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Same as MOD 0, plus increased cold water duration capability.

Same as MOD 0.

Same as MOD 0.

18-3.1 Gas Flow Path. The gas flow path of the MK 25 UBA is shown in Figure 18-2.

The gas is exhaled by the diver and directed by the mouthpiece one-way valves into the exhalation hose. The gas then enters the carbon dioxide-absorbent canister, which is packed with a NAVSEA-approved carbon dioxide-absorbent material. The carbon dioxide is removed by passing through the CO2-absorbent bed and chemically combining with the CO2-absorbent material in the canister. Upon leaving the canister the used oxygen enters the breathing bag. When the diver inhales, the gas is drawn from the breathing bag through the inhalation hose and back into the diver's lungs. The gas flow described is entirely breath activated. As the diver exhales, the gas in the UBA is pushed forward by the exhaled gas,

Figure 18-2. Gas Flow Path of the MK 25.

18-3.1.1 Breathing Loop. The demand valve adds oxygen to the breathing bag of the UBA from the oxygen cylinder only when the diver empties the bag on inhalation. The demand valve also contains a manual bypass knob to allow for manual filling of the breathing bag during rig setup and as required. There is no constant flow of fresh oxygen to the diver. This feature of the MK 25 UBA makes it essential that nitrogen be purged from the apparatus prior to the dive. If too much nitrogen is present in the breathing loop, the breathing bag may not be emptied and the demand valve may not add oxygen even when metabolic consumption by the diver has reduced the oxygen in the UBA to dangerously low levels (see paragraph 18-2.2).

Table 18-2. Average Breathing Gas Consumption.

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