Mk 21

The MK 21 MOD 1 is an open-circuit, demand, diving helmet (Figure 8-1). The maximum working depth for air diving operations using the MK 21 MOD 1 system is 190 fsw. The MK 21 MOD 1 system may be used up to 60 fsw without an Emergency Gas Supply (EGS). An EGS is mandatory at depths deeper than 60 fsw and when diving inside a wreck or enclosed space. The Diving Supervisor may elect to use an EGS that can be man-carried or located outside the wreck or enclosed space and connected to the diver with a 50 to 150 foot whip. Planned air dives below 190 fsw require CNO approval.

8-2.1 Operation and Maintenance. The technical manual for the MK 21 MOD 1 is

NAVSEA S6560-AG-OMP-010, Technical Manual, Operation and Maintenance Instructions, Underwater Breathing Apparatus MK 21 MOD 1 Surface-Supported Diving System. To ensure safe and reliable service, the MK 21 MOD 1 system must be maintained and repaired in accordance with PMS procedures and the MK 21 MOD 1 operation and maintenance manual.

8-2.2 Air Supply. Air for the MK 21 MOD 1 system is supplied from the surface by either an air compressor or a bank of high-pressure air flasks as described in paragraph 8-6.2.3.

Figure 8-1. MK 21 MOD 1 SSDS.

8-2.2.1 Emergency Gas Supply Requirements. The emergency breathing supply valve provides an air supply path parallel to the nonreturn valve and permits attachment of the EGS whip. The EGS system consists of a steel 72 (64.7 cubic-foot [minimum]) scuba bottle with either a K- or J- valve and a first-stage regulator set at 135 ± 5 psi over bottom pressure. A relief valve set at 180 ± 5 psi over bottom pressure must be installed on the first-stage regulator to prevent rupture of the low-pressure hose should the first-stage regulator fail. The flexible low-pressure hose from the first-stage regulator attaches to the emergency supply valve on the helmet sideblock. A submersible pressure gauge is also required on the first-stage regulator.

When using an EGS whip 50 to 100 feet in length, set at manufacturer's recommended pressure, but not lower than 135 psi. If the diving scenario dictates leaving the EGS topside, adjust the first-stage regulator to 150 psig.

8-2.2.2 Flow Requirements. When the MK 21 MOD 1 system is used, the air supply system must be able to provide an average sustained flow of 1.4 acfm to the diver. The air consumption of divers using the MK 21 MOD 1 varies between 0.75 and 1.5 acfm when used in a demand mode, with occasional faceplate and mask clearing. When used in a free-flow mode, greater than eight acfm is consumed.

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