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All rebreathers contain the following minimum ele- ments: • Pressure reducing regulator(s) • At least one flexible inhalation bag (breathing bag) • Large diameter, low-pressure breathing hoses • Scrubber canister and some type of carbon dioxide- absorbent system (either a chemical or mechanical device) Another highly desirable element for rebreathers is a carbon dioxide sensor. The inability to monitor carbon Rebreathers 14-3 FIGURE 14.6 Closed-Circuit Mixed-Gas System SCHEMATIC dioxide in rebreathers has been a significant problem in the design of rebreathers to date. A carbon dioxide sensor allows direct monitoring of the CO 2 level in the breathing loop and provides valuable information for the calculation of a diver’s decompression profile. Recent advances in car- bon dioxide sensor design may someday allow such sen- sors to be a standard component of all rebreathers (see Figure 14.9). 14.3 HOW REBREATHERS WORK Although normal air contains 20.9 percent oxygen, the body does not consume all of the oxygen inhaled. How much oxygen is used in each breath depends primarily on how hard the diver is working. As the diver moves into deeper water and the partial pressure of oxygen increases, the actual percentage of oxy- gen in the gas mixture needs to be reduced. For example, a saturation diver working at 400 fsw (122 msw) might be breathing a gas mixture of 96% helium and only 4% oxy- gen. By reusing the inert gas in the mixture and replacing the small amount of oxygen that is metabolically con- sumed, a diver using a closed-circuit system can stay under water for a significant period of time. Most rebreathers use a chemical absorbent that is based upon a substance known as “soda lime” (see Figure 14.10). The chemical reaction of combining the carbon dioxide in the diver’s exhaled breath with the absorbent produces heat and moisture as by-products, both of which are generally beneficial to the diver. The heat can be useful in helping to keep the diver’s core temperature warm while the moisture helps prevent dehydration. 14-4 NOAA Diving Manual FIGURE 14.8 MK 16 Closed-Circuit System with Shroud Removed FIGURE 14.7 MK 16 Closed-Circuit System FIGURE 14.9 Oxygen Galvanic Sensor and Carbon Dioxide Sensor FIGURE 14.10 Chemical Absorbent O 2 Sensor CO 2 Sensor

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