The Halcyon Passive, Variable Ratio-Biased Addition Semi-Closed rebreather is a unique design of semi-closed

rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen i ...

using a depth-compensated passive gas addition system.DUI Halcyon PVR-BASC Manual Passive addition implies that in steady state operation (at a constant depth) addition of fresh feed gas is a response to low volume of gas in the loop - the gas is injected when the top of the counterlung activates a demand type addition valve, which provides feed gas as long as the diver continues to inhale. The mechanism discharges gas to the environment in proportion to breathing volume to induce this gas feed. In the PVR-BASC the discharged gas volume is a function of depth and breath volume. The passive addition occurs during each inhalation as the loop volume is deficient by the amount discharged. The volumetric ratio of discharged gas to exhaled gas volume varies with depth, and decreases as the pressure increases. At the surface one quarter of the volume of an exhaled breath is discharged. As the ambient pressure increases with depth, the volume of the inner counterlumg is reduced and the reduced discharge both provides an oxygen addition more closely matched to usage, and saves a considerable amount of gas. Gas is added to the loop after the

scrubber Scrubber systems (e.g. chemical scrubbers, gas scrubbers) are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. An early application of a carbon dioxide sc ...

, and this means that the fresh gas is immediately available for breathing. It also means that the calculation of inhalation gas

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

fraction must take this into account. This also reduces exposure of the scrubber absorbent medium to cold gas, and increases dwell time of exhaled gas in the scrubber, both of which improve the efficiency of CO₂ absorption. Gas discharge takes place before the remaining exhaled gas reaches the scrubber, so the discharged gas does not waste scrubber capacity. The fresh gas is added when the counterlung cover bottoms out towards the end of a breath, so the fresh gas is largely inhaled into respiratory

dead space ''Dead Space'' is a science fiction/ horror media franchise created by Glen Schofield and Michael Condrey, developed by Visceral Games, and published and owned by Electronic Arts. The franchise's chronology is not presented in a linear format; e ...

, and most of the mixing occurs after exhalation. Excess gas due to expansion during ascent is vented through the overpressure relief valve. The proportionality of the discharge counterlung volume to depth is lost below a depth of around 90 m (10 bar) so the gas saving below this depth is not as great as for shallower dives. The slightly higher feed rates will push the partial pressure of oxygen in the loop closer to the feed gas value.Kellon, J. (1996), ''Passive Semi-Closed Circuit Rebreather Diver Manual'', International Association of Nitrox Technical Divers. (IANTD)

Counterlung

The PVR-BASC has a hinged

bellows A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtig ...

counterlung, similar to that of the Interspiro DCSC, and like the DCSC the top cover of the counterlung is weighted to improve breathing effort. However unlike the DCSC, the PVR-BASC uses an internal secondary bellows to discharge a portion of the exhaled gas during the inhalation part of the breathing cycle, when the main counterlung closes and pumps out the contents of the inner bellows. The reduction in volume of the inner counterlung with depth results in an approximation of a mass discharge proportional to breathing volume.

Harness

The PVR-BASC is designed to be carried on a standard Hogarthian

backplate and wing A backplate and wing (often abbreviated as BP&W or BP/W) is a type of scuba harness with an attached buoyancy compensation device (BCD) which establishes neutral buoyancy underwater and positive buoyancy on the surface. Unlike most other BCDs ...

harness.

Gas supply

The frame is designed to carry a

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on each side. The only restrictions on size are what the diver can carry, and the scrubber endurance. The cylinders are normally rigged with open circuit regulators in the standard DIR configuration for

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, and a supply hose with quick connector on each for the rebreather supply.

Loop oxygen fraction

The rebreather maintains a substantially constant breathing gas composition at any given depth regardless of the level of exertion, but gas consumption will vary with exertion.

