(7 per watch)


1. Depth limits are based on considerations of working time, decompression obligation, oxygen tolerance and nitrogen narcosis. The expected duration of the gas supply, the expected duration of the carbon dioxide absorbent, the adequacy of thermal protection, or other factors may also limit both the depth and the duration of the dive.

2. An on-station chamber is defined as a certified and ready chamber at the dive site.

Operations deeper than 300 fsw usually require Deep Diving Systems (DDSs). The decompression obligation upon the diver is of such length that in-water decompression is impractical. Using a personnel transfer capsule (PTC) to transport divers to a deck decompression chamber (DDC) increases the margin of diver safety and support-ship flexibility.

13-3.4 Bottom Time Requirements. The nature of the operation may influence the bottom time requirements of the diver. An underwater search may be best undertaken by using multiple divers with short bottom times or by conducting a single bounce dive simply to identify a submerged object. Other tasks, such as underwater construction work, may require numerous dives with long bottom times requiring surface-supplied or saturation diving techniques. Although primarily intended to support deep diving operations, saturation diving systems may be ideal to support missions as shallow as 150 fsw where the nature of the work is best accomplished using several dives with extended bottom times. Under these conditions, time is saved by eliminating in-water decompression obligations for each diver and by reducing the number of dive team changes, thus compensating for the increased logistical complexity such operations entail.

13-3.5 Environment. Environmental conditions play an important role in planning mixed-gas diving operations. Environmental factors, such as those addressed in Chapter 6, should be considered when planning such operations. Mixed-gas diving operations often involve prolonged dives requiring lengthy decompression and travels that carry divers great distances from a safe haven. Special attention should therefore be given to preventing diver hypothermia. Mixed-gas diving apparatus are designed to minimize thermal stress, but the deepest, longest helium-oxygen dives place the greatest stress on the diver. Exposure to extreme surface conditions prior to the dive may leave the diver in a thermally compromised state. A diver who has been exposed to adverse environmental conditions should not be considered for mixed-gas diving until complete rewarming of the diver has taken place, as shown by sweating, normal pulse, and return of normal core temperature. Subjective thermal comfort does not accurately indicate adequate rewarming.

13-3.6 Mobility. Some diving operations may dictate the use of a diving method that is selected as a result of special mobility requirements in addition to depth, bottom time and logistical requirements. The MK 21 MOD 1 is the preferred method when operations require mobility in the water column (see Figure 13-1).

Figure 13-1. Searching Through Aircraft Debris on the Ocean Floor.

For missions where mobility is an essential operating element and depth and bottom time requirements are great, closed-circuit diving may be the only available option. Such diving is frequently required by special warfare and/or explosive ordnance disposal (EOD) personnel.

13-3.7 Equipment Selection. Equipment and supplies available for mixed-gas diving operations by U.S. Navy personnel have been tested under stringent conditions to ensure that they will perform according to design specifications under the most difficult conditions that may be encountered. Several types of equipment are available for mixed-gas operations. Equipment selection is based upon the chosen diving method, depth of the dive and the operation to be performed. Table 13-3 outlines the differences between equipment configurations.


Principal Applications

Minimum Personnel



Restrictions and Depth Limits

MK 21 MOD 1 (Notes 1 & 3)

Deep search, inspection and repair.

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