The closed-circuit oxygen UBAs currently used by U.S. Navy combat swimmers
are the MK 25 MOD 0, MOD 1, and MOD 2 (Draeger LAR V UBA). Refer to
Table 18-1 for the operational characteristics of the MK 25.
table 18-1 MK 25 Equipment Information.
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figure 18-1 Diver in Draeger
LAR V UBA.
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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 and upon inhalation the one-way valves in the hoses allow fresh gas to be pulled
into the diver’s lungs from the breathing bag.
figure 18-2 Gas Flow Path of the MK 25.
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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).
TThe operational duration of the MK 25
UBA may be limited by either the oxygen supply or the canister duration. Refer to
Table 18-2 for the breathing gas consumption rates for the MK 25 U
table 18-2 Average Breathing Gas Consumption.
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The MK 25 oxygen bottle is charged to 3,000 psig (200 BAR).
The oxygen supply may be depleted in two ways: by the diver’s metabolic
consumption or by the loss of gas from the UBA. A key factor in maximizing the
duration of the oxygen supply is for the diver to swim at a relaxed, comfortable
pace. A diver swimming at a high exercise rate may have an oxygen consumption
of two liters per minute (oxygen supply duration = 150 minutes) while one swimming
at a relaxed pace may have an oxygen consumption of one liter per minute
(oxygen supply duration = 300 minutes).
The canister duration is dependent on water temperature, exercise
rate, and the mesh size of the NAVSEA-approved carbon dioxide absorbent.
(Table 18-3 lists NAVSEA-approved absorbents.) The canister will function
adequately as long as the UBA has been set up properly. Factors that may cause
the canister to fail early are discussed under carbon dioxide buildup in paragraph
18-2.3.
Dives should be planned so as not to exceed the canister duration limits. Oxygen
pressure is monitored during the dive by the UBA oxygen pressure gauge,
displayed in bars. The duration of the oxygen supply will be dependent on the factors discussed in paragraph 18-5.2 and must be estimated using the anticipated
swim speed and the expertise of the divers in avoiding gas loss.
table 18-3 NAVSEA-Approved Sodalime CO2 Absorbents
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Caution should be used when packing the carbon dioxide
canister to ensure the canister is completely filled with carbon dioxide-absorbent
material to minimize the possibility of channeling. Channeling allows the diver’s
exhaled carbon dioxide to pass through channels in the absorbent material without
being absorbed, resulting in an ever-increasing concentration of carbon dioxide in
the breathing bag, leading to hypercapnia. Channeling can be avoided by
following the canister-packing instructions provided by the specific MK 25 Operation
and Maintenance Manual. Basic precautions include orienting the canister
vertically and filling the canister to approximately 1/3 full with the approved
absorbent material and tapping the sides of the canister with the hand or a rubber
mallet. This process should be repeated by thirds until the canister is filled to the
fill line scribed on the inside of the absorbent canister. Mashing the material with a
balled fist is not recommended as it may cause the approved absorbent material to
fracture, thereby producing dust which would then be transported through the
breathing loop to the diver’s lungs while breathing the UBA.
Additional concerns include ensuring
water is not inadvertently introduced into the canister by leaving the
mouthpiece in the “dive” position when on the surface or through system leaks.
The importance of performing the tightness and dip test while performing predive
setup procedures cannot be overemphasized. When water combines with the absorbent
material, it creates strong caustic solution commonly referred to as
“caustic cocktail,” which is capable of producing chemical burns in the diver’s
mouth and airway. In the event of a “caustic cocktail,” the diver should immediately
maintain a heads-up attitude in the water column, depress the manual bypass
knob on the demand valve, and terminate the dive.
References for Additional Information.
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MK 25 MOD 0 (UBA LAR V) Operation and Maintenance Manual, NAVSEA
Publication SS-600-AJ-MMO-010, Change 1, August 1, 1985
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MK 25 MOD 1 Operation and Maintenance Manual, NAVSEA Publication
SS-600-A2-MMO-010, 31 August, 1996
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MK 25 MOD 2 Operation and Maintenance Manual, NAVSEA Publication
SS-600-A3-MMO-010/53833
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Marine Corps TM 09603B-14 & P/1
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Evaluation of the Draeger LAR V Pure Oxygen Scuba; NEDU Report 11-75
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Evaluation of the Modified Draeger LAR V Closed-Circuit Oxygen
Rebreather; NEDU Report 5-79
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Unmanned Evaluation of Six Closed-Circuit Oxygen Rebreathers; NEDU
Report 3-82