Gallery

File:PVR-BASC BOV and breathing hoses P5167681.JPG, Halcyon PVR-BASC BOV/DSV and breathing hoses File:PVR-BASC Bailout valve P5167687.JPG, Halcyon PVR-BASC Bailout valve/Dive-surface valve File:PVR-BASC Bailout valve P5167688.JPG, Halcyon PVR-BASC BOV/DSV Bailout valve/Dive-surface valve File:PVR-BASC Bailout valve P5167689.JPG, Halcyon PVR-BASC BOV/DSV Bailout valve/Dive-surface valve File:PVR-BASC breathing hose connector P5167690.JPG, Halcyon PVR-BASC breathing hose connector File:PVR-BASC breathing hose connector P5167692.JPG, Halcyon PVR-BASC BOV/DSV breathing hose connector File:Halcyon PVR-BASC side view P5167697.JPG, Halcyon PVR-BASC BOV/DSV side view with cover File:Halcyon PVR-BASC top end view P5167699.JPG, Halcyon PVR-BASC BOV/DSV top end view with cover File:PVR-BASC P5167701.JPG, Halcyon PVR-BASC BOV/DSV with cover File:PVR-BASC P5167703.JPG, Halcyon PVR-BASC BOV/DSV with cover File:Halcyon PVR-BASC P5167705.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - closed, from top/front File:Halcyon PVR-BASC P5167706.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from above File:Halcyon PVR-BASC P5167708.JPG, Halcyon PVR-BASC semi-closed circuit rebreather -open from above File:Halcyon PVR-BASC P5167709.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from left File:Halcyon PVR-BASC P5167710.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from right File:Halcyon PVR-BASC P5167711.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from top/front File:Halcyon PVR-BASC P5167712.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from top/front File:Halcyon PVR-BASC P5167713.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open from top/front File:Halcyon PVR-BASC P5167715.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from left File:Halcyon PVR-BASC P5167716.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - open, from right File:Halcyon PVR-BASC scrubber P5167719.JPG, Halcyon PVR-BASC semi-closed circuit rebreather scrubber - assembled and mounted File:Halcyon PVR-BASC P5167720.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - bellows counterlung File:Halcyon PVR-BASC scrubber components P5167721.JPG, Halcyon PVR-BASC semi-closed circuit rebreather scrubber components File:Halcyon PVR-BASC scrubber components P5167722.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - Scrubber internal components File:Halcyon PVR-BASC sorb canister P5167724.JPG, Halcyon PVR-BASC semi-closed circuit rebreather absorbent canister File:Halcyon PVR-BASC scrubber casing interior P5167726.JPG, Halcyon PVR-BASC semi-closed circuit rebreather scrubber casing interior and secondary water trap File:Halcyon PVR-BASC scrubber spring P5167728.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - scrubber medium retaining screen and spring File:Halcyon PVR-BASC overpressure dump valve P5167729.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - overpressure dump valve File:Halcyon PVR-BASC counterlung detail P5167730.JPG, Halcyon PVR-BASC semi-closed circuit rebreather - bellows counterlung detail File:Halcyon PVR-BASC P5167731.JPG, Halcyon PVR-BASC semi-closed circuit rebreather File:Halcyon PVR-BASC breathing hose connector P5167732.JPG, Halcyon PVR-BASC semi-closed rebreather. breathing hose connector File:Halcyon PVR-BASC manual valves and drain pump P5167733.JPG, Halcyon PVR-BASC semi-closed rebreather Manual gas supply valves and loop drain pump File:Halcyon PVR-BASC underside P5167734.JPG, Halcyon PVR-BASC semi-closed rebreather - underside File:Halcyon PVR-BASC inderside P5167737.JPG, Halcyon PVR-BASC semi-closed rebreather - underside, showing gas supply connectors, hoses and manual valves, and loop drain pump File:Halcyon PVR-BASC underside P5167738.JPG, Halcyon PVR-BASC semi-closed rebreather underside, showing bolts for connection to backplate File:Halcyon PVR-BASC Breathing hose connector P5167746.JPG, Halcyon PVR-BASC semi-closed rebreather breathing hose connector File:Halcyon PVR-BASC breathing hose connector cover P5167745.JPG, Halcyon PVR-BASC semi-closed rebreather. breathing hose connector cover

References

{{Underwater diving, divequ Rebreathers