CHAPTER 8 — Surface Supplied Air Diving Operations
Change A 8-3
= 28.38 scf
Air required for ascent to reach the first stop: For this example, you need to
determine ascent time and average depth. Ascent time is 7 minutes (rounded up
from 6 minutes 20 seconds) from 190 fsw to the surface at 30 feet per minute.
Average depth is calculated as follows:
= 38.01 scf
Total Vr = 28.38 + 38.01
= 66.39 scf
2. The next step is to convert the required scf to an equivalent cylinder pressure in
psig. In this example, we are using an 80 ft3 aluminum cylinder to support this
dive. Refer to Table 7-1 for cylinder data used in this calculation:
Where:
FV
=
Floodable Volume (scf) = 0.399 scf
14.7
=
Atmospheric Pressure (psi)
Pm
=
Minimum cylinder pressure
Minimum Cylinder Pressure = First stage regulator setting + bottom
pressure at final stop: [135 psig + (0 fsw x 0.445 psi)] = 135 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.
NOTE
When planning a dive, calculations are based on 1.4 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
average depth
190
2
---------
95fsw
=
=
Ascent Vr
95
33
+
33
------------------
1.4
7
=
psig required
Vr
FV
--------
14.7
Pm
+
=
66.39
0.399
-------------
14.7
135
+
=
2580.95(round to 2600 psig)
=
