Best known by the acronym pSCR, Passive Semi Closed Rebreather, this rebreather is also identified by the CMR acronym, Constant Mass Ratio rebreather and CVR, Constant Volume Ratio rebreather.
A distinctive feature of the pSCR is its completely mechanical operation controlled by the diver's breathing. In a nutshell, at every inhalation a percentage of exhaust gas is eliminated. The cycle is repeated until the reduction in the volume occupied by the gas triggers the injectors which refill it. This occurs on average every 3 or 4 breaths when remaining at a constant depth.
The pSCR is a simple and intuitive rebreather as it is the diver's breath that controls the expulsion of the exhaust gas and its refilling. By its nature, it is a particularly solid piece of equipment, also due to the absence of any electrical or electronic part; it is intended for cave diving or particularly long dives.
The pSCR is a rebreather because it recycles a portion of the exhaled gas; it is semi-closed because it does not recycle all the exhaled gas and it is passive since it expels and supplies gas only according to the diver's breathing.