Oxygen feed options

n some rebreathers, e.g. the Siebe Gorman Salvus, the oxygen cylinder has oxygen supply mechanisms in parallel. One is constant flow; the other is a manual on-off valve called a bypass valve; both feed into the same hose which feeds the counterlung.In the Salvus there is no second stage and the gas is turned on and off at the cylinder.

Others such as the USN Mk25 UBA are supplied via a demand valve on the counterlung. This will add gas at any time that the counterlung is emptied and the diver continues to inhale. Oxygen can also be added manually by a button which activates the demand valve.

Some simple oxygen rebreathers had no automatic supply system, but only the manual feed valve, and the diver had to operate the valve at intervals to refill the breathing bag as the volume of oxygen decreased below a comfortable level.