Halcyon PVR-BASC 1 Halcyon PVR-BASC Schematic diagram of the breathing loop and gas supply pneumatics of the Halcyon PVR-BASC Rebreather.1 DSV/BOV with non-return valves 2 Exhalation hose 3 Primary water trap 4 Manual drain pump 5 Overpressure relief valve 6 Main bellows counterlung 7 Discharge bellows counterlung (variable volume) 8 Discharge valve 9 Axial flow scrubber with secondary water trap 10 Feed gas demand valves 11 Inhalation hose 12 Feed gas supply cylinder 13 Cylinder valve 14 High pressure regulator (1st stage) 15 Submersible pressure gauge 16 Intermediate pressure gas connectors 17 Drain port for cleaning and drying counterlung The Halcyon Passive, Variable Ratio-Biased Addition Semi-Closed rebreather is a unique design of semi-closed rebreather using a depth-compensated passive gas addition system. [1] 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 disharged. 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, and this means that the fresh gas is immediately available for breathing. It also means that the calculation of inhalation gas oxygen 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 CO 2 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, 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 90m (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. [2]
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Halcyon PVR-BASC 1
Halcyon PVR-BASC
Schematic diagram of the breathing loop and gas supply pneumatics of the HalcyonPVR-BASC Rebreather.1 DSV/BOV with non-return valves 2 Exhalation hose 3 Primarywater trap 4 Manual drain pump 5 Overpressure relief valve 6 Main bellows counterlung
7 Discharge bellows counterlung (variable volume) 8 Discharge valve 9 Axial flowscrubber with secondary water trap 10 Feed gas demand valves 11 Inhalation hose 12Feed gas supply cylinder 13 Cylinder valve 14 High pressure regulator (1st stage) 15
Submersible pressure gauge 16 Intermediate pressure gas connectors 17 Drain port forcleaning and drying counterlung
The Halcyon Passive, VariableRatio-Biased Addition Semi-Closedrebreather is a unique design ofsemi-closed rebreather using adepth-compensated passive gasaddition system.[1] Passive additionimplies that in steady state operation(at a constant depth) addition of freshfeed gas is a response to low volume ofgas in the loop - the gas is injectedwhen the top of the counterlungactivates a demand type addition valve,which provides feed gas as long as thediver continues to inhale. Themechanism discharges gas to theenvironment in proportion to breathingvolume to induce this gas feed.
In the PVR-BASC the discharged gasvolume is a function of depth andbreath volume. The passive additionoccurs during each inhalation as theloop volume is deficient by the amountdisharged. The volumetric ratio ofdischarged gas to exhaled gas volumevaries with depth, and decreases as thepressure increases.
At the surface one quarter of thevolume of an exhaled breath is discharged. As the ambient pressure increases with depth, the volume of the innercounterlumg 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, and this means that the fresh gas is immediately available for breathing. Italso means that the calculation of inhalation gas oxygen fraction must take this into account. This also reducesexposure of the scrubber absorbent medium to cold gas, and increases dwell time of exhaled gas in the scrubber, bothof which improve the efficiency of CO2 absorption.
Gas discharge takes place before the remaining exhaled gas reaches the scrubber, so the discharged gas does notwaste 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, 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 90m (10 bar) so thegas saving below this depth is not as great as for shallower dives. The slightly higher feed rates will push the partialpressure of oxygen in the loop closer to the feed gas value.[2]
CounterlungThe PVR-BASC has a hinged bellows counterlung, similar to that of the Interspiro DCSC, and like the DCSC the topcover of the counterlung is weighted to improve breathing effort. However unlike the DCSC, the PVR-BASC usesan internal secondary bellows to discharge a portion of the exhaled gas during the incalation part of the preathingcylcle, when the main counterlung closes and pumps out the contents of the inner bellows. The reduction in volumeof the inner counterlung with depth results in an approximation of a mass discharge proportional to breathingvolume.
HarnessThe PVR-BASC is designed to be carried on a standard Hogarthian backplate and wing harness.
Gas supplyThe frame is designed to carry a cylinder on each side. The only restrictions on size are what the diver can carry, andthe scrubber endurance. The cylinders are normally rigged with open circuit regulators in the standard DIRconfiguration for bailout, and a supply hose with quick connector on each for the rebreather supply.
Loop oxygen fractionThe rebreather maintains a substantially constant breathing gas composition at any given depth regardless of thelevel of exertion, but gas consumption will vary with exertion.
References[1][1] DUI Halcyon PVR-BASC Manual[2] Kellon, J. (1996), Passive Semi-Closed Circuit Rebreather Diver Manual, International Association of Nitrox Technical Divers. (IANTD)