CHAPTER 18 Closed-Circuit Oxygen UBA Diving
Change A 18-13
Evaluation of the Modified Draeger LAR V Closed-Circuit Oxygen
Rebreather; NEDU Report 5-79
Unmanned Evaluation of Six Closed-Circuit Oxygen Rebreathers; NEDU
CLOSED-CIRCUIT OXYGEN EXPOSURE LIMITS
The U.S. Navy closed-circuit oxygen exposure limits have been extended and
revised to allow greater flexibility in closed-circuit oxygen diving operations. The
revised limits are divided into two categories: Transit with Excursion Limits and
Single Depth Limits.
Transit with Excursion Limits Table. The Transit with Excursion Limits (Table
18-4) call for a maximum dive depth of 20 fsw or shallower for the majority of the
dive, but allow the diver to make a brief excursion to depths as great as 50 fsw.
The Transit with Excursion Limits is normally the preferred mode of operation
because maintaining a depth of 20 fsw or shallower minimizes the possibility of
CNS oxygen toxicity during the majority of the dive, yet allows a brief downward
excursion if needed (see Figure 18-3). Only a single excursion is allowed.
Single-Depth Oxygen Exposure Limits Table. The Single-Depth Limits (Table
18-5) allow maximum exposure at the greatest depth, but have a shorter overall
exposure time. Single-depth limits may, however, be useful when maximum
bottom time is needed deeper than 25 fsw.
Oxygen Exposure Limit Testing. The Transit with Excursion Limits and Single-
Depth Limits have been tested extensively over the entire depth range and are
acceptable for routine diving operations. They are not considered exceptional
exposure. It must be noted that the limits shown in this section apply only to
closed-circuit 100-percent oxygen diving and are not applicable to deep mixed-gas
diving. Separate oxygen exposure limits have been established for deep, helium-
oxygen mixed-gas diving.
Individual Oxygen Susceptibility Precautions. Although the limits described in
this section have been thoroughly tested and are safe for the vast majority of indi-
viduals, occasional episodes of CNS oxygen toxicity may occur. This is the basis
for requiring buddy lines on closed-circuit oxygen diving operations.
Table 18-4. Excursion Limits.