When diving with an open-circuit UBA, ppO2 increases with depth. With a
closed-circuit UBA, ppO2 remains constant at a preset level regardless of depth.
Therefore, standard U.S. Navy decompression tables cannot be used.
Use of Constant ppO2 Decompression Tables. Closed-circuit UBA users must
use constant ppO2 decompression tables Oxygen in Nitrogen (air diluent), and
Oxygen in Helium (Helium-Oxygen diluent). Closed-circuit, mixed-gas UBA
decompression tables (Table 17-14 and Table 17-15) are included at the end of this
chapter.
Monitoring ppO2. During decompression, it is very important to frequently
monitor the secondary display and ensure a 0.7 ppO2 is maintained as closely as
possible. Always use the appropriate decompression table when surfacing, even if UBA malfunction has significantly altered the ppO2.
NOTE Surface decompression is not authorized for MK 16 operations.
Appropriate surface decompression tables have not been developed for
constant 0.7 ata ppO2 closed-circuit diving.
Rules for Using 0.7 ata Constant ppO2 in Nitrogen and in Helium
Decompression Tables.
NOTE The rules using the 0.7 ata ppO2 tables are the same for nitrogen and
helium; however, the tables are not interchangeable.
-
These tables are designed to be used with MK 16 UBA (or any other constant
ppO2 closed-circuit UBA) with an oxygen setpoint of 0.7 ata or higher.
-
When using helium as the inert gas, the amount of nitrogen must be minimized
in the breathing loop. Flush the UBA well with helium-oxygen using proper
purge procedure in the MK 16 UBA O&M manual.
-
Tables are grouped by depth and within each depth group is a limit line. These
tables are designed to be dived to the limit line. Schedules below the limit line
provide for unforeseen circumstances when a diver might experience an inadvertent
downward excursion or for an unforeseen reason overstay the planned
bottom time.
-
Tables/schedules are selected according to the maximum depth obtained during
the dive and the bottom time (time from leaving the surface to leaving the
bottom).
-
General rules for using these tables are the same as for standard air tables:
1. Enter the table at the listed depth that is exactly equal to or is next greater
than the maximum depth attained during the dive.
2. Select the bottom time from those listed for the selected depth that is
exactly equal to or is next greater than the bottom time of the dive.
3. Never attempt to interpolate between decompression schedules.
4. Use the decompression stops listed for the selected bottom time.
5. Ensure that the diver's chest is maintained as close as possible to each
decompression depth for the number of minutes listed.
6. Maximum ascent rate is 30 feet per minute.
7. Begin timing each stop on arrival at the decompression stop depth and
resume ascent when the specified time has elapsed. Do not include ascent
time as part of stop time.
8. The last stop may be taken at 20 fsw if desired. After completing the
prescribed 20-fsw stop, remain at any depth between 10 fsw and 20 fsw
inclusive for the 10-fsw stop time as noted in the appropriate
decompression table.
9. Always use the appropriate decompression table when surfacing even if UBA malfunction has significantly altered ppO2.
-
In emergency situations (e.g., UBA flood-out or failure), immediately ascend
to the first decompression stop according to the original decompression schedule
if deeper than the first stop, and shift to the Emergency Breathing System
(EBS). The subsequent decompression is modified according to the diluent gas
originally breathed.
-Helium-Oxygen Diluent. Follow the original HeO2 decompression
schedule without modification while breathing air.
-Nitrogen-Oxygen (Air) Diluent. Double all remaining decompression
stops while breathing air. If the switch to emergency air is made
while at a decompression stop, then double the remaining time at that
stop and all shallower stops. If a planned decompression dive falls
within a no-decompression limit and a switch to EBS has occurred, a
mandatory 10-minute stop at 20 fsw is required.
-
When selecting the proper decompression table, all dives within the past 12
hours must be considered. Repetitive dives are allowed. Repetitive diving
decompression procedures vary depending on the breathing medium(s)
selected for past dives and for the current dive. If a dive resulted in breathing
from the EBS then no repetitive dives shall be made within the next 12 hours.
Refer to the following tables:
-Table 17-8a for Repetitive
Dive Procedures for Various Gas Mediums.
-Figure 17-7 for the Dive Worksheet for Repetitive 0.7 ata Constant Partial Pressure Oxygen in Nitrogen Dives.
-Table 17-9 for the No-Decompression Limits and Repetitive Group
Designation. Table for No-Decompression 0.7 ata Constant Partial
Pressure Oxygen in Nitrogen Dives.
-Table 17-10 for the Residual Nitrogen Timetable for Repetitive 0.7 ata Constant Partial Pressure Oxygen in Nitrogen Dives.
-
PPO2 Variances. The ppO2 in the MK 16 UBAs is expected to vary slightly from
0.6 - 0.9 ata for irregular brief intervals. This does not constitute a malfunction.
The decompression tables were calculated and tested using functioning or prototype
MK 16 UBAs. When addition of oxygen to the UBA is manually controlled,
ppO2 should be maintained in accordance with techniques and emergency procedures
listed in the MK 16 O&M manual.
The Diving Supervisor and medical personnel should recognize that a diver who
has been breathing a mixture with ppO2 lower than 0.6 ata for any length of time
may have a greater risk of developing decompression sickness. Such a diver
requires observation after surfacing, but need not be treated unless symptoms of
decompression sickness occur.
