The following terms are frequently used when conducting diving operations and
discussing the decompression tables.
Descent time is the total elapsed time from when the divers leave
the surface to the time they reach the bottom. Descent time is rounded up to the
next whole minute.
The repetitive group designation is a letter used to
indicate the amount of residual nitrogen remaining in a diver’s body following a
previous dive.
Residual nitrogen is the nitrogen gas still dissolved in a
diver’s tissues after surfacing.
Residual nitrogen time is the time that must be added to
the bottom time of a repetitive dive to compensate for the nitrogen still in solution
in a diver’s tissues from a previous dive. Residual nitrogen time is expressed in
minutes.
A single dive refers to any dive conducted more than 12 hours after a
previous dive.
A single repetitive dive is a dive for which the bottom
time used to select the decompression schedule is the sum of the residual nitrogen
time and the actual bottom time of the dive.
The surface interval is the time a diver has spent on the surface
following a dive. It begins as soon as the diver surfaces and ends as soon as he
starts his next descent.
Bottom time is the total elapsed time from when the divers leave the
surface to the time they begin their ascent from the bottom. Bottom time is
measured in minutes and is rounded up to the next whole minute.
A decompression table is a structured set of decompression
schedules, or limits, usually organized in order of increasing bottom times
and depths.
A decompression schedule is a specific decompression
procedure for a given combination of depth and bottom time as listed in a
decompression table. It is normally indicated as feet/minutes.
A decompression stopis a specified depth where a diver
must remain for a specified length of time (stop time).
The following terms are used to indicate the depth of a dive:
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Maximum depth is the deepest depth attained by the diver plus the
pneumofathometer correction factor (Table 9-1). When conducting scuba
operations, maximum depth is the deepest depth gauge reading.
-
Stage depthis the pneumofathometer reading taken when the divers are on the
stage just prior to leaving the bottom. Stage depth is used to compute the
distance and travel time to the first stop, or to the surface if no stops are
required.
TABLE 9-1 Pneumofathometer Correction Factors.
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The equivalent single dive bottom time is
the time used to select a schedule for a single repetitive dive. This time is
expressed in minutes.
The maximum time that can be
spent at a given depth that safe ascent can be made directly to the surface at a
prescribed travel rate with no decompression stops is the unlimited/no-decompression
or No “D” limit (Table 9-6).
TABLE 9-6 Unlimited/No-Decompression Limits and Repetitive Group Designation Table for Unlimited/No-
Decompression Air Dives.
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A repetitive dive is any dive conducted more than 10 minutes and
within 12 hours of a previous dive.
