The Surface-Supplied Helium-Oxygen Decompression Table (Table 14-7) is used
to decompress divers from surface-supplied helium-oxygen dives. The table is in a
depth-time format similar to the U.S. Navy Air Decompression Table and is used in a similar fashion. One additional table, the Emergency Procedures Decompression
Table (Table 14-1), is used under emergency conditions (see paragraph
14-4.4).
The Surface-Supplied
Helium-Oxygen Decompression Table (Table 14-7) specifies the maximum and
minimum concentrations of oxygen allowable in the helium-oxygen mixture at
depth. Select a gas mixture for the dive that is compatible with the deepest depth
anticipated for the dive.
14-4.4).
To select the proper decompression table and
schedule, measure the deepest depth reached by the diver and enter the table at the
exact or next greater depth. When using an air-filled pneumofathometer to
measure depth, the observed depth reading must be corrected as shown in Table
14-2. It is also important that the pneumofathometer be at mid-chest level. Enter
the table at the maximum or next greater depth. It is also important that the pneumofathometer
be at mid-chest level. The bottom time is measured as the time from
leaving the surface to leaving the bottom, rounded up to the next whole minute,
except as noted in paragraph 14-3.2. Enter the table at the exact or next greater
bottom time.
Example. The diver’s pneumofathometer reads 250 fsw. In the depth range of
201-300 fsw, the pneumofathometer underestimates the diver’s true depth by 4 fsw. To determine the true depth, 4 fsw must be added to the pnuemofathometer
reading. The diver’s true depth is 254 fsw.
table 14-2 Pneumofathometer Correction Factors.
|
The descent rate is not critical, but it should not exceed 75 fsw/min.
The ascent rate is at a constant rate of 30 fsw/minute. The ascent time between
stops is included in the time of the subsequent stop, except when reaching the first
stop and when the shift to 100 percent oxygen is made.
For dives as deep as 200 fsw, decompression is
taken on the bottom mixture up to the 50-fsw water stop (40-fsw if 40 fsw is the
first stop) and the diver is then shifted to 100 percent oxygen. For dives greater
than 200 fsw, decompression is taken on the bottom mixture to the 100-fsw water
stop (or next shallower stop if there is no 100-fsw stop) and the diver is then
shifted to a 60 percent helium/40 percent oxygen mixture. Upon arrival at the 50-
fsw water stop, the diver is shifted to 100 percent oxygen. Surface decompression
may be taken after completing a portion of the 40-fsw oxygen stop on all dives, as
described in paragraphs 14-3.7 and 14-3.8.
The rate of ascent to the first stop and between
subsequent stops is 30 fsw/minute. Minor variations in the rate of travel between
20 and 40 fsw/minute are acceptable.
If the divers arrive early at the first stop:
1. Begin timing the first stop when the required travel time has been completed.
2. If the first stop is 50 or 40 fsw and arrival at the stop is early, shift to oxygen
and begin stop time when the required travel time has been completed.
-
Delay less than 1 minute. Delays in arrival at the first stop of less than 1
minute may be ignored.
-
Delay less than 1 minute. For delays in excess of 1 minute:
1. Add the total delay to the bottom time.
2. Recalculate the required decompression.
—If no change in schedule is required, continue on the planned decompression.
—If a change in schedule is required and the new schedule calls for
a decompression stop or stops deeper than the diver’s current
depth, perform any missed deeper stops at the diver’s current
depth. Do not go deeper.
Example. If the delay time to arrival at the first stop is 3 minutes and 25 seconds,
round up to the next whole minute and add 4 minutes to the bottom time. Recheck
the decompression table to see if the decompression stop depths or times have
changed.
Ascent time between stops is not critical as it is included
in the time of the next stop.
-
Delay less than 1 minute.When the delay is less than 1 minute, disregard the
delay.
-
Delay greater than 1 minute leaving a stop deeper than 50 fsw. Add the delay
to the bottom time and recalculate the required decompression. If a new
schedule is required, pick up the new schedule at the present stop. Ignore any
missed stops or time deeper than the present stop.
