CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-1
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 ex-
halation 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 pre-
vents 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 escap-
ing 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 com-
pared 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 appro-
priate 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.
Closed-circuit oxygen divers are subject to many of the same medical problems as
other divers. Volume 5 provides in-depth coverage of all medical considerations.
Only the diving disorders that merit special attention for closed-circuit oxygen
divers are addressed in this chapter.
Figure 18-1.
Diver in Draeger
18-2 U.S. Navy Diving Manual—Volume 4
Oxygen Toxicity.
Breathing oxygen at high partial pressures may have toxic
effects in the body. Relatively brief exposure to elevated oxygen partial pressure,
when it occurs at depth or in a pressurized chamber, can result in CNS oxygen
toxicity causing CNS-related symptoms. High partial pressures of oxygen are
associated with many biochemical changes in the brain, but which of the changes
are responsible for the signs and symptoms of CNS oxygen toxicity is presently
The off-effect, a hazard associated with CNS oxygen toxicity, may
occur several minutes after the diver comes off gas or experiences a reduction of
oxygen partial pressure. The off-effect is manifested by the onset or worsening of
CNS oxygen toxicity symptoms. Whether this paradoxical effect is truly caused by
the reduction in partial pressure or whether the association is coincidental is
Pulmonary Oxygen Toxicity.
Pulmonary oxygen toxicity, causing lung irritation
with coughing and painful breathing, can result from prolonged exposure to
elevated oxygen partial pressure. This form of oxygen toxicity produces symp-
toms of chest pain, cough, and pain on inspiration that develop slowly and become
increasingly worse as long as the elevated level of oxygen is breathed. Although
hyperbaric oxygen may cause serious lung damage, if the oxygen exposure is
discontinued before the symptoms become too severe, the symptoms will slowly
abate. This form of oxygen toxicity is generally seen during oxygen recompres-
sion treatment and saturation diving, and on long, shallow, in-water oxygen
Symptoms of CNS Oxygen Toxicity.
In diving, the most serious effects of oxygen
toxicity are CNS symptoms. The most hazardous is a sudden convulsion which
can result in drowning or arterial gas embolism. The symptoms of CNS oxygen
toxicity may occur suddenly and dramatically, or they may have a gradual, almost
imperceptible onset. The mnemonic device VENTIDC is a helpful reminder of
these common symptoms.
Visual symptoms. Tunnel vision, a decrease in the diver’s peripheral vision,
and other symptoms, such as blurred vision, may occur.
Ear symptoms. Tinnitus is any sound perceived by the ears but not resulting
from an external stimulus. The sound may resemble bells ringing, roaring, or a
machinery-like pulsing sound.
Nausea or spasmodic vomiting. These symptoms may be intermittent.
Twitching and tingling symptoms. Any of the small facial muscles, lips, or
muscles of the extremities may be affected. These are the most frequent and
clearest symptoms.
Irritability. Any change in the divers mental status; including confusion,
agitation, and anxiety.
Dizziness. Symptoms include clumsiness, incoordination, and unusual fatigue.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-3
Convulsions. The first sign of CNS oxygen toxicity may be a convulsion that
occurs with little or no warning.
The most serious symptom of CNS oxygen toxicity is convulsion. Refer to
Chapter 3 for a complete description of a convulsive episode. The following
factors should be noted regarding an oxygen convulsion:
The diver is unable to carry on any effective breathing during the convulsion.
After the diver is brought to the surface, there will be a period of unconscious-
ness or neurologic impairment following the convulsion; these symptoms are
indistinguishable from those of arterial gas embolism.
No attempt should be made to insert any object between the clenched teeth of
a convulsing diver. Although a convulsive diver may suffer a lacerated tongue,
this trauma is preferable to the trauma that may be caused during the insertion
of a foreign object. In addition, the person providing first aid may incur signif-
icant hand injury if bitten by the convulsing diver.
There may be no warning of an impending convulsion to provide the diver the
opportunity to return to the surface. Therefore, buddy lines are essential to safe
closed-circuit oxygen diving.
Causes of CNS Oxygen Toxicity.
Factors that increase the likelihood of CNS
oxygen toxicity are:
Increased partial pressure of oxygen. At depths less than 25 fsw, a change in
depth of five fsw increases the risk of oxygen toxicity only slightly, but a sim-
ilar depth increase in the 30-fsw to 50-fsw range may significantly increase the
likelihood of a toxicity episode.
Increased time of exposure
Prolonged immersion
Stress from strenuous physical exercise
Carbon dioxide buildup. The increased tendency toward CNS oxygen toxicity
may occur before the diver is aware of any symptoms of carbon dioxide
Cold stress resulting from shivering or an increased exercise rate as the diver
attempts to keep warm.
Systemic diseases that increase oxygen consumption. Conditions associated
with increased metabolic rates (such as certain thyroid or adrenal disorders)
tend to cause an increase in oxygen sensitivity. Divers with these diseases
should be excluded from oxygen diving.
18-4 U.S. Navy Diving Manual—Volume 4
Treatment of Nonconvulsive Symptoms.
The stricken diver should alert his dive
buddy and make a controlled ascent to the surface. The victim’s life preserver
should be inflated (if necessary) with the dive buddy watching him closely for
progression of symptoms.
Treatment of Underwater Convulsion.
The following steps should be taken when
treating a convulsing diver:
Assume a position behind the convulsing diver. Release the victim’s weight
belt unless he is wearing a dry suit, in which case the weight belt should be left
in place to prevent the diver from assuming a face-down position on the
Leave the victim’s mouthpiece in his mouth. If it is not in his mouth, do not
attempt to replace it; however, if time permits, ensure that the mouthpiece is
switched to the SURFACE position.
Grasp the victim around his chest above the UBA or between the UBA and his
body. If difficulty is encountered in gaining control of the victim in this
manner, the rescuer should use the best method possible to obtain control. The
UBA waist or neck strap may be grasped if necessary.
Make a controlled ascent to the surface, maintaining a slight pressure on the
divers chest to assist exhalation.
If additional buoyancy is required, activate the victim’s life jacket. The rescuer
should not release his own weight belt or inflate his own life jacket.