Emergency Breathing System (EBS). The Emergency Breathing System provides
an alternate breathing source for decompressing diver(s) in the event of a
MK 16 failure. The two types of EBS available for use are EBS Type I and EBS
Type II MK 1 Mod 0. The systems have been designed and tested as an accurate
method for topside to control and monitor breathing gas being supplied to a
diver(s) during decompression. The EBS shall be deployed whenever MK 16 decompression
diving is anticipated. In the event of MK 16 failure or malfunction,
the diver(s) will transfer to the EBS as soon as possible and continue to use the
EBS to complete the decompression profile. It is to be used only for its designed
purpose as discussed in paragraph 17-10.3 as an emergency breathing source and
not as a surface-supplied diving system.
EBS Type I. The EBS type I was designed and is intended to be used only in
support of diving up to 200 fsw. NAVSEA Operation and Maintenance manual
S9592-AN-MMO-010 provides detailed equipment descriptions, reference data,
and information on operation and maintenance. This type of EBS is a non-certified
system (Figure 17-8)
EBS Type II MK 1 Mod 0. The EBS II is a certified surface-supplied, in-water
emergency life-support system, with capabilities to support two divers during
decompression for dive profiles to 300 fsw (Figure 17-9). The EBS II enables
voice communication capabilities between topside personnel and divers while the
divers are using the MK 24 FFM (Figure 17-10). PEO MINEWAR technical
manual SS600-AL-MMA-010 provides detailed equipment descriptions, reference
data, and information on operation and maintenance.
Required Gas Supply for the EBS. When a decompression dive is planned, the
Diving Supervisor must calculate the volume of gas required should a diver be
required to breathe from the EBS throughout decompression.
Calculating EBS Gas Requirements. The following steps may be used to calculate EBS gas requirements (Figure
17-11):
1. Determine decompression profiles from appropriate closed-circuit mixed-gas UBA decompression tables using 0.7 ata
constant partial pressure of oxygen.
2. Multiply the time of each decompression stop by the gas consumption rates
(scfm) in Table 17-11 to obtain total volume
required per stop. Table
17-11 assumes a light work rate (gas consumption = 0.63 acfm).
3. Total the volumes required per stop to obtain total volume for decompression.
The total should be rounded up to the
nearest whole scf.
4. Multiply the total volume for decompression by a safety factor of 10 percent
and add the product to the volume for
decompression for total air volume
required.
The volume of gas available in the EBS I may be obtained from Table
17-12 when
twin 80-cubic foot scuba bottles are employed or from the following formula
when other EBS configurations are used.
EBS volume available =
0.399 * 2 *( 2800-250)
14.7
= 138.4 scf
The following floodable volume numbers are provided for reference:
Scuba 72 0.420
Scuba 80 0.399
Scuba 90 0.398
K Bottle 1.620
EBS II Bottle 0.926
The volume of gas available in the EBS II may be obtained from its O&M technical
manual, Appendix B.
The above formula may be rearranged as shown below to determine the minimum
bank pressure that will just provide the required EBS volume (VR):
EBS volume available = VR*14.7
+ PR
FV X N
EBS Deployment Procedures.
1. When directed by the Diving Supervisor, the EBS tender shall attach the EBS
to either a descent line or the diver’s marker float and lower the EBS to 10 fsw
below the diver’s first decompression stop
2. Upon arrival at the EBS, the diver(s) shall signal arrival at the EBS (one on the
lazy shot). The EBS tender shall report signal receipt to the Diving Supervisor,
who will control the divers’ ascent to the first decompression stop and
continue to control their ascent and stops throughout in-water decompression.
Omitted Decompression. Certain emergencies may interrupt or prevent specified
decompression. UBA failure, exhausted diluent or oxygen gas supply, and bodily
injury are examples that constitute such emergencies. Omitted decompression
must be made up to avoid later difficulty. Table 17-13 contains specific guidance
for the initial management of omitted decompression in an asymptomatic MK 16
diver. For further information on omitted decompression, see Chapter 21.
At 20 fsw or Shallower. If the deepest decompression stop omitted is 20 fsw or
shallower, the diver may be returned to the water stop at which the omission
occurred.
-
If the surface interval was less than 1 minute, add 1 minute to the stop time
and resume the planned decompression at the point of interruption.
-
If the surface interval was greater than 1 minute, compute a new decompression
schedule by multiplying the 20- and/or 10-foot stop time(s) by 1.5. After arrival at the decompression stop at the Diving Supervisor’s discretion the
oxygen partial pressure may be manually adjusted to 1.3 ata (increased-rate
oxygen supply depletion shall be taken into consideration).
-
Ascend on the new decompression schedule. Alternatively, the diver may be
removed from the water and treated on Treatment Table 5 (Figure 21-7) if the
surface interval is less than 5 minutes, or Treatment Table 6 (Figure 21-8) if
the surface interval is greater than 5 minutes.
Deeper than 20 fsw .If the deepest decompression stop omitted is deeper than 20
fsw, a more serious situation exists. The use of a recompression chamber when
immediately available is mandatory.
-
If less than 30 minutes of decompression were missed and the surface interval
is less than 5 minutes, treat the diver on Treatment Table 5.
-
If less than 30 minutes of decompression were missed but the surface interval
exceeds 5 minutes, treat the diver on Treatment Table 6.
-
If more than 30 minutes of decompression were missed, treat the diver on
Treatment Table 6 regardless of the length of the surface interval.
