Mixed gas manual closed circuit rebreather.
Manual addition of oxygen only as it is needed to compensate diver´s actually metabolic consumption rate, automatically addition of diluent, usually air, as descends to replace lost volume by increasing ambient pressure.
Three oxygen sensors display the PPO2 in the loop, but there is no electronic controll in the system.
The main computer is the diver´s brain.
MOD is based on diluent.
Dive time is limited by oxygen exposure.