The term closed-circuit oxygen rebreather describes a specialized type of underwater breathing apparatus (UBA). In this type of UBA, all exhaled gas is kept within the rig. As it is exhaled, the gas is carried via the exhalation hose to an absorbent canister through a carbon dioxide-absorbent bed that removes the carbon dioxide by chemically reacting with the carbon dioxide produced as the diver breathes. After the unused oxygen passes through the canister, the gas travels to the breathing bag where it is available to be inhaled again by the diver. The gas supply used in such a rig is pure oxygen, which prevents inert gas buildup in the diver and allows all of the gas carried by the diver to be used for metabolic needs. Closed-circuit oxygen UBAs offer advantages valuable to special warfare, including stealth (no escaping bubbles), extended operating duration, and less weight than open-circuit air scuba. Weighed against these advantages are the disadvantages of increased hazards to the diver, greater training requirements, and greater expense. However, when compared to a closed-circuit mixed-gas UBA, an oxygen UBA offers the advantages of reduced training and maintenance requirements, lower cost, and reduction in weight and size.


This chapter provides general guidance for MK 25 diving operations and procedures. For detailed operation and maintenance instructions, see appropriate technical manual (see Appendix 1B for manual reference numbers).


This chapter covers MK 25 UBA principles of operations, operational planning, dive procedures, and medical aspects of closed-circuit oxygen diving.

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