-
Delays up to 5 minutes in leaving the 50-fsw and 40-fsw oxygen stops.
Ignore the delay. Longer delays may be associated with an increased risk of
oxygen toxicity and should be avoided.
Disregard any delays in travel from
40 fsw to the surface during surface decompression unless the diver exceeds the 5-
minute interval. When the diver exceeds the 5-minute interval, the diver shall be
treated for omitted decompression (see paragraph 14-4.10).
Limited gas supplies or system constraints may force some surface-supplied
helium-oxygen dives to be performed at oxygen partial pressures greater than 1.3
ata. Such dives place the diver at increased risk for CNS oxygen toxicity on the
bottom and require NAVSEA concurrence and CNO approval. Bottom times shall
be limited to those shown in Table 14-4.
The formula for calculating oxygen partial
pressure is:
| |
|
|
|
%O2 |
|
D+33 |
| ppO2 |
|
= |
|
-------- |
* |
------- |
| |
|
|
|
100 |
|
33 |
Where:
| ppo2 |
|
= |
|
Oxygen partial pressure in ata |
| %O2 |
|
= |
|
Oxygen percentage in the mixture |
| D |
|
= |
|
Diver’s depth in fsw |
Example. A diver is at 250 fsw breathing a 17.0 percent oxygen mixture. The
oxygen partial pressure is:
| |
|
|
|
172 |
|
250+33 |
| ppO2 |
|
= |
|
-------- |
* |
------- |
| |
|
|
|
100 |
|
33 |
| |
|
= |
|
1.46 ata |
To dive in accordance with this section:
1. Determine the bottom time that will be required to complete the task.
2. From Table 14-4, select the oxygen partial pressure that corresponds to this
bottom time. If the bottom time is not exactly equal to the times listed in the
table, round to the next longer bottom time.
3. Determine the deepest depth that will be attained by the diver during the dive.
4. Calculate the maximum oxygen percentage that can be used by rearranging the
oxygen partial pressure equation to solve for the maximum oxygen percentage
that can be used:
| |
|
|
|
ppO2*33 |
|
|
| %O2 |
|
= |
|
-------- |
* |
100 |
| |
|
|
|
D+33 |
|
|
Sample Problem. A dive to a maximum depth of 270 fsw will require 35 minutes
of bottom time. Determine the maximum oxygen percentage that can be used for
this dive.
1. Round the 35-minute bottom time to 40 minutes, the next longer bottom time
given in Table 14-7.
2. The maximum allowable oxygen partial pressure for this bottom time is 1.50
ata.
3. Calculate the maximum oxygen percentage:
| |
|
|
|
1.50*33 |
|
|
| %O2 |
|
= |
|
------- |
* |
100 |
| |
|
|
|
270+33 |
|
|
| |
|
= |
|
=16.34 ata |
table 14-4 .Oxygen Partial Pressure Exposure Limits for Surface-Supplied HeO2
Diving.
|
table 14-7
Any gas mixture between the calculated maximum and minimum
values shown in the decompression table may be used to make the dive under the
provisions of this section.
Figure 14-7 provides the
proper format for charting surface-supplied helium-oxygen dives.
figure 14-7 .HeO2 Diving Chart.
|
To prevent hypoxia, a special descent procedure is required
when the bottom mixture contains less than 16 percent oxygen:
1. Place the diver on the surface on air.
2. Make the appropriate predive checks.
3. Have the diver descend to 20 fsw.
4. At 20 fsw, shift the diver to the bottom mix and ventilate. The diver is allowed
10 minutes at 20 fsw to shift to the bottom mixture and perform equipment
checks.
5. Confirm the diver is on bottom mix, then perform a final leak check.
6. Have the diver begin descent. On the diving chart, note the time from leaving
the surface to leaving 20 fsw in case the dive must be aborted during descent.
7. Start counting bottom time:
-
If the diver spends 10 minutes or less at 20 fsw, bottom time starts when the
diver leaves 20 fsw.
-
If the diver spends more than 10 minutes at 20 fsw, bottom time starts at the
10-minute mark.
