Closedcircuit 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.

18-4.1 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.

Table 18-4. Excursion Limits.


Maximum Time

21-40 fsw

15 minutes

41-50 fsw

5 minutes

18-4.2 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.

18-4.3 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.

18-4.4 Individual Oxygen Susceptibility Precautions. Although the limits described in this section have been thoroughly tested and are safe for the vast majority of individuals, occasional episodes of CNS oxygen toxicity may occur. This is the basis for requiring buddy lines on closed-circuit oxygen diving operations.

Figure 18-3. Example of Transit with Excursion. Table 18-5. Single-Depth Oxygen Exposure Limits.


Maximum Oxygen Time

25 fsw

240 minutes

30 fsw

80 minutes

35 fsw

25 minutes

40 fsw

15 minutes

50 fsw

10 minutes

18-4.5 Transit with Excursion Limits. A 20 foot maximum depth for transit with one excursion, if necessary, will be the preferred option in most combat swimmer operations. When operational considerations necessitate a descent to deeper than 20 fsw for longer than allowed by the excursion limits, the appropriate single-depth limit should be used (paragraph 18-4.6).

18-4.5.1 Transit with Excursion Limits Definitions. The following definitions are illustrated in Figure 18-3:

Transit is the portion of the dive spent at 20 fsw or shallower.

Excursion is the portion of the dive deeper than 20 fsw.

Excursion time is the time between the diver's initial descent below 20 fsw and his return to 20 fsw or shallower at the end of the excursion.

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-oxygen 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 prescribed 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 continued 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


excursion limit for 40 fsw. He may continue the dive, but he must maintain his depth at 25 fsw or less and make no additional excursions.

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