Construction of Counterlung

The counterlung is built into an insulated body to both constrain the counterlung and minimise the surface area of the thin external membrane and thus reduce heat loss.

The diaphragm is connected to the body by a hinge so that it pivots and the assembly acts as a bellows. An internal membrane separates the supply and return chambers of the counterlung. The diaphragm and the internal membrane work together to ensure that the volume of gas in the supply chamber is always the same as the volume of gas in the return chamber. This volume is in turn determined by the angle of the diaphragm.

The pressure drop in the hoses from the supply chamber to the diver and back to the return chamber means that the pressure in the supply chamber is always higher than in the return chamber. If the two chambers were not constrained the supply chamber would tend to be always full and the return chamber always empty. With the two chambers constrained so that they fill and empty together, the work of breathing of the reclaim system is reduced significantly when compared to the counterlung’s use as part of a rebreather.

Supply and Return Shut Off Valves

A shut off valve in the supply chamber of the double counterlung shuts off the flow of gas from the supply umbilical hose when the counterlung is full. This valve remains fully open when the breathing circuit has the optimum volume of gas. If there is more than the optimum volume of gas in the reclaim circuit the counterlung becomes full at the end of exhalation and shuts off the supply before gas is vented from the counterlung through the over pressure valve. The supply shut off valve opens as the diver starts to inhale.

The size of the diaphragm and the mechanical advantage that it provides means a very small increase in pressure in the counterlung is required to hold the shut off valve closed.

In a similar way the return chamber has a shut off valve that shuts off the flow of gas to the return umbilical hose when the counterlung is empty. Again, because of the diaphragm size and the mechanical advantage, it takes a very small increase in pressure to open the return shut off valve.

When either of the valves shuts off, the drop in flow is detected on the surface and used as a signal to add or vent gas from the circuit as appropriate and so maintain the optimum volume of gas in the reclaim circuit.

Counterbalance Weight to Minimise Hydrostatic Imbalance

Hydrostatic imbalance is the difference between the water pressure acting on the outside of the lungs and the pressure in the lungs at the end of exhalation. The effect that this has can be experienced by standing up to your neck in water so that the pressure acting on the lungs is greater than the pressure of the air that you are breathing.

To minimise hydrostatic imbalance, the counterlung is placed as close to the diver’s lungs as possible. The hydrostatic imbalance is further reduced by mounting a counterweight on the counterlung diaphragm. When the diver is facing down, the water pressure acting on the lungs is greater than the water pressure acting on the counterlung so the counterweight pushes down on the diaphragm and so increases the pressure in the counterlung to compensate. Similarly, when the diver is on his back, the pressure of the water on the counterlung is higher than the water pressure on the lungs so the counterweight pulls on the diaphragm, reducing the pressure in the counterlung to compensate.