Passive addition semi-closed circuit

This type of rebreather works on the principle of adding fresh gas to compensate for reduced volume in the breathing circuit. A portion of the respired gas is discharged that is in some way proportional to usage. Generally it is a fixed volumetric fraction of the respiratory flow, but more complex systems have been developed which exhaust a close approximation of a ratio to the surface respiratory flow rate. These are described as depth compensated or partially depth compensated systems. Gas addition is triggered by low counterlung volume.