page_title: Rebreather
section_title: Passive addition semi-closed circuit
hierarchical_section_title: Rebreather / System variants / Semi-closed circuit rebreathers / Passive addition semi-closed circuit
caption_attribution_description: English: Schematic diagram of the breathing gas circuit of a passive addition semi-closed circuit rebreather 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
context_page_description: A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, and, for covert military use by frogmen or observation of underwater life, eliminating the bubbles produced by an open circuit system. A rebreather is generally understood to be a portable unit carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system.
  Rebreather technology may be used where breathing gas supply is limited, such as underwater or in space, where the environment is toxic or hypoxic, as in firefighting, mine rescue and high-altitude operations, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent or anaesthetic gases.
context_section_description: This type of rebreather works on the principle of adding fresh gas to compensate for reduced volume in the breathing circuit. A portion of the respired gas is discharged that is in some way proportional to usage. Generally it is a fixed volumetric fraction of the respiratory flow, but more complex systems have been developed which exhaust a close approximation of a ratio to the surface respiratory flow rate. These are described as depth compensated or partially depth compensated systems. Gas addition is triggered by low counterlung volume.
  The simple case of a fixed ratio discharge can be achieved by concentric bellows counterlungs, where the exhaled gas expands both the counterlungs, and while the larger volume outer bellows discharges back to the loop when the diver inhales the next breath, the inner bellows discharges its contents to the surroundings, using non return valves to ensure a one-directional flow.  The amount processed during each breath depends on the tidal volume of that breath.
  Towards the end of inhalation the bellows bottoms out and activates an addition valve, in much the way that a regulator diaphragm activates the demand valve, to make up the gas discharged by the inner bellows. This type of rebreather therefore tends to operate at a minimal volume.
  The fixed ratio systems usually discharge between 10% (1/10) and 25% (1/4) of the volume of each breath overboard. As a result, gas endurance is from 10 times to four times that of open circuit, and depends on breathing rate and depth in the same way as for open circuit. Oxygen fraction in the loop depends on the discharge ratio, and to a lesser extent on the breathing rate and work rate of the diver. As some gas is recycled after breathing, the oxygen fraction will always be lower than that of the make-up gas, but can closely approximate the make-up gas after a loop flush, so the gas is generally chosen to be breathable at maximum depth, which allows it to be used for open circuit bailout. The loop gas oxygen fraction will increase with depth, as the mass rate of oxygen used metabolically remains almost constant with a change in depth. This is the opposite tendency of what is done in a closed circuit rebreather, where the oxygen partial pressure is controlled to be more or less the same within limits throughout the dive. The fixed ratio system has been used in the DC55 and Halcyon RB80 rebreathers. Passive addition rebreathers with small discharge ratios may become hypoxic near the surface when moderate oxygen fraction supply gas is used.
  The depth compensating systems discharge a portion of the diver's tidal volume which varies in inverse proportion to the absolute pressure. At the surface they generally discharge between 20% (1/5) and 33% (1/3) of each breath, but that decreases with depth, to keep the oxygen fraction in the loop approximately constant and reduce gas consumption. A fully depth compensated system will discharge a volume of gas, inversely proportional to pressure, so that the volume discharged at 90m depth (10 bar absolute pressure) will be 10% of the surface discharge. This system will provide an approximately fixed oxygen fraction regardless of depth, when used with the same make-up gas, because the effective mass discharge remains constant.
  Partially depth compensating systems are part way between the fixed ratio and the depth compensating systems. They provide a high discharge ratio near the surface, but the discharge ratio is not fixed either as a proportion of respired volume or mass. Gas oxygen fraction is more difficult to calculate, but will be somewhere between the limiting values for fixed ratio and fully compensated systems. The Halcyon PVR-BASC uses a variable volume inner bellows system to compensate for depth.
caption: Schematic diagram of the breathing gas circuit of a passive addition semi-closed circuit rebreather.1 Dive/surface valve with loop non-return valves2 Exhalation hose3 Counterlung fore-chamber4 Non-return valve to discharge bellows5 Discharge bellows6 Overpressure valve7 Main counterlung bellows8 Addition valve9 Scrubber (axial flow)10 Inhalation hose11 Breathing gas storage cylinder12 Cylinder valve13 Regulator first stage14 Submersible pressure gauge15 Bailout demand valve
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