CHAPTER 18 — Closed-Circuit Oxygen UBA Diving
Change A 18-15
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-oxy-
gen time).
18-4.5.2
Transit with Excursion Rules. A diver who has maintained a transit depth of 20
fsw or shallower may make one brief downward excursion as long as he observes
these rules:
Maximum total time of dive (oxygen time) may not exceed 240 minutes.
A single excursion may be taken at any time during the dive.
The diver must have returned to 20 fsw or shallower by the end of the pre-
scribed excursion limit.
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 15 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 20 fsw to the time at which they finish the excursion
must be 15 minutes or less.
18-4.5.3
Inadvertent Excursions. If an inadvertent excursion should occur, one of the
following situations will apply:
If the depth and/or time of the excursion exceeds the limits in Table 18-4 or if
an excursion has been taken previously, the dive must be aborted and the diver
must return to the surface.
If the excursion was within the allowed excursion limits, the dive may be con-
tinued 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
