The BK2 - a novel depth-compensated passive addition rebreather (by Jürgen Bohnert)
Bo. August 12, 2001

There can be no doubt that passive addition rebreathers have paid off with the best safety record in cave diving due to the simple and reliable mechanism of operation. This sort of rebreathers is based on the simple fact that breathing rate and oxygen consumption are linearly linked. By dumping a certain percentage of gas during each breathing cycle and replacing it with fresh gas via a regulator the oxygen fraction of the breathing mix will remain fairly constant at a given depth. Since a constant dump fraction of e.g. 10 % independent of depth will lead to a dangerously low oxygen fraction near the surface, the best mode of operation for a passive rebreather is using a depth-compensation. This means that a relatively large amount of gas is dumped at the surface (usually 25 to 33 %) while proportionally less is dumped at greater depths (e.g. 10 % at 90 m).

The novel rebreather BK2 makes use of this principle. It was designed by HFGOK-members Jürgen Bohnert and Andreas Kücha for penetrating longer cave sumps. Given the fact that the BK2 is a fully functional depth-compensated rebreather its size is remarkably small. It therefore allows for a double rebreather arrangement mounted on the back. The double BK2 setup has been extensively used both for deep open-water dives as well as for some longer cave dives on the Suabian Jura. It is capable of at least 8 h of operation and provides the diver with a fully functional bail-out rebreather. This rebreather automatically removes water which could theoretically enter the loop via minor leakages and thus provides the diver with a fully operational system should the other rebreather fail.
A BK2-MPEG-Video (36 sec, 5,6 MB)  can be downloaded from this site showing the dual rig in action.


Photo: Jürgen Bohnert

Andreas Kücha before a dive with the double BK2 setup. The drive gas is stored in two 20 liter nitrox tanks attached to the rig. Stage tanks can easily be connected to the rebreathers by quick release couplings.


Photo: Jürgen Bohnert

The BK2 double rig is being checked by Andreas Kücha prior to a dive. Since the rebreathers are not relying on oxygen sensors or any sort of electronic gear the task load ist dramatically dimished. The work of breathing is very low and compares best to a standard open circuit regulator. The whole rig is about the same size as the old Halcyon rig but consists of two independent rebreathers.


Photo: Andreas Kücha

Getting ready to go for a longer cave dive into a resurgence on the Suabian Jura using the double BK2. Cave dives on a rebreather offer several obvious advantages over open circuit gear. Only a minor amount of body heat is lost due to the recycling of gas, moreover additional heat is produced by the soda lime thus enabling very long exposures. Less silt is stirred up from the cave ceiling since only a small fraction of bubbles is generated compared to an open circuit system. And last but not least: in an emergency (e.g. loosing the line, scooter breakdown) the huge amount of gas provides the diver with a considerable safety margin to sort things out.


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