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.
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.
table 18-4 Excursion Limits.
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FIGURE 18-3
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, N2O2, or HeO2). In this
case, the rules found in the paragraph 9-13 apply.
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.
-
CNS Oxygen Toxicity in Closed-Circuit Scuba Divers; NEDU Report 11-84
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CNS Oxygen Toxicity in Closed-Circuit Scuba Divers II; NEDU Report 3-85
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CNS Oxygen Toxicity in Closed-Circuit Scuba Divers III; NEDU Report 5-86
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Diving with Self-Contained Underwater Operating Apparatus; NEDU Report
11-54
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Symptoms of Oxygen Poisoning and Limits of Tolerance at Rest and at Work;
NEDU Report 1-47
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“Oxygen Poisoning in Man”; K. W. Donald; British Medical Journal, 1947;
1:667-672, 712-717
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
TABLE 18-5
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, heliumoxygen
mixed-gas diving
Although the limits described in
this section have been thoroughly tested and are safe for the vast majority of individuals,
occasional episodes of CNS oxygen toxicity may occur. This is the basis
for requiring buddy lines on closed-circuit oxygen diving operations.
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 (paragraph
18-4.6).
The following definitions are illustrated
in Figure 18-3:
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Transit is the portion of the dive spent at 25 fsw or shallower.
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Excursion is the portion of the dive deeper than 25 fsw.
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Excursion time is the time between the diver’s initial descent below 25 fsw
and his return to 25 fsw or shallower at the end of the excursion.
-
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-oxygen
time).
figure 18-3 Example of Transit with Excursion.
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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:
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Maximum total time of dive (oxygen time) may not exceed 240 minutes.
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A single excursion may be taken at any time during the dive.
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The diver must have returned to 25 fsw or shallower by the end of the prescribed
excursion limit.
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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.
table 18-4
If an inadvertent excursion should occur, one of the
following situations will apply:
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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
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If the excursion was within the allowed excursion limits, the dive may be continued
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 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 long he was below 25 fsw, the dive must be
aborted.
table 18-5 Single-Depth Oxygen Exposure Limits.
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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.
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-oxygen
time).
-
The depth for the dive used to determine the allowable exposure time is determined
by the maximum depth attained during the dive. For intermediate depth,
the next deeper depth limit will be used.
The Single-Depth Limits are provided in Table 18-5. No
excursions are allowed when using these limits.
Example. 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.
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
Interval.
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.
-
Successive Oxygen Dive. A successive oxygen dive is one that follows a previous
oxygen dive after an Off-Oxygen Interval of more than 10 minutes but less
than 2 hours.
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 exposure
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 oxygen time is permitted within a 24-hour period.
Example. 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
allowed.
Solution. 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 excursion
may be taken because the maximum depth of the previous dive was 19 fsw
Example. 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.
Solution. 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 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 Limit
table 18-6 Adjusted Oxygen Exposure Limits for Successive Oxygen Dives.
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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.
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 closedcircuit
oxygen rig.
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 generally
restricted to special operations. Decompression procedures for multiple-UBA
diving must be in accordance with approved procedures.
Example. 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.
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.
Example. 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.
