15-4 U.S. Navy Diving Manual — Volume 4 15-2.3.4 Diaphragm Assembly. A diaphragm assembly or counter lung is used in all closed circuit UBAs to permit free breathing in the circuit. The need for such devices can be readily demonstrated by attempting to exhale and inhale into an empty bottle. The bottle, similar to the recirculation system without a bag, is unyielding and presents extreme back pressure. In order to compensate, flexible diaphragms or a breathing bag must be placed in the UBA circuit with a maximum displacement equal to the combined volume of both lungs. Constant buoyancy is inherent in the system because the gas reservoir acts counter to normal lung action. In open-circuit scuba, diver buoyancy decreases during exhalation due to a decrease in lung volume. In closed-circuit scuba, expansion of the breathing bag keeps buoyancy constant. On inhalation, the process is reversed. This cycle is shown in Figure 15-3 . The flexible gas reservoir must be located as close to the diver’s chest as possible to minimize hydrostatic pressure differences between the lungs and the reservoir as the diver changes attitude in the water. Figure 15 - 3. UBA Breathing Bag Acts to Maintain the Diver ’s Constant Buoyancy by Responding Counter to Lung Displacement. 15-2.3.5 Recirculation System. Optimal performance of the recirculation system depends on proper maintenance of equipment, proper filling with fresh absorbent, and accurate metering of oxygen input. To ensure efficient carbon dioxide removal throughout the dive, personnel must carefully limit dive time to the specified canister duration. Any factor that reduces the efficiency of carbon dioxide removal increases the risk of carbon dioxide poisoning.

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