Oxygen levels in closed circuit UBAs during descent

Closed circuit rebreathers used by divers recirculate the respiratory gases through a scrubber to remove exhaled carbon dioxide. For systems used in deep diving applications, oxygen is mixed with a diluent gas to maintain a preset partial pressure of oxygen (PO2) that is within the physiologically acceptable limits for the diver. These systems accurately maintain the oxygen set point while the diver swims at constant depth. However, as a diver increases depth, the gas volume in his lungs and his underwater breathing apparatus (UBA) will be compressed. To avoid damage, a diluent gas, a breathable mixture of oxygen and some inert component such as nitrogen or helium, must be supplied to the breathing circuit to maintain constant volume. Because the composition of this diluent will normally be different from the desired circuit breathing mixture, it is important that the addition of this diluent will not have harmful physiological effects on the diver. The effects of UBA circuit volume, diver descent rate, oxygen consumption rate, and diluent composition on the oxygen partial pressures within the breathing circuit during descent are investigated in this article. Exposures to oxygen partial pressures in excess of 2.0 atmospheres absolute (ata) are shown to be possible during dives when using closed circuit UBAs with present PO2 levels of 0.7 ata. These elevated oxygen exposures are shown to diminish when a diver reaches depth, but could last in excess of 18 min during a dive to 200 ft of seawater.