Upon reaching the surface, inflate the victim’s life jacket if not previously
Remove the victim’s mouthpiece and switch the valve to SURFACE to
prevent the possibility of the rig flooding and weighing down the victim.
Signal for emergency pickup.
Once the convulsion has subsided, open the victims airway by tilting his head
back slightly.
Ensure the victim is breathing. Mouth-to-mouth breathing may be initiated if
If an upward excursion occurred during the actual convulsion, transport to the
nearest chamber and have the victim evaluated by an individual trained to
recognize and treat diving-related illness.
Oxygen Deficiency (Hypoxia).
Oxygen deficiency, or hypoxia, is the condition in
which the partial pressure of oxygen is too low to meet the metabolic needs of the
body. Chapter 3 contains an in-depth description of this disorder. In the context of
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-5
closed-circuit oxygen diving, the cause of hypoxia may be considered to be the
result of too much inert gas (nitrogen) in the breathing loop. Although all cells in
the body need oxygen, the initial symptoms of hypoxia are a manifestation of
central nervous system dysfunction.
Causes of Hypoxia with the MK 25 UBA.
If a diver begins breathing from a MK
25 UBA with too low an oxygen fraction in the breathing loop, hypoxia may
develop. A diver can become hypoxic in a rig that uses pure oxygen. Oxygen is
added to the UBA only on a demand basis as the breathing bag is emptied on inha-
lation. If, as the diver consumes the oxygen in the UBA, there is sufficient
nitrogen in the breathing loop to prevent the breathing bag from being emptied, no
oxygen will be added and the diver may become hypoxic even though he has suffi-
cient gas volume in the breathing bag for normal inhalation. If a diver waiting to
begin a dive finishes his purge with a low level of oxygen (e.g., 25 percent) in the
breathing loop and the oxygen fraction remains at 25 percent, there will be no
problem. As the diver consumes oxygen, the oxygen fraction in the breathing loop
will begin to decrease, as will the gas volume in the breathing bag. If the breathing
bag is emptied and the UBA begins to add oxygen before a dangerously low frac-
tion of oxygen is obtained, hypoxia may be avoided. If the diver begins with a
very full breathing bag, however, the gas volume in the bag may decrease two or
three liters without adding any oxygen. In this case, the oxygen fraction may drop
to ten percent or lower and hypoxia may result. The risk of this happening is
greatest when the diver is on the surface before the dive starts because as the diver
descends to the transit depth of 15-25 fsw, two things happen: (1) pure oxygen is
added to the rig to maintain volume as the diver descends and the oxygen fraction
in the rig increases and (2) the pressure increase causes a rise in the partial pres-
sure of the oxygen.
Underwater Purge.
If the diver conducts an underwater purge or purge under
pressure at depth, no descent may be required following the purge procedure and
the pressure-related increase in oxygen fraction as described above would not
occur. Therefore, in the under-pressure purge procedure strict adherence to
prescribed procedures is extremely important to ensure an adequate oxygen frac-
tion in the rig.
MK 25 UBA Purge Procedure.
The possibility of hypoxia developing in the situa-
tion described above led to the development of a detailed purge procedure for the
MK 25 UBA to ensure that the oxygen fraction in the breathing loop is sufficiently
high to prevent such an occurrence. This is accomplished by using the purging
procedures described in the appropriate MK 25 Operation and Maintenance
Symptoms of Hypoxia.
Hypoxia due to a low oxygen content in the breathing gas
may have no warning symptoms prior to loss of consciousness. Other symptoms
that may appear include confusion, incoordination, dizziness, and convulsion. It is
important to note that if symptoms of unconsciousness or convulsion occur at the
beginning of a closed-circuit oxygen dive, hypoxia, not oxygen toxicity, is the
most likely cause.
18-6 U.S. Navy Diving Manual—Volume 4
Treatment of Hypoxia.
Treatment for a suspected case of hypoxia consists of the
If the diver becomes unconscious or incoherent at depth, the dive buddy
should add oxygen to the stricken diver’s UBA.
The diver must be brought to the surface. Remove the mouthpiece and allow
the diver to breathe fresh air. If unconscious, check breathing and circulation,
maintain an open airway and administer 100-percent oxygen.
If the diver surfaces in an unconscious state, transport to the nearest chamber
and have the victim evaluated by an individual trained to recognize and treat
diving-related illness. If the diver recovers fully with normal neurological
function, he does not require immediate treatment for arterial gas embolism.
Carbon Dioxide Toxicity (Hypercapnia).
Carbon dioxide toxicity, or hyper-
capnia, is an abnormally high level of carbon dioxide in the body tissues.
Hypercapnia is generally the result of a buildup of carbon dioxide in the breathing
supply or in the body. Inadequate ventilation (breathing volume) by the diver or
failure of the carbon dioxide-absorbent canister to remove carbon dioxide from the
exhaled gas will cause a buildup to occur.
Symptoms of Hypercapnia.
Symptoms of hypercapnia are:
Increased rate and depth of breathing
Labored breathing (similar to that seen with heavy exercise)
NOTE Symptoms are dependent on the partial pressure of carbon dioxide,
which is a factor of both the fraction of carbon dioxide and the absolute
pressure. Thus, symptoms would be expected to increase as depth
It is important to note that the presence of a high partial pressure of oxygen may
reduce the early symptoms of hypercapnia. As previously mentioned, elevated
levels of carbon dioxide may result in an episode of CNS oxygen toxicity on a
normally safe dive profile.
Treating Hypercapnia.
To treat hypercapnia:
Increase ventilation if skip-breathing is a possible cause.
Decrease exertion level.
Abort the dive. Return to the surface and breathe air.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-7
During ascent, while maintaining a vertical position, the diver should activate
his bypass valve, adding fresh gas to his UBA. If the symptoms are a result of
canister floodout, an upright position decreases the likelihood that the diver
will sustain chemical injury (paragraph 18-2.4).
If unconsciousness occurs at depth, the same principles of management for
underwater convulsion as described in paragraph 18-2.1.6 apply.
