The U.S. Navy closed-circuit mixed-gas UBA is a
constant partial-pressure-of-oxygen rebreather. To
conserve the gas supply and extend underwater
duration, the efficiency of gas use is improved by:
-
Removing carbon dioxide produced by metabolic
action of the body.
-
Adding pure oxygen to the breathing gas to
replace the oxygen consumed.
-
Recirculating the breathing gas for reuse.
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.
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.
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 must be of relatively large diameter (minimum one-inch ID) to minimize
breathing resistance.
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.
Diaphram Assembly. A diaphram assembly or counter lung is used in all closedcircuit
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.
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 canister duration. Any factor that reduces the efficiency of carbon dioxide removal
increases the risk of carbon dioxide poisoning.
CAUTION The MK 16 UBA
provides no visual warning of excess CO2 problems.The diver should be
aware of CO2 toxicity symptoms.
Gas Addition, Exhaust, and Monitoring. In addition to the danger of carbon
dioxide toxicity, the closed-circuit UBA diver encounters the potential hazards of
hypoxia and central nervous system (CNS) oxygen toxicity (see Volume 5). It is
essential that these hazards be avoided. The UBA must control the partial pressure
of oxygen (ppO2) in the breathing medium within narrow limits for safe operation
and be monitored frequently by the diver.
Hypoxia can occur when there is insufficient oxygen in the recirculation circuit to
meet metabolic requirements. If oxygen is not added to the breathing circuit, the
oxygen in the loop will be gradually consumed over a period of 2-5 minutes, at
which point the oxygen in the mixture is incapable of sustaining life.
CNS oxygen toxicity can occur whenever the oxygen partial pressure in the
diver’s breathing medium exceeds specified concentration and exposure time
limits. Consequently, the UBA must function to limit the ppO2 level to the appropriate
value.
The closed-circuit mixed-gas UBA uses a direct control method of maintaining
oxygen concentration in the system, rather than the indirect method of a preset
mass flow, common to semi-closed apparatus.
Advantages of
Closed-Circuit Mixed-Gas UBA. While functionally simpler in
principle, the closed-circuit mixed-gas UBA tends to be more complex
than the semi-closed UBA because of the oxygen analysis and control
circuits required. Offsetting this complexity, however, are several
inherent advantages:
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Aside from mixed or diluent gas addition during
descent, the only gas required at depth is oxygen to make up for
metabolic consumption.
-
The partial pressure of oxygen in the system is automatically controlled
throughout the dive to a preset value. No adjustment is required during a dive
for variations in depth and work rate.
-
No inert gas leaves the system except by accident or during ascent, making the
closed-circuit UBA relatively bubble-free and well-suited for SPECWAR and
EOD operations requiring low acoustic signature.
