During the 12-hour period after an air dive, the quantity of residual nitrogen in
divers’ bodies will gradually be reduced to its normal level. If the divers are to
make a second dive within this period (repetitive dive), they must consider their
residual nitrogen level when planning for the dive.
The procedures for conducting a repetitive dive are summarized in Figure 9-7.
Upon completing the first dive, the divers are assigned a repetitive group designation
from either the Standard Air Decompression Table or the Unlimited/No-
Decompression Table. This designation relates directly to the residual nitrogen
level upon surfacing. As nitrogen passes out of the diver’s tissues and blood, their
repetitive group designation changes. By using the Residual Nitrogen Timetable
(Table 9-7), this designation may be determined at any time during the surface
interval.
To determine the decompression schedule for a repetitive dive using either the
unlimited/no-decompression, standard air, or surface decompression table:
1. Determine the residual nitrogen level just prior to leaving the surface of the of
the repetitive dive (based on the repetitive dive depth), using the Residual
Nitrogen Timetable. This level is expressed as residual nitrogen time, in
minutes.
2. Add this time to the actual bottom time of the repetitive dive to get the bottom
time of the Equivalent Single Dive.
3. Conduct decompression from the repetitive dive using the depth and bottom
time of the equivalent single dive to select the appropriate decompression
schedule. Avoid equivalent single dives requiring the use of Exceptional
Exposure decompression schedules.
Always use a systematic Repetitive Dive Worksheet, shown in Figure 9-8, when
determining the decompression schedule for a repetitive dive. If still another dive
follows the repetitive dive, insert the depth and bottom time of the first equivalent
single dive in Part One of the second Repetitive Dive Worksheet.
FIGURE 9-7 Repetitive Dive Flowchart.
|
FIGURE 9-8 Repetitive Dive Worksheet.
|
TABLE 9-7 Residual Nitrogen Timetable for Repetitive Air Dives.
|
The quantity of residual
nitrogen in a diver’s body immediately after a dive is expressed by the repetitive
group designation assigned from either the Standard Air Decompression Schedule
or the Unlimited/No-Decompression Table. The upper portion of the Residual
Nitrogen Timetable is composed of various intervals between 10 minutes and 12
hours. These are expressed in hours and minutes (2:21 = 2 hours, 21 minutes).
Each interval has a minimum time (top limit) and a maximum time (bottom limit)
Residual nitrogen times corresponding to the depth of the repetitive dive are given
in the body of the lower portion of the table. To determine the residual nitrogen
time for a repetitive dive:
1. Locate the diver’s repetitive group designation from the previous dive along
the diagonal line above the table.
2. Read horizontally to the interval where the diver’s surface interval lies. The
time spent on the surface must be between or equal to the limits of the selected
interval.
3. Read vertically down to the new repetitive group designation. This corresponds
to the present quantity of residual nitrogen in the diver’s body.
4.Continue down in this same column to the row representing the depth of the
repetitive dive. The time given at the intersection is the residual nitrogen time,
in minutes, to be applied to the bottom time of the repetitive dive.
A repetitive dive is planned to 98 fsw for an estimated bottom time of
15 minutes. The previous dive was to a depth of 100 (100+1=101) fsw with a
bottom time of 48 minutes. The diver’s surface interval is 6 hours 26 minutes
(6:26). Determine the proper decompression schedule.
1. Use the 110/50 schedule of the Standard Air Decompression Table to find the
residual nitrogen time of the previous dive. Read across the 50-minute bottom
time row to find the repetitive group designator of M.
2. RMove to the Residual Nitrogen Timetable for Repetitive Air Dives.
3. Enter the table on the diagonal line at M.
4.Read horizontally across the line until reaching the surface interval coinciding
with the diver’s surface interval of 6 hours 26 minutes. The diver’s surface
interval falls within the limits of the 6:19/9:28 column.
5. Read vertically down the 6:19/9:28 column until reaching the depth coinciding
with the repetitive dive depth of 100 fsw to find the residual nitrogen time of 7
minutes.
6. Add the 7 minutes of residual nitrogen time to the estimated bottom time of 15
minutes to obtain the single equivalent dive time of 22 minutes.
7. The diver will be decompressed on the 100/22 No-Decompression schedule.
Figure 9-9 depicts the dive profile for the first dive, Figure 9-10 shows the Repetitive
Dive Worksheet, and Figure 9-11 shows the dive profile for the repetitive
dive.
FIGURE 9-9 Dive Profile.
|
FIGURE 9-10 Completed Repetitive Dive Worksheet.
|
FIGURE 9-11 Dive Profile for Repetitive Dive.
|
An exception to this table occurs when the repetitive dive is
made to the same or greater depth than that of the previous dive. This is referred to
as the RNT Exception Rule. In such cases, the residual nitrogen time may be
longer than the bottom time of the previous dive. A diver’s body cannot contain
more residual nitrogen than it was originally exposed to. To obtain the equivalent
single dive time, simply add the bottom time of the previous dive to that of the
repetitive dive. (All of the residual nitrogen passes out of a diver’s body after 12
hours, so a dive conducted after a 12-hour surface interval is not a repetitive dive.)
