Gas Addition, Exhaust, and Monitoring. In addition to the danger of carbon dioxide toxicity, the closed-circuit UBA diver encounters the potential hazards of hypoxia and central nervous system (CNS) oxygen toxicity (see Volume 5). It is essential that these hazards be avoided. The UBA must control the partial pressure of oxygen (ppO2) in the breathing medium within narrow limits for safe operation and be monitored frequently by the diver.
Hypoxia can occur when there is insufficient oxygen in the recirculation circuit to meet metabolic requirements. If oxygen is not added to the breathing circuit, the oxygen in the loop will be gradually consumed over a period of 2-5 minutes, at which point the oxygen in the mixture is incapable of sustaining life.
CNS oxygen toxicity can occur whenever the oxygen partial pressure in the diver's breathing medium exceeds specified concentration and exposure time limits. Consequently, the UBA must function to limit the ppO2 level to the appropriate value.
The closed-circuit mixed-gas UBA uses a direct control method of maintaining oxygen concentration in the system, rather than the indirect method of a preset mass flow, common to semi-closed apparatus.
17-2.3 Advantages of Closed-Circuit Mixed-Gas UBA. While functionally simpler in principle, the closed-circuit mixed-gas UBA tends to be more complex than the semi-closed UBA because of the oxygen analysis and control circuits required. Offsetting this complexity, however, are several inherent advantages:
Aside from mixed or diluent gas addition during descent, the only gas required at depth is oxygen to make up for metabolic consumption.
The partial pressure of oxygen in the system is automatically controlled throughout the dive to a preset value. No adjustment is required during a dive for variations in depth and work rate.
No inert gas leaves the system except by accident or during ascent, making the closed-circuit UBA relatively bubble-free and well-suited for SPECWAR and EOD operations requiring low acoustic signature.
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