The Interspiro DCSC (Demand Controlled Semi-Closed Circuit) is a semi-closed circuit nitrox rebreather manufactured by Interspiro of Sweden for military applications. Interspiro, formerly a division of AGA, has been manufacturing self-contained breathing apparatus for diving, firefighting, and rescue applications since the 1950s.
History
The first Interspiro rebreather was the ACSC (alternating closed and semi-closed circuit rebreather), developed and marketed in the 1980s. In the 1990s, this design was developed further to become the DCSC, also intended for mine countermeasures operations.
Construction
The gas supply is carried in a 5-litre, 200-bar aluminium cylinder mounted horizontally at the bottom of the unit, with the valve to the diver's left. The reserve valve and bypass valve are also on the left. The fairing case holding the components is clipped to a tubular harness frame and can be released by pulling a knob on the lower right.
The scrubber is a radial-flow cylindrical design with inward flow, carrying a 2.5 kg charge of absorbent. The counterlung is a wedge-shaped bellows hinged on the lower edge, with the angle between the top and bottom covers proportional to the internal volume. The change in top plate angle as the diver breathes controls the gas addition mechanism. The bellows volume is approximately 4.5 litres, and the total loop volume is approximately 7 litres.
The approved operating depth range is from 0 to 57 metres. Nitrox 28% is used for depths below about 30 metres, and 46% for shallower depths. The unit has a mass of approximately 33 kg.
Operating Principle
The DCSC is an active-addition semi-closed circuit rebreather, but it has more in common with passive-addition systems in that the amount of feed gas supplied is a function of the diver's breathing rate. Unlike most passive-addition rebreathers, the gas feed mass flow rate is independent of depth, and unlike most active-addition systems, it is not a constant mass flow.
The Interspiro DCSC is the only rebreather using this gas mixture control principle that has been marketed. The principle of operation is to add a mass of oxygen proportional to the volume of each breath. This approach is based on the assumption that the volumetric breathing rate of a diver is directly proportional to metabolic oxygen consumption, which experimental evidence indicates is close enough to work.
Fresh gas addition is made by controlling the pressure in a dosage chamber proportional to the counterlung bellows volume. The dosage chamber is filled with fresh gas to a pressure proportional to bellows volume, with the highest pressure when the bellows is in the empty position. When the bellows fills during exhalation, gas is released from the dosage chamber into the breathing circuit, proportional to the volume in the bellows during exhalation, and is fully released when the bellows is full. Excess gas is dumped to the environment through the overpressure valve after the bellows is full.
The volume of the dosage chamber is matched to a specific supply gas mixture and is changed when the gas is changed. The DCSC uses two standard nitrox mixtures (28% and 46%) and has two corresponding dosage chambers.
Alarms and Warnings
To prevent a silent feed-gas supply failure that could lead to hypoxia, there is a controllable flow restriction in the inhalation side of the loop, operated by pressure from the supply gas in the dosage mechanism. If the supply pressure falls, the flow warning system imposes a restriction to the inhalation gas flow, similar to the effect of a low supply pressure on an open-circuit demand valve. The diver can then activate the reserve mechanism on the cylinder valve, which allows the last 25 bar from the cylinder to be used and de-activates the warning restriction.
Oxygen Partial Pressure in the Breathing Loop
The gas calculation for the DCSC differs from other semi-closed circuit rebreathers. The steady-state oxygen fraction in the breathing circuit can be calculated from the formula:
FO₂loop = (Qfeed × FO₂feed − VO₂) / (Qfeed − VO₂)
where Qfeed is the flow rate of fresh gas supplied, FO₂feed is the oxygen fraction of the supply gas, and VO₂ is the oxygen uptake flow rate of the diver.
The analysis shows that the oxygen fraction in the loop has no dependency on depth or on oxygen uptake, and since the dosage ratio is constant once the gas has been selected, the remaining variations are due to variations in the extraction ratio. This means the DCSC has theoretically the most stable oxygen fraction of the semi-closed rebreathers and is a reasonable approximation of open circuit for decompression purposes.
The unit has been used by the Swedish armed forces for over 15 years with a good safety record. However, a large decompression stress when using air tables for decompression on dives using a 28% nitrox supply gas has been indicated by the presence of high venous gas emboli (VGE) scores post-dive. Oxygen fraction in the loop was not monitored during these tests.
Gas Endurance
The reserve valve is activated at about 25 bar. A 5-litre cylinder at 200 bar provides about 875 litres of free gas at 1 bar available for the dive. At a respiratory minute volume (RMV) of 30 L/min for a diver working moderately hard, using 28% nitrox with a dosage ratio of 0.6, the gas will last approximately 48 minutes. The 46% nitrox with a dosage ratio of 0.3 will last approximately 97 minutes. A 15 L/min RMV for light work will double these times.
Scrubber Endurance
The scrubber capacity is 2.5 kg of soda lime. Using a conservative value of 100 litres of CO₂ per kg, the capacity of the scrubber is 250 litres of CO₂. At an extraction rate of 1/20 and a dosage rate of 0.3, some 146 litres of carbon dioxide may be produced by the diver, showing that endurance is not limited by the scrubber.
Successor
Interspiro later developed the IS-Mix rebreather, a complete mine clearance system built on the well-proven ACSC and DCSC systems, incorporating more than 30 years of experience.
