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CHAPTER 15 — EC-UBA Diving 15-3 n The diver strictly adheres to and immediately implements all operational and emergency procedures. 15-2.2 Advantages of EC-UBA. While functionally simpler in principle, the EC-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: n Aside from mixed or diluent gas addition during descent, the only gas required at depth is oxygen to make up for metabolic consumption. n 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. n No inert gas leaves the system except by accident or during ascent, making the closed circuit UBA relatively bubble-free and well suited for EOD and SPECWAR operations. 15-2.3 Recirculation and Carbon Dioxide Removal. The diver’s breathing medium is recirculated in a closed circuit UBA to remove carbon dioxide and permit reuse of the inert diluent and unused oxygen in the mixture. The basic recirculation system consists of a closed loop that incorporates inhalation and exhalation hoses and associated check valves, a mouthpiece or full face mask (FFM), a carbon dioxide removal unit, and a diaphragm assembly ( Figure 15-2 ). 15-2.3.1 Recirculating Gas. Recirculating gas is normally moved through the circuit by the natural inhalation-exhalation action of the diver’s lungs. Because the lungs can produce only small pressure differences, the entire circuit must be designed for minimum flow restriction. 15-2.3.2 Full Face Mask. The FFM uses an integral oral-nasal mask or T-bit to reduce dead space and the possibility of rebreathing carbon dioxide-rich gas. Similarly, check valves used to ensure one-way flow of gas through the circuit must be close to the diver’s mouth and nose to minimize dead space. All breathing hoses in the system must be of relatively large diameter to minimize breathing resistance. 15-2.3.3 Carbon Dioxide Scrubber. Carbon dioxide is removed from the breathing circuit in a watertight canister filled with an approved carbon dioxide-absorbent material. The bed of carbon dioxide-absorbent material chemically combines with the diver’s exhaled carbon dioxide, while allowing the unused oxygen and diluent to pass through it. If the canister is improperly filled, channels may form in the absorbent granules permitting gas to bypass the absorbent material and allow the build up of carbon dioxide in the UBA. The canister design must also provide a low flow resistance for the gas while ensuring maximum contact between the gas and the absorbent.

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