B; Gavin, WA Jr |year=2001 |title=Comparison of predicted and measured oxygen levels in a semi-closed underwater breathing apparatus. |journal=OCEANS, 2001 |publisher=MTS/IEEE Conference and Exhibition |volume=3 |url=http://archive.rubicon-foundation.org/7960 |accessdate=2013-05-16}}</ref> The volume may be low because the internal bellows has discharged a part of the previous breath to the environment, or because an increase in depth has caused the contents to be compressed, or a combination of these causes. The oxygen used by the diver also slowly decreases the volume of gas in the loop.

The steady state partial pressure, <math>F_{O_2loop}</math>, in a passive addition loop can be calculated from the formula:<ref name="DC55" >[http://www.teknosofen.com/dc55_tech.htm Le Spirotechnique DC55<!-- Bot generated title -->]</ref>

:<math>F_{O_2loop}=\frac{(P_{amb}*K_{bellows}*K_E+1)F_{O_2feed}-1}{P_{amb}*K_{bellows}*K_E}</math>

Where:
:<math>P_{amb}</math> = Ambient pressure,
:<math>K_{bellows}</math> = Bellows ratio - the ratio between the volume of expired air in the counterlungs and the amount dumped,
:<math>K_E</math> = Extraction ratio (ratio of minute ventilation and oxygen uptake), which usually falls in the range of 17 to 25 with a normal value of about 20 for healthy humans. Values as low as 10 and as high as 30 have been measured.<ref name="Morrison">{{cite book |author= Morrison, J.B; Reimers, S.D |title=Bennett and Elliott's Physiology and Medicine of Diving |edition= 3rd |publisher= Best Publishing Company |year= 1982 |ISBN= 0941332020}}</ref> Variations may be caused by the diet of the diver, the dead space of the diver and equipment, raised levels of carbon dioxide, or raised work of breathing and tolerance to carbon dioxide. 
:<math>F_{O_2feed}</math> = Oxygen fraction of the supply gas,
in a consistent system of units.

Oxygen consumption and feed rate are strongly related, and the steady state oxygen concentration in the loop is independent of oxygen uptake and is likely to remain within fairly close tolerances of the calculated value for a given depth.

The oxygen fraction of the gas in the circuit will approximate the feed gas more closely for greater depth.

The deficit between inhaled FO<sub>2</sub> and feed gas FO<sub>2</sub> is a function of the bellows ratio and depth.<ref name="pmid21384764">{{cite journal |author=Frånberg O, Ericsson M, Larsson A, Lindholm P |title=Investigation of a demand-controlled rebreather in connection with a diving accident |journal=Undersea & Hyperbaric Medicine : Journal of the [[Undersea and Hyperbaric Medical Society]], Inc |volume=38 |issue=1 |pages=61–72 |year=2011 |pmid=21384764 |url=http://archive.rubicon-foundation.org/9756 |accessdate=2013-05-16}}</ref> It is large near the surface and decreases with increase in depth. The inhaled FO<sub>2</sub> remains fairly steady at any depth for a considerable range of workload, although supply gas will be used more quickly at higher workloads. The deficit will vary at constant depth with the ratio of respiratory minute ventilation to rate of oxygen consumption, as will occur in hyper- or hypoventilation.<ref name="pmid21384764" />

This deficit can reduce the partial pressure of oxygen in the loop to levels that will not support life, particularly at shallow depths, and there is a risk that the diver may ascend to a depth where the mix will be hypoxic.
The variation in oxygen deficit means that the range of depth at which the rebreather is safe to dive is significantly less than for the same supply gas on open circuit.

==References==
{{Reflist}}

{{Underwater breathing apparatus}}

[[Category:Underwater diving]]