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.