If it is necessary to bring the diver back to the surface from 20
fsw:
1. Shift the diver from the bottom mixture to air.
2. Ventilate the diver.
3. Confirm the diver is on air.
4. Have the diver begin ascent.
5. When the diver reenters the water the 10-minute period begins again.
For dives deeper than 200 fsw in which the bottom mixture contains less than 16
percent oxygen, it is necessary to shift from the bottom mixture to 60 percent
helium/40 percent oxygen at 100 fsw during decompression or the next shallower
stop if there is no 100-fsw decompression stop. Ventilate each MK 21 MOD 1
diver using the following procedures.
1. Ventilate each diver and listen for the gas-flow change over the communications.
2. Once a gas-flow change is heard, continue to vent for an additional 10 seconds.
If a gas-flow change cannot be heard, ventilate for a minimum of 20
seconds.
The time required to effect the shift over to 40 percent oxygen is not critical.
All
dives except no-decompression dives require a shift to 100 percent oxygen at the
50-fsw stop, or at the 40-fsw stop if there is no 50-fsw stop. Upon arrival at the
stop, ventilate each MK 21 MOD 1 diver with oxygen following these steps:
1. Ventilate each diver and listen for the gas-flow change over the communications.
2. Once a gas-flow change is heard, continue to vent for an additional 10 seconds.
If a gas-flow change cannot be heard, ventilate for a minimum of 20
seconds.
Verify the diver’s voice change. Time at the stop begins when the diver is
confirmed to be on oxygen. When 50 fsw is the first oxygen stop, the ascent time
from 50 fsw to 40 fsw is included in the time of the 40-fsw stop.
For normal in-water decompression, the
diver surfaces from 40 fsw during the last minute of the 40-fsw stop. Ascent rate is
40 fsw/min. For example, if the 40-fsw stop is 68 minutes, the diver remains at 40
fsw for 67 minutes. During the last minute, he travels to the surface at 40 fsw/
minute. Figure 14-1 shows the diving chart for this dive; the in-water decompression
dive profile is shown in Figure 14-2.
figure 14-1 . HeO2 Diving Chart.
|
figure 14-2 . In-Water Decompression Dive Profile for a 249 fsw/:18 Dive.
|
There are two types of surface
decompression procedures, Normal SUR D and Emergency SUR D. Normal SURD procedures are preferred over in-water decompression procedures in routine
operations. Normal SUR D procedures improve the diver’s comfort and safety but
increase total decompression time and oxygen consumption. Emergency SUR Ds
are used for handling CNS oxygen toxicity symptoms, systems failures and other
emergency conditions. Emergency surface decompression allows the diver to be
removed from the water in the shortest possible time.
A diver is eligible for normal surface
decompression if he has been on oxygen at 40 fsw for a length of time equal to that
of the 50-fsw stop. If there is no 50-fsw stop, 10 minutes on oxygen at 40 fsw is
required.
Example. If the 50-fsw stop time is 12 minutes, the diver must remain on oxygen
at 40 fsw for 12 minutes before normal surface decompression can be
implemented.
To initiate normal surface decompression:
1. Bring the diver to the surface at 40 fsw/min and undress him.
2. Place the diver in the recompression chamber.
3. Compress on air to 40 fsw at a maximum compression rate of 80 fsw/min and
place the diver on 100 percent oxygen by mask. The interval from leaving 40
fsw in the water to arriving at 40 fsw in the chamber cannot exceed 5 minutes.
4. At 40 fsw in the chamber, the diver breathes oxygen for 30-minute periods
separated by 5-minute air breaks. The number of oxygen periods required
depends on the time of the 40-fsw water stop as indicated in Table 14-3.
5. When the last oxygen breathing period has been completed, return the diver to
breathing chamber air.
6. Ascend to the surface from 40 fsw in the chamber at a rate of 30 feet per
minute.
A normal surface decompression dive chart is shown in Figure 14-3. A normal
surface decompression dive profile is shown in Figure 14-4.
table 14-3 . Recompression Chamber Breathing Requirements
|
figure 14-3 . HeO2 Diving Chart.
|
figure 14-4 . Normal Surface Decompression Dive Profile for a 249 fsw/:18 Dive.
|
A diver is eligible for emergency
surface decompression if he is on oxygen at the 40-fsw water stop and is within 5
minutes of repeating the 50-fsw stop time. If there is no 50-fsw stop, 5 minutes on
oxygen at 40 fsw is required.
Example. If the 50-fsw stop time is 12 minutes, the diver must remain at 40 fsw
breathing oxygen for 7 minutes before emergency surface decompression can be
initiated.
The emergency surface decompression procedure is identical to the normal
surface decompression procedure except that the length of the first oxygen
breathing period at 40 fsw in the recompression chamber is lengthened from 30
minutes to 40 minutes. An emergency surface decompression dive chart is shown
Figure 14-5; the profile is shown in Figure 14-6.
figure 14-5 . HeO2 Diving Chart.
|
figure 14-6 . Emergency Surface Decompression Dive Profile for a 249 fsw/:18 Dive.
|
Follow these procedures when a dive must be
aborted during descent
.
-
Add any time spent shifting gases at 20 fsw to the bottom time to derive a
corrected bottom time.
-
Enter the table at the deepest depth attained by the diver and select the
schedule corresponding to the corrected bottom time.
-
Decompress according to the indicated schedule.
-
No correction of the bottom time for time spent at 20 fsw is needed.
-
Enter the table at the deepest depth attained by the diver and select the
schedule corresponding to the bottom time.
-
Decompress according to the indicated schedule.
In many instances the diver will be observed to fall
within the no-decompression limits when the above procedures are followed.
-
If the diver falls within the no-decompression limits and is breathing at least
16 percent oxygen, surface the diver at 30 fsw/minute.
-
If the diver falls within the no-decompression limits but is breathing less than
16 percent oxygen:
1. Bring the diver to 20 fsw at 30 fsw/minute.
2. Shift the diver to air and ventilate.
3.Surface the diver from 20 fsw when it is confirmed that the diver is breathing
air.
Example. A diver intends to dive to 300 fsw. Five minutes is spent at 20 fsw
shifting from air to an 88 percent helium/12 percent oxygen mixture. Descent is
then begun at 60 fsw/min, but at 80 fsw the diver signals for a hold because he is
unable to clear. After 2 minutes at 80 fsw, the dive is aborted. The bottom time is
3 minutes: 1 minute for the descent and 2 minutes at 80 fsw. The corrected bottom
time is 8 minutes: 5 minutes at 20 fsw and 3 minutes of bottom time. The table
shows an 80 fsw dive for 8 minutes is well within the no-decompression limit of
25 minutes. The diver should ascend to 20 fsw at 30 fsw/min, shift to air, ventilate,
and then ascend directly to the surface.
Example. A diver intends to dive to 200 fsw on an 84 percent helium/16 percent
oxygen mixture. The diver descends from the surface to 60 fsw at 60 fsw/min. No
time is spent at 20 fsw shifting gases. At 60 fsw the diver signals for a hold and
after 3 minutes at 60 fsw, the dive is aborted. The bottom time and the corrected
bottom time are both 4 minutes: zero minutes at 20 fsw, 1 minute for descent, and
3 minutes at depth. The table shows a 60 fsw dive for 4 minutes is well within the
no-decompression limit of 40 minutes. This diver may ascend directly to the
surface at 30 fsw/min.
Example. A diver intends to dive to 300 fsw. Eight minutes is spent at 20 fsw
shifting from air to an 88 percent helium/12 percent oxygen mixture. Descent is
begun at 60 fsw/min but at 140 fsw the diver signals for a hold. After 2 minutes at
140 fsw the dive is aborted. The corrected bottom time is 12 minutes: 8 minutes at
20 fsw, 2 minutes of descent time from 20 fsw to 140 fsw and 2 minutes at 140
fsw. Decompression would take place on the 140-fsw/20-minute schedule.