NOTE If carbon dioxide toxicity is suspected, the dive should be aborted even
if symptoms dissipate upon surfacin
. The decrease in symptoms may
be a result of the reduction in partial pressure, in which case the
symptoms will reappear if the diver returns to depth.
Avoiding Hypercapnia.
To minimize the risk of hypercapnia:
Use only an approved carbon dioxide absorbent in the UBA canister.
Follow the prescribed canister-filling procedure to ensure that the canister is
correctly packed with carbon dioxide absorbent.
Dip test the UBA carefully before the dive. Watch for leaks that may result in
canister floodout.
Do not exceed canister duration limits for the water temperature.
Ensure that the one-way valves in the supply and exhaust hoses are installed
and working properly.
Swim at a relaxed, comfortable pace.
Avoid skip-breathing. There is no advantage to this type of breathing in a
closed-circuit rig and it may cause elevated blood carbon dioxide levels even
with a properly functioning canister.
Chemical Injury.
The term “chemical injury” refers to the introduction of a caustic
solution from the carbon dioxide scrubber of the UBA into the upper airway of a
Causes of Chemical Injury.
The caustic alkaline solution results from water
leaking into the canister and coming in contact with the carbon dioxide absorbent.
When the diver is in a horizontal or head-down position, this solution may travel
through the inhalation hose and irritate or injure his upper airway.
Symptoms of Chemical Injury.
The diver may experience rapid breathing or
headache, which are symptoms of carbon dioxide buildup in the breathing gas.
This occurs because an accumulation of the caustic solution in the canister may be
impairing carbon dioxide absorption. If the problem is not corrected promptly, the
alkaline solution may travel into the breathing hoses and consequently be inhaled
or swallowed. Choking, gagging, foul taste, and burning of the mouth and throat
18-8 U.S. Navy Diving Manual—Volume 4
may begin immediately. This condition is sometimes referred to as a “caustic
cocktail.” The extent of the injury depends on the amount and distribution of the
Management of a Chemical Incident.
If the caustic solution enters the mouth,
nose, or face mask, the diver must take the following steps:
Immediately assume an upright position in the water.
Depress the manual bypass valve continuously and make a controlled ascent to
the surface, exhaling through the nose to prevent overpressurization.
Should signs of system flooding occur during underwater purging, abort the
dive and return to open-circuit or mixed-gas UBA if possible.
Using fresh water, rinse the mouth several times. Several mouthfuls should then
be swallowed. If only sea water is available, rinse the mouth, but do not swallow.
Other fluids may be substituted if available, but the use of weak acid solutions
(vinegar or lemon juice) is not recommended. Do not attempt to induce vomiting.
As a result of the chemical injury, the diver may have difficulty breathing properly
on ascent. He should be observed for signs of an arterial gas embolism and treated
if necessary. A victim of a chemical injury should be evaluated by a Diving
Medical Officer or a Diving Medical Technician/Special Operations Technician as
soon as possible. Respiratory distress which may result from the chemical trauma
to the air passages requires immediate hospitalization.
NOTE Performance of a careful dip test durin
predive set up is essential to
detect system leaks. Additionally, dive buddies should check each other
carefully before leavin
the surface at the start of a dive.
Middle Ear Oxygen Absorption Syndrome.
Middle ear oxygen absorption syn-
drome refers to the negative pressure that may develop in the middle ear following
a long oxygen dive. Gas with a very high percentage of oxygen enters the middle
ear cavity during the course of an oxygen dive. Following the dive, the oxygen is
slowly absorbed by the tissues of the middle ear. If the Eustachian tube does not
open spontaneously, a negative pressure relative to ambient may result in the
middle ear cavity. Symptoms are often noted the morning after a long oxygen
dive. Middle ear oxygen absorption syndrome is difficult to avoid but usually does
not pose a significant problem because symptoms are generally minor and easily
eliminated. There may also be fluid (serous otitis media) present in the middle ear
as a result of the differential pressure.
Symptoms of Middle Ear Oxygen Absorption Syndrome.
Symptoms of middle
ear oxygen absorption syndrome are:
The diver may notice mild discomfort and hearing loss in one or both ears.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-9
There may also be a sense of pressure and a moist, cracking sensation as a
result of fluid in the middle ear.
Treating Middle Ear Oxygen Absorption Syndrome.
Equalizing the pressure in
the middle ear using a normal Valsalva maneuver (paragraph 3-8.3.1) or the
diver’s procedure of choice (e.g., swallowing, yawning) will usually relieve the
symptoms. Discomfort and hearing loss resolve quickly, but the middle ear fluid is
absorbed more slowly. If symptoms persist, a Diving Medical Technician or
Diving Medical Officer shall be consulted.
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.
Gas Flow Path.
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 CO
bed and chemically combining with the CO
-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 divers 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,
Table 18-1. MK 25 Equipment Information.
Advantages Disadvantages
Restrictions and
Depth Limits
MK 25 MOD 0 Special Warfare
only. Shallow
search and
5 No surface bubbles.
Minimum support.
Long duration. Porta-
bility. Mobility.
Limited to shallow
depths. CNS O
toxicity hazards. No
voice communica-
tions. Limited physical
and thermal protection.
Normal: 25 fsw for 240
m. Maximum: 50 fsw for
10 m. No excursion
allowed when using
Single Depth Diving
MK 25 MOD 1 Same as MOD 0. 5 Same as MOD 0, plus
low magnetic signa-
ture, increased cold
water duration capa-
Same as MOD 0. Same as MOD 0.
MK 25 MOD 2 Same as MOD 0. 5 Same as MOD 0, plus
increased cold water
duration capability.
Same as MOD 0. Same as MOD 0.
18-10 U.S. Navy Diving Manual—Volume 4
and upon inhalation the one-way valves in the hoses allow fresh gas to be pulled
into the divers lungs from the breathing bag.
Breathing Loop.
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
Figure 18-2.
Gas Flow Path of the MK 25.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-11
Operational Duration of the MK 25 UBA.
The 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 UBA.
Oxygen Supply.
The MK 25 oxygen bottle is charged to 3,000 psig (200 BAR).
The oxygen supply may be depleted in two ways: by the divers 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 swim-
ming at a relaxed pace may have an oxygen consumption of one liter per minute
(oxygen supply duration = 300 minutes).
