1. Calculate minimum manifold pressure (MMP).
Round up to 223 psig
2. Calculate standard cubic feet (scf) of air available. The formula for calculating the scf of air available is:
Flask pressure = 3,000 psig Minimum flask pressure = 220 psig 223 psig
Capacity of flasks = 8 cffv Number of flasks = 4
„ , , 3000- (220 + 223) _ . scf available = -J4-7-- x 8 x 4
= 5566.26 scf (round down to 5566)
3. Calculate scf of air required to make the dive. You will need to calculate the air required for the bottom time, the air required for each decompression stop, and the air required for the ascent. The formula for calculating the air required is:
D = Depth (feet) V = acfm needed per diver N = Number of divers T = Time at depth (minutes)
Bottom time: 30 minutes
„ . , 130 + 33 , . _ on scf required = ——— x 1.4 x 3 x 30 33
Decompression stops: A dive to 130 fsw for 30 minutes requires the following decompression stops:
■ 3 minutes at 20 fsw scf required = 20,+ 33 x 1.4 x 3 x 3 33
■ 18 minutes at 10 fsw scf required = 10,+ 33 x 1.4 x 3 x 18 33
Ascent time: 5 minutes (rounded up from 4 minutes 20 seconds) from 130 fsw to the surface at 30 feet per minute.
average depth = -^p = 65 feet scf required = 65,+ 33 x 1.4 x 3 x 5 33
Total air required = 622.36 + 20.24 + 98.51 + 62.36 = 803.48 scf (round to 804 scf)
4. Calculate the air remaining at the completion of the dive to see if there is sufficient air in the air supply flasks to make the dive.
scf remaining = scf available - scf required = 5609 scf - 804 scf = 4805 scf
More than sufficient air is available in the air supply flasks to make this dive.
NOTE Planned air usage estimates will vary from actual air usage. The air requirements for a standby diver must also be taken into account for all diving operations. The Diving Supervisor must note initial volume/pressure and continually monitor consumption throughout dive. If actual consumption exceeds planned consumption, the Diving Supervisor may be required to curtail the dive in order to ensure there is adequate air remaining in the primary air supply to complete decompression.
The MK 20 MOD 0 is a surface-supplied UBA consisting of a full face mask, diver communications components, equipment harness, and an umbilical assembly (Figure 8-2). One of its primary uses is in enclosed spaces, such as submarine ballast tanks. The MK 20 MOD 0 is authorized for use to a depth of 60 fsw with surface-supplied air and must have an Emergency Gas Supply when used for enclosed space diving.
Operation and Maintenance. Safety considerations and working procedures are covered in Chapter 6. NAVSEA SS600-AK-MM0-010 Technical Manual, Operