8-4 Change A
U.S. Navy Diving ManualVolume 2
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 regu-
lator (demanding more air than is available). For those systems not capable of
sustaining 165 psi overbottom due to design limitations, 135 psi overbottom is
acceptable. Table 8-1 shows the MK 21 MOD 1 overbottom pressure
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
= 77.87 psig (round to 78)
2. Determine the overbottom pressure for the MK 21 MOD 1 system (see Table
8-1). Because the operating depth is 175 fsw, the overbottom pressure is 165
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.
These calculations are based on an assumption of an average of 1.4
acfm diver air consumption over the total time of the dive. Higher con-
sumption over short periods can be expected based on diver work rate.
Table 8-1. MK 21 MOD 1 Over Bottom Pressure Requirements
Pressure in psig
* Not approved for use with a double exhaust kit installed. Instead use a minimum of 135 psig.
** For diver life support systems not capable of sustaining 165 psig over bottom due to system design
limitations, 135 psig is authorized.