Figure 17-2. MK 16 MOD 0 UBA Functional Block Diagram.
17-2.1 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 diaphram assembly.
17-2.1.1 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.
17-2.1.2 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
17-2.1.3 Carbon Dioxide Scrubber. Carbon dioxide is removed from the breathing circuit in a watertight canister filled with a NAVSEA-approved carbon dioxide-absorbent material located in the backpack of the UBA. 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. Inadvertent wetting of the absorbent material produces a caustic solution. Water produced by the reaction between carbon dioxide and the carbon dioxide absorbent, or by the diver himself, is collected by moisture absorbent pads above and below the canister. A major limiting factor for the MK 16 is the CO2 absorbent capability. Absorbent duration is directly related to the environmental operating temperature and depth. Absorbent duration decreases as temperature decreases and as depth increases.
The canister design must provide low flow resistance while ensuring maximum contact between the gas and the absorbent. Flow resistance is minimized in the MK 16 UBA by employing a radially-designed canister to reduce gas flow distance. If the canister is improperly filled, channels may be formed through the absorbent granules permitting the gas to bypass the absorbent and allowing carbon dioxide to build up in the UBA.
17-2.1.4 Diaphram Assembly. A diaphram 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 17-3.
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.
The MK 16 UBA uses a single reservoir built into a streamlined backpack assembly. Using a single reservoir located within the backpack affords minimum encumbrance to the diver and maximum protection for the reservoir.
17-2.1.5 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
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