The use of manual addition in oxygen rebreathers is the oldest form of adding oxygen to the loop. When the diver is breathing oxygen is metabolised and the counter lung volume will decrease. To keep the counter lung filled oxygen must be added. In the older models the diver had to press a button to increase the volume. Modern methods of adding oxygen to the loop use the principle of constant mass flow or on demand systems. A disadvantage of the manual  system was the risk of a malfunction of the addition system. The diver would surface due to a too big volume in the loop or suffer a lack of oxygen. Both are not wanted for hidden operations.