Canister Duration.
The canister duration is dependent on water temperature, exer-
cise 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
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
Table 18-2. Average Breathing Gas Consumption.
(Heavy Work)
(100% O
72.5 psi
(4.9 BAR)
15-17 psi/min
(1-1.2 BAR)
(See Note)
Heavy work is not recommended for the MK 25.
Table 18-3. NAVSEA-Approved Sodalime CO
Name Vendor NSN
High Performance Sodasorb, Regular W.R. Grace 6810-01-113-0110
Sofnolime 4-8 Mesh NI, L Grade O.C. Lugo 6810-01-113-0110
Sofnolime 8-12 Mesh NI, D Grade O.C. Lugo 6810-01-412-0637
Sofnolime 8-12 is only approved for use in the MK 16 UBA.
18-12 U.S. Navy Diving Manual—Volume 4
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.
Packing Precautions.
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 divers
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 Oper-
ation 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 divers lungs while breathing the UBA.
Preventing Caustic Solutions in the Canister.
Additional concerns include en-
suring 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 ab-
sorbent 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 immedi-
ately 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.
MK 25 MOD 0 (UBA LAR V) Operation and Maintenance Manual, NAVSEA
Publication SS-600-AJ-MMO-010, Change 1, August 1, 1985
MK 25 MOD 1 Operation and Maintenance Manual, NAVSEA Publication
SS-600-A2-MMO-010, 31 August, 1996
MK 25 MOD 2 Operation and Maintenance Manual, NAVSEA Publication
Marine Corps TM 09603B-14 & P/1
Evaluation of the Draeger LAR V Pure Oxygen Scuba; NEDU Report 11-75
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-13
Evaluation of the Modified Draeger LAR V Closed-Circuit Oxygen
Rebreather; NEDU Report 5-79
Unmanned Evaluation of Six Closed-Circuit Oxygen Rebreathers; NEDU
Report 3-82
The U.S. Navy closed-circuit oxygen exposure limits have been extended and
revised to allow greater flexibility in closed-circuit oxygen diving operations. The
revised limits are divided into two categories: Transit with Excursion Limits and
Single Depth Limits.
Transit with Excursion Limits Table.
The Transit with Excursion Limits (Table
18-4) call for a maximum dive depth of 25 fsw or shallower for the majority of the
dive, but allow the diver to make a brief excursion to depths as great as 50 fsw.
The Transit with Excursion Limits is normally the preferred mode of operation
because maintaining a depth of 25 fsw or shallower minimizes the possibility of
CNS oxygen toxicity during the majority of the dive, yet allows a brief downward
excursion if needed (see Figure 18-3). Only a single excursion is allowed.
Single-Depth Oxygen Exposure Limits Table.
The Single-Depth Limits (Table
18-5) allow maximum exposure at the greatest depth, but have a shorter overall
exposure time. Single-depth limits may, however, be useful when maximum
bottom time is needed deeper than 25 fsw.
Oxygen Exposure Limit Testing.
The Transit with Excursion Limits and Single-
Depth Limits have been tested extensively over the entire depth range and are
acceptable for routine diving operations. They are not considered exceptional
exposure. It must be noted that the limits shown in this section apply only to
closed-circuit 100-percent oxygen diving and are not applicable to deep mixed-gas
diving. Separate oxygen exposure limits have been established for deep, helium-
oxygen mixed-gas diving.
Individual Oxygen Susceptibility Precautions.
Although the limits described in
this section have been thoroughly tested and are safe for the vast majority of indi-
viduals, occasional episodes of CNS oxygen toxicity may occur. This is the basis
for requiring buddy lines on closed-circuit oxygen diving operations.
Table 18-4. Excursion Limits.
Depth Maximum Time
26-40 fsw 15 minutes
41-50 fsw 5 minutes
18-14 U.S. Navy Diving Manual—Volume 4
Transit with Excursion Limits.
A transit with one excursion, if necessary, will be
the preferred option in most combat swimmer operations. When operational
considerations necessitate a descent to deeper than 25 fsw for longer than allowed
by the excursion limits, the appropriate single-depth limit should be used (para-
graph 18-4.6).
Transit with Excursion Limits Definitions.
The following definitions are illus-
trated in Figure 18-3:
Transit is the portion of the dive spent at 25 fsw or shallower.
Excursion is the portion of the dive deeper than 25 fsw.
Excursion time is the time between the divers initial descent below 25 fsw
and his return to 25 fsw or shallower at the end of the excursion.
Figure 18-3.
Example of Transit with Excursion.
Table 18-5. Single-Depth Oxygen Exposure Limits.
Depth Maximum Oxygen Time
25 fsw 240 minutes
30 fsw 80 minutes
35 fsw 25 minutes
40 fsw 15 minutes
50 fsw 10 minutes
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-15
Oxygen time is calculated as the time interval between when the diver begins
breathing from the closed-circuit oxygen UBA (on-oxygen time) and the time
when he discontinues breathing from the closed-circuit oxygen UBA (off-oxy-
gen time).
Transit with Excursion Rules.
A diver who has maintained a transit depth of 25
fsw or shallower may make one brief downward excursion as long as he observes
these rules:
Maximum total time of dive (oxygen time) may not exceed 240 minutes.
A single excursion may be taken at any time during the dive.
The diver must have returned to 25 fsw or shallower by the end of the pre-
scribed excursion limit.
The time limit for the excursion is determined by the maximum depth attained
during the excursion (Table 18-4). Note that the Excursion Limits are different
from the Single-Depth Limits.
Example: Dive Profile Using Transit with Excursion Limits.
A dive mission calls
for a swim pair to transit at 25 fsw for 45 minutes, descend to 36 fsw, and
complete their objective. As long as the divers do not exceed a maximum depth of
40 fsw, they may use the 40-fsw excursion limit of 15 minutes. The time at which
they initially descend below 25 fsw to the time at which they finish the excursion
must be 15 minutes or less.
Inadvertent Excursions.
