Counterlung equipment

Some important devices are attached directly to the counterlungs.  The automatic diluent valve (ADV), overpressure valve, manual bypass valves.

Counterlung equipment

The ADV is seen in position 1. As you descend, the water pressure increases and compresses the air in the counterlungs (and of course in your lungs) to an increasingly smaller and smaller volume to the point that there is not enough air in the loop when you want to inhale. At that moment, the sequence comes to the ADV. The empty counterlung  is completely compressed, thus pressing the valve button on the inner side of the ADV, causing diluent to flow into the counterlung. The volume of air in the circuit is replenished and you can continue your descent without interruption.

Some divers leave the ADV in operation throughout the entire time of a dive. They use it only for descending to depth, then close it and during the dive add diluent as needed by means of the button on the manual diluent bypass valve (2).

Conversely, ambient pressure decreases as you ascend. Air expands and you suddenly have too much; the counterlungs are fully inflated, but you still need to exhale. For this purpose, the Liberty has an overpressure valve (3) which starts to expel air into the water when reaching the defined overpressure in the loop. This is the only situation when bubbles are seen with divers using a rebreather.

The manual oxygen bypass valve (4) is situated above the overpressure valve. In older types of rebreathers, such a valve was used in the last few meters of ascent. The diver switched off automatic oxygen regulation and added oxygen manually because rebreathers had a tendency to overinflate in small depths. Of course, the CCR Liberty’s control unit knows that at a depth of three meters it is not possible to achieve 1.6 bars of partial pressure of oxygen and thus will not try to do that. However, by expelling all gas from the breathing loop and manually adding oxygen, at the final shallow decompression stop you can accelerate the transition to (nearly) pure oxygen, which is necessary for effective decompression.

All of the valves are attached with a bayonet connector and can be easily removed from the counterlungs during maintenance and cleaning; just turn and pull. The breathing-loop hoses are attached to the head and counterlungs in the same manner. Securing these components against accidental turning and removal when diving is simple – the turn angle is 1/3 revolution and this suffices to ensure that turning is reliable prevented by the length of the connected hose when you are wearing the apparatus. Mechanical safeguards against turning are used only on the connector for the overpressure valve, which you can rotate during a dive and thus set the opening overpressure, and on the connector for the manual diluent bypass valve, from which the hose can be disconnected during a dive by turning the valve downward and then connecting offboard gas, which is a supply of gas in a separate pressure tank used during the most extreme dives.

See also Breathing loop, Water trap and Gas distribution

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