- 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 discharged. The volumetric ratio of discharged gas to exhaled gas volume varies with depth, and decreases as the pressure increases.[2]
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 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, 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.[3]