If an inadvertent excursion should occur, one of the
following situations will apply:
If the depth and/or time of the excursion exceeds the limits in paragraph
18-4.5.2 or if an excursion has been taken previously, the dive must be aborted
and the diver must return to the surface.
If the excursion was within the allowed excursion limits, the dive may be con-
tinued to the maximum allowed oxygen dive time, but no additional
excursions deeper than 25 fsw may be taken.
The dive may be treated as a single-depth dive applying the maximum depth
and the total oxygen time to the Single-Depth Limits shown in Table 18-5.
Example 1.
A dive pair is having difficulty with a malfunctioning compass. They
have been on oxygen (oxygen time) for 35 minutes when they notice that their
depth gauge reads 55 fsw. Because this exceeds the maximum allowed oxygen
exposure depth, the dive must be aborted and the divers must return to the surface.
Example 2.
A diver on a compass swim notes that his depth gauge reads 32 fsw.
He recalls checking his watch 5 minutes earlier and at that time his depth gauge
read 18 fsw. As his excursion time is less than 15 minutes, he has not exceeded the
18-16 U.S. Navy Diving Manual—Volume 4
excursion limit for 40 fsw. He may continue the dive, but he must maintain his
depth at 25 fsw or less and make no additional excursions.
NOTE If the diver is unsure how lon
he was below 25 fsw, the dive must be
Single-Depth Limits.
The term Single-Depth Limits does not mean that the entire
dive must be spent at one depth, but refers to the time limit applied to the dive
based on the maximum depth attained during the dive.
Single-Depth Limits Definitions.
The following definitions apply when using the
Single-Depth Limits:
Oxygen time is calculated as the time interval between when the diver begins
breathing from the closed-circuit oxygen UBA (on-oxygen time) and the time
when he discontinues breathing from the closed-circuit oxygen UBA (off-oxy-
gen time).
The depth for the dive used to determine the allowable exposure time is deter-
mined by the maximum depth attained during the dive. For intermediate depth,
the next deeper depth limit will be used.
Depth/Time Limits.
The Single-Depth Limits are provided in Table 18-5. No
excursions are allowed when using these limits.
Twenty-two minutes (oxygen time) into a compass swim, a dive pair
descends to 28 fsw to avoid the propeller of a passing boat. They remain at this
depth for 8 minutes. They now have two choices for calculating their allowed
oxygen time: (1) they may return to 25 fsw or shallower and use the time below 25
fsw as an excursion, allowing them to continue their dive on the Transit with
Excursion Limits to a maximum time of 240 minutes; or (2) they may elect to
remain at 28 fsw and use the 30-fsw Single-Depth Limits to a maximum dive time
of 80 minutes.
Exposure Limits for Successive Oxygen Dives.
If an oxygen dive is conducted
after a previous closed-circuit oxygen exposure, the effect of the previous dive on
the exposure limit for the subsequent dive is dependent on the Off-Oxygen
Definitions for Successive Oxygen Dives.
The following definitions apply when
using oxygen exposure limits for successive oxygen dives.
Off-Oxygen Interval. The interval between off-oxygen time and on-oxygen
time is defined as the time from when the diver discontinues breathing from
his closed-circuit oxygen UBA on one dive until he begins breathing from the
UBA on the next dive.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-17
Successive Oxygen Dive. A successive oxygen dive is one that follows a previ-
ous oxygen dive after an Off-Oxygen Interval of more than 10 minutes but less
than 2 hours.
Off-Oxygen Exposure Limit Adjustments.
If an oxygen dive is a successive
oxygen dive, the oxygen exposure limit for the dive must be adjusted as shown in
Table 18-6. If the Off-Oxygen Interval is 2 hours or greater, no adjustment is
required for the subsequent dive. An oxygen dive undertaken after an Off-Oxygen
Interval of more than 2 hours is considered to be the same as an initial oxygen
exposure. If a negative number is obtained when adjusting the single-depth expo-
sure limits as shown in Table 18-6, a 2-hour Off-Oxygen Interval must be taken
before the next oxygen dive.
NOTE A maximum of 4 hours oxy
en time is permitted within a 24-hour period.
Ninety minutes after completing a previous oxygen dive with an
oxygen time of 75 minutes (maximum dive depth 19 fsw), a dive pair will be
making a second dive using the Transit with Excursion Limits. Calculate the
amount of oxygen time for the second dive, and determine whether an excursion is
The second dive is considered a successive oxygen dive because the
Off-Oxygen Interval was less than 2 hours. The allowed exposure time must be
adjusted as shown in Table 18-6. The adjusted maximum oxygen time is 165
minutes (240 minutes minus 75 minutes previous oxygen time). A single excur-
sion may be taken because the maximum depth of the previous dive was 19 fsw.
Seventy minutes after completing a previous oxygen dive (maximum
depth 28 fsw) with an oxygen time of 60 minutes, a dive pair will be making a
second oxygen dive. The maximum depth of the second dive is expected to be 25
fsw. Calculate the amount of oxygen time for the second dive, and determine
whether an excursion is allowed.
First compute the adjusted maximum oxygen time. This is determined
by the Single-Depth Limits for the deeper of the two exposures (30 fsw for 80
Table 18-6. Adjusted Oxygen Exposure Limits for Successive Oxygen Dives.
Adjusted Maximum Oxygen Time Excursion
Transit with
Excursion Limits
Subtract oxygen time on previous
dives from 240 minutes
Allowed if none taken on
previous dives
Single-Depth Limits 1. Determine maximum oxygen
time for deepest exposure.
2. Subtract oxygen time on
previous dives from maximum
oxygen time in Step 1 (above)
No excursion allowed when
using Single-Depth Limits to
compute remaining oxygen
18-18 U.S. Navy Diving Manual—Volume 4
minutes), minus the oxygen time from the previous dive. The adjusted maximum
oxygen time for the second dive is 20 minutes (80 minutes minus 60 minutes
previous oxygen time). No excursion is permitted using the Single-Depth Limits.
Exposure Limits for Oxygen Dives Following Mixed-Gas or Air Dives.
When a
subsequent dive must be conducted and if the previous exposure was an air or MK
16 dive, the exposure limits for the subsequent oxygen dive require no adjustment.
Mixed-Gas to Oxygen Rule.
