NOTE When planning a dive calculations are based on 14 acfm

To satisfactorily support the MK 21 MOD 1 system, the air supply must:

■ Replenish the air consumed from the system (average rate of flow)

■ Replenish the air at a rate sufficient to maintain the required pressure

■ Provide the maximum rate of flow required by the diver

8-2.2.3 Pressure Requirements. Because the MK 21 MOD 1 helmet is a demand-type system, the regulator has an optimum overbottom pressure that ensures the lowest possible breathing resistance and reduces the possibility of overbreathing the regulator (demanding more air than is available). The optimum overbottom pressure for all dives shallower than 130 fsw is 135 psi. For those systems which cannot maintain 135 psig when diving shallower than 60 fsw, 90 psi is permissible. The manifold supply pressure requirement for dives 130-190 fsw is 165 psi. For those systems not capable of sustaining 165 psi overbottom due to design limitations, 135 psi overbottom is acceptable.

This ensures that the air supply will deliver air at a pressure sufficient to overcome bottom seawater pressure and the pressure drop that occurs as the air flows through the hoses and valves of the mask.

Sample Problem 1. Determine the air supply manifold pressure required to dive the MK 21 MOD 1 system to 175 fsw.

1. Determine the bottom pressure at 175 fsw:

Bottom pressure at 175 fsw = 175 x .445 psi = 77.87 psig (round to 78)

2. Determine the overbottom pressure for the MK 21 MOD 1 system (see paragraph 8-2.2.3). Because the operating depth is 175 fsw, the overbottom pressure is 165 psig.

3. Calculate the minimum manifold pressure (MMP) by adding the bottom pressure to the overbottom pressure:

The minimum manifold pressure for a 175-fsw dive must be 243 psig.

Sample Problem 2. Determine if air from a bank of high-pressure flasks is capable of supporting two MK 21 MOD 1 divers and one standby diver at a depth of 130 fsw for 30 minutes. There are 5 flasks in the bank; only 4 are on line. Each flask has a floodable volume of 8 cubic feet and is charged to 3,000 psig.

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