Maintaining safe oxygen levels in semiclosed underwater breathing apparatus.

Semiclosed circuit underwater breathing apparatus (UBAs) are far more conserving of limited gas supplies than either steady-flow, open circuit or demand, open circuit systems. Semiclosed systems recirculate most of the diver's breathing gases, while injecting only a small percentage of the total circuit flow with a fresh makeup supply. The fresh, injected gas supply delivers the necessary makeup to compensate for the oxygen that the diver consumes during each pass through the lungs. This gas-conserving feature makes the utility for semiclosed UBAs for long-duration, deep diving applications more practical than either of the open circuit designs. However, although more gas conservative, these systems maintain variable oxygen levels during a dive that can fluctuate widely with changes in the diver's activity level, depth of the dive, makeup gas composition, or variations in the total circuit volume. It is essential that the oxygen levels in the circuit always remain within physiologically safe limits; neither too low to avoid the onset of hypoxia, nor too high to risk oxygen toxicity. This article presents an analytical assessment of the oxygen levels in semiclosed UBAs. This model allows the designers of semiclosed circuits to predict the effects of UBA circuit volumes, makeup gas compositions, injection rates for the makeup gas, diver depths, and initial circuit oxygen levels on the time-dependent levels of oxygen in these circuits. By using this analytical tool, judicious selection of these design variables can result in the most efficient utilization of the fresh gas supply carried by the diver, and ensure the safe operation of these UBA designs.