If the previous dive used a mixed-gas breathing mix
having an oxygen partial pressure of 1.0 ata or greater, the previous exposure must
be treated as a closed-circuit oxygen dive as described in paragraph 18-4.7. In this
case, the Off-Oxygen Interval is calculated from the time the diver discontinued
breathing the previous breathing mix until he begins breathing from the closed-
circuit oxygen rig.
Oxygen to Mixed-Gas Rule.
If a diver employs the MK 25 UBA for a portion of
the dive and another UBA that uses a breathing gas other than oxygen for another
portion of the dive, only the portion of the dive during which the diver was
breathing oxygen is counted as oxygen time. The use of multiple UBAs is gener-
ally restricted to special operations. Decompression procedures for multiple-UBA
diving must be in accordance with approved procedures.
A dive scenario calls for three swim pairs to be inserted near a harbor
using a SEAL Delivery Vehicle (SDV). The divers will be breathing compressed
air for a total of 3 hours prior to leaving the SDV. No decompression is required as
determined by the Combat Swimmer Multilevel Dive (CSMD) procedures. The
SDV will surface and the divers will purge their oxygen rigs on the surface, take a
compass bearing and begin the oxygen dive. The Transit with Excursion Limits
rules will be used. There would be no adjustment necessary for the oxygen time as
a result of the 3 hour compressed air dive.
Oxygen Diving at High Elevations.
The oxygen exposure limits and procedures
as set forth in the preceding paragraphs may be used without adjustment for
closed-circuit oxygen diving at altitudes above sea level.
Flying After Oxygen Diving.
Flying is permitted immediately after oxygen diving
unless the oxygen dive has been part of a multiple-UBA dive profile in which the
diver was also breathing another breathing mixture (air, N
, or HeO
). In this
case, the rules found in the paragraph 9-13 apply.
Combat Operations.
The oxygen exposure limits in this section are the only
limits approved for use by the U.S. Navy and should not be exceeded in a training
or exercise scenario. Should combat operations require a more severe oxygen
exposure, an estimate of the increased risk of CNS oxygen toxicity may be
obtained from a Diving Medical Officer or the Naval Experimental Diving Unit.
The advice of a Diving Medical Officer is essential in such situations and should
be obtained whenever possible.
References for Additional Information.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-19
CNS Oxygen Toxicity in Closed-Circuit Scuba Divers; NEDU Report 11-84
CNS Oxygen Toxicity in Closed-Circuit Scuba Divers II; NEDU Report 3-85
CNS Oxygen Toxicity in Closed-Circuit Scuba Divers III; NEDU Report 5-86
Diving with Self-Contained Underwater Operating Apparatus; NEDU Report
Symptoms of Oxygen Poisoning and Limits of Tolerance at Rest and at Work;
NEDU Report 1-47
“Oxygen Poisoning in Man”; K. W. Donald; British Medical Journal, 1947;
1:667-672, 712-717
Certain factors must be taken into consideration in the planning of the oxygen dive
operation. The following gives detailed information on specific areas of planning.
Operating Limitations.
Diving Officers and Diving Supervisors must consider the
following potential limiting factors when planning closed-circuit oxygen combat
swimmer operations:
UBA oxygen supply (paragraph 18-3.2)
UBA canister duration (NAVSEA 10560 ltr ser 00C35/3215, 22 Apr 96)
Oxygen exposure limits (paragraphs 18-4.7 and 18-4.8)
Thermal factors (Chapter 11 and Chapter 19)
Maximizing Operational Range.
The operational range of the UBA may be maxi-
mized by adhering to these guidelines:
Whenever possible, plan the operation using the turtleback technique, in
which the diver swims on the surface part of the time, breathing air where
Use tides and currents to maximum advantage. Avoid swimming against a
current when possible.
Ensure that oxygen bottles are charged to a full 3,000 psig (200 bar) before the
Minimize gas loss from the UBA by avoiding leaks and unnecessary depth
Maintain a comfortable, relaxed swim pace during the operation. For most
divers, this is a swim speed of approximately 0.8 knot. At high exercise rates,
18-20 U.S. Navy Diving Manual—Volume 4
the faster swim speed is offset by a disproportionately higher oxygen con-
sumption, resulting in a net decrease in operating range. High exercise rates
may reduce the oxygen supply duration below the canister carbon dioxide
scrubbing duration and become the limiting factor for the operation (paragraph
Ensure divers wear adequate thermal protection. A cold diver will begin shiv-
ering or increase his exercise rate, either of which will increase oxygen
consumption and decrease the operating duration of the oxygen supply.
WARNING The MK 25 does not have a carbon dioxide-monitorin
capability. Failure
to adhere to canister duration operations plannin
could lead to uncon-
sciousness and/or death.
Training and requalification dives shall be performed with the following
considerations in mind:
Training dives shall be conducted with equipment that reflects what the diver
will be required to use on operations. This should include limpets, demoli-
tions, and weapons as deemed appropriate.
Periodic classroom refresher training shall be conducted in oxygen diving pro-
cedures, CNS oxygen toxicity and management of diving accidents.
Develop a simple set of hand signals, including the following signals:
Match swim pairs according to swim speed.
If long duration oxygen swims are to be performed, work-up dives of gradu-
ally increasing length are recommended.
Personnel Requirements.
The following topside personnel must be present on all
training and exercise closed-circuit oxygen dives:
Diving Supervisor/Boat Coxswain
Standby diver/surface swimmer with air (not oxygen) scuba
Diving Medical Technician/Special Operations Technician (standby diver
Emergency Surface
Speed Up
Slow Down
Feel Strange
Ear Squeeze
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-21
Equipment Requirements.
The operational characteristics of the MK 25 UBA are
shown in Table 18-7. Equipment requirements for training and exercise closed-
circuit oxygen dives are shown in Table 18-8. Several equipment items merit
special consideration as noted below:
Motorized Chase Boat
. A minimum of one motorized chase boat must be
present for the dive. Safe diving practice in many situations, however, would
require the presence of more than one chase boat (e.g., night operations). The
Diving Supervisor must determine the number of boats required based on the
diving area, medical evacuation plan and number of personnel participating in
the dive. When more than one safety craft is used, communications between
support craft should be available.
Buddy Lines
. Because the risk is greater that a diver will become unconscious
or disabled during a closed-circuit oxygen dive than during other types of
dives, buddy lines are required equipment for oxygen dives. In a few special
diving scenarios, when their use may hinder or endanger the divers, buddy
lines may not be feasible. The Diving Supervisor must carefully consider each
situation and allow buddy lines to be disconnected only when their use will
impede the performance of the mission.
Depth Gauge
. The importance of maintaining accurate depth control on oxy-
gen swims mandates that a depth gauge be worn by each diver.
Transport and Storage of Prepared UBA.
Once the UBA has been set up, the
mouthpiece valve must be placed in the SURFACE position and the oxygen-
supply valve turned off. In this configuration, the rig is airtight and the carbon
dioxide absorbent in the canister is protected from moisture which can impair
carbon dioxide absorption. Two weeks is the maximum allowable time a rig may
be stored from preparation to the time the rig is used.
Table 18-7. Equipment Operational Characteristics.
Diving Equipment
Normal Working Limit
(fsw) (Notes 1 and 2)
Maximum Working
Limit (fsw) (Note 1)
MK 25 UBA 25 (Note 3) 50 None 5
1. Depth limits are based on considerations of workin
time, decompression obli
ation, oxy
en tolerance and nitro
narcosis. The expected duration of the
as supply, the expected duration of the carbon dioxide absorbent, the adequacy of
thermal protection or other factors may also limit both the depth and the duration of the dive.
2. A Divin
Medical officer is required on site for all dives exceedin
the normal workin
3. The normal depth limit for closed-circuit oxy
en divin
operations should be 25 fsw. The option of makin
an excursion to
reater depth (down to 50 fsw), if required durin
a dive, is acceptable and not considered “exceptional exposure.” A
Medical officer is not required on site for an excursion or a sin
le-depth dive.
18-22 U.S. Navy Diving Manual—Volume 4
High temperatures during transport and storage will not adversely affect approved
absorbent; however, storage temperatures below freezing may decrease
performance and should be avoided. Should additional carbon dioxide absorbents
other than those provided in Table 18-3 be approved for use in closed-circuit
UBAs, the manufacturer’s recommendations regarding storage temperatures shall
be followed.
In the event an operation calls for an oxygen dive followed by a surface interval
and a second oxygen dive, the UBA shall be sealed during the surface interval as
described above. It is not necessary to change carbon dioxide absorbent in the
UBA before the second dive as long as the combined oxygen time of both dives
does not exceed the canister duration limits.
Predive Precautions.
The following items shall be determined prior to the diving
Means of communicating with the nearest available Diving Medical Officer.
Table 18-8. Closed-Circuit Oxygen Diving Equipment.
A. General
1. Motorized chase boat*
2. Radio (radio communications with parent unit,
chamber, medevac units, and support craft
when feasible)
3. Hi
h-intensity, wide-beam li
ht (ni
4. Dive fla
s and/or dive li
hts as required
B. Diving Supervisor
1. Dive watch
2. Dive pair list
3. Recall devices
4. Copy of Oxy
en Exposure Limits
5. Copy of Air Tables
C. Standby Diver
1. Compressed-air scuba
2. Wei
ht belt (if needed)
3. Approved life jacket
4. Face mask
5. Fins
6. Appropriate thermal protection
7. Dive knife
8. Flare
9. Tendin
10. Depth
11. Dive watch
D. Diving Medical Technician
1. Self-inflatin
-mask ventilator with medium
adult mask
2. Oro-pharyn
eal airway, adaptable to mask used
3. First aid kit/portable O
4. Two canteens of fresh water for treatin
chemical injury
E. Divers
1. Approved life jacket
2. Wei
ht belt
3. Face mask
4. Fins
5. Dive knife
6. Flare
7. Dive watch
8. Appropriate thermal protection
9. Whistle
10. Buddy line (one per pair)*
11. Depth
e (lar
e face; accurate at shallow
depths; one per diver)*
12. Compass (one per pair if on compass course)
1. Gloves
2. Buoy (one per pair)
3. Slate with writin
* See para
raph 18-5.5
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-23
Location of the nearest functional recompression chamber. Positive confirma-
tion of the chambers availability must be obtained prior to diving.
Nearest medical facility for treatment of injuries or medical problems not
requiring recompression therapy.
Optimal method of transportation to recompression chamber or medical facil-
ity. If coordination with other units for aircraft/boat/vehicle support is
necessary, the Diving Supervisor must know the frequencies, call signs and
contact personnel needed to make transportation available in case of emer-
gency. A medical evacuation plan must be included in the Diving Supervisor
The preparation of a checklist similar to that found in Chapter 6 is
When operations are to be conducted in the vicinity of ships, the guidelines
provided in the Ship Repair Safety Checklist (Chapter 6) and appropriate
Naval Special Warfare Group instructions shall be followed.
Notification of intent to conduct diving operations must be sent to the appro-
priate authority in accordance with local directives.
This section provides the predive procedures for closed-circuit oxygen dives.
Equipment Preparation.
The predive set up of the MK 25 (Draeger LAR V) is
performed using the appropriate checklist from the appropriate MK 25 (UBA
LAR V) Operation and Maintenance Manual. Transport and storage guidelines
found in paragraph 18-5.6 shall be followed.
Diving Supervisor Brief.
The Diving Supervisor brief shall be given separately
from the overall mission brief and shall focus on the diving portion of the opera-
tion with special attention to the items shown in Table 18-9.
Diving Supervisor Check.
First Phase.
The Diving Supervisor check is accomplished in two stages. As the
divers set up their rigs prior to the dive, the Diving Supervisor must ensure that the
steps in the set up procedure are accomplished properly. The Diving Supervisor
checklist [see MK 25 (UBA LAR V) Operation and Maintenance Manual] is
completed during this phase.
Second Phase.
The second phase of the Diving Supervisor check is done after the
divers are dressed. At this point, the Diving Supervisor must check for the
following items:
Adequate oxygen pressure
18-24 U.S. Navy Diving Manual—Volume 4
Proper functioning of hose one-way valves
Loose-fitting waist strap
Proper donning of UBA, life jacket and weight belt. The weight belt is worn so
it may be easily released
Presence of required items such as compasses, depth gauges, dive watches,
buddy lines, and tactical equipment
The diver is required to perform a purge procedure prior to or during any dive in
which closed-circuit oxygen UBA is to be used. The purge procedure is designed
to eliminate the nitrogen from the UBA and the divers lungs as soon as he begins
breathing from the rig. This procedure prevents the possibility of hypoxia as a
result of excessive nitrogen in the breathing loop. The gas volume from which this
excess nitrogen must be eliminated is comprised of more than just the UBA
breathing bag. The carbon dioxide-absorbent canister, inhalation/exhalation hoses,
and diver’s lungs must also be purged of nitrogen.
Purge Procedure.
Immediately prior to entering the water, the divers shall carry
out the appropriate purge procedure. It is both difficult and unnecessary to elimi-
nate nitrogen completely from the breathing loop. The purge procedure need only
raise the fraction of oxygen in the breathing loop to a level high enough to prevent
the diver from becoming hypoxic, as discussed in paragraph 18-2.2. For the MK
Table 18-9. Diving Supervisor Brief.
A. Dive Plan
1. Operatin
2. Distance, bearin
s, transit lines
3. Dive time
4. Known obstacles or hazards
B. Environmental
1. Weather conditions
2. Water/air temperatures
3. Water/air visibility
4. Divin
medical technician
C. Special Equipment for:
1. Divers (include thermal
2. Divin
3. Standby Diver
4. Divin
medical technician
D. Review of Hand Signals
E. Communications
1. Frequencies
2. Call si
F. Emergency Procedures
1. Symptoms of O
Toxicity - review in detail
2. Symptoms of CO
buildup - review in detail
3. Review mana
ement of underwater convulsion,
nonconvulsive O
hit, CO
buildup, hypoxia,
chemical injury, unconscious diver
4. UBA malfunction
5. Lost swim-pair procedures
6. Medical evacuation plan
nearest available chamber
nearest Divin
Medical Officer (DMO)
transportation plan
recovery of other swim pairs
G. Review of Purge Procedure
H. Times for Operations
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-25
25 UBA, this value has been determined to be 45 percent. For further information
on purge procedures, see paragraph 18-7.4.
If the dive is part of a tactical scenario that requires a turtleback phase, the purge
must be done in the water after the surface swim, prior to submerging. If the
tactical scenario requires an underwater purge procedure, this will be completed
while submerged after an initial subsurface transit on open-circuit scuba or other
UBA. When the purge is done in either manner, the diver must be thoroughly
familiar with the purge procedure and execute it carefully with attention to detail
so that it may be accomplished correctly in this less favorable environment.
Turtleback Emergency Descent Procedure.
This procedure is approved for
turtleback emergency descents:
Open the oxygen supply.
Exhale completely, clearing the mouthpiece with the dive/surface valve in the
surface position.
Put the dive/surface valve in the DIVE position and make the emergency
Immediately upon reaching depth, perform purging under pressure
(pressurized phase) (IAW the appropriate MK 25 Technical Manual).
Avoiding Purge Procedure Errors.
The following errors may result in a danger-
ously low percentage of oxygen in the UBA and should be avoided:
Exhaling back into the bag with the last breath rather than to the atmosphere
while emptying the breathing bag.
Underinflating the bag during the fill segment of the fill/empty cycle.
Adjusting the waist strap of the UBA or adjustment straps of the life jacket too
tightly. Lack of room for bag expansion may result in underinflation of the bag
and inadequate purging.
Breathing gas volume deficiency caused by failure to turn on the oxygen-sup-
ply valve prior to underwater purge procedures.
References for Additional Information.
The following references provide infor-
mation on the LAR V purge procedures:
Purging Procedures for the Draeger LAR V Underwater Breathing Appara-
tus; NEDU Report 5-84
Underwater Purging Procedures for the Draeger LAR V UBA; NEDU Report
18-26 U.S. Navy Diving Manual—Volume 4
MK 25 UBA (LAR V) Operation and Maintenance Manual; NAVSEA SS600-
AJ-MMO-010, Change 1, January 1, 1985
General Guidelines.
During the dive, the divers shall adhere to the following
Know and observe the oxygen exposure limits.
Observe the UBA canister limit for the expected water temperature [see
NAVSEA 10560 ltr ser 00C35/3215, 22 Apr 96].
Wear the appropriate thermal protection.
Use the proper weights for the thermal protection worn and for equipment
Wear a depth gauge to allow precise depth control. The depth for the pair of
divers is the greatest depth attained by either diver.
Dive partners check each other carefully for leaks at the onset of the dive. This
should be done in the water after purging, but before descending to transit
Swim at a relaxed, comfortable pace as established by the slower swimmer of
the pair.
Maintain frequent visual or touch checks with buddy.
Be alert for any symptoms suggestive of a medical disorder (CNS oxygen tox-
icity, carbon dioxide buildup, etc.).
Use tides and currents to maximum advantage.
Swim at 25 fsw or shallower unless operational requirements dictate
Use the minimum surface checks consistent with operational necessity.
Minimize gas loss from the UBA.
Do not use the UBA breathing bag as a buoyancy compensation device.
Do not perform additional purges during the dive unless the mouthpiece is
removed and air is breathed.
If an excursion is taken, the diver not using the compass will note carefully the
starting and ending time of the excursion.
CHAPTER 18 — Closed-Circuit Oxygen UBA Diving 18-27
UBA Malfunction Procedures.
The diver shall be thoroughly familiar with the
malfunction procedures unique to his UBA. These procedures are described in the
appropriate UBA MK 25 Operational and Maintenance Manual.
The ascent rate shall never exceed 30 feet per minute.
UBA postdive procedures should be accomplished using the appropriate checklist
from the appropriate UBA MK 25 Operation and Maintenance Manual.
Document all dives performed by submitting a Combined Diving Log and
Mishap/Injury Report.
18-28 U.S. Navy Diving Manual—Volume 4