The "on demand principle" is a common way of adding oxygen
to the counter lung. The system is in use since 1942, direct after Emile
Gagnan and Cousteau's introduction op the IP regulator. When a certain under
pressure is achieved in the Cl the regulator will add oxygen until pressure
difference closes the valve. Split counter lungs are used to lower the WOB
(work of breathing). The hydrostatic pressure helps by means of an exhale
lung to press the gas through the scrubber. When the diver needs to breath,
he breathes direct from the pre filled inhale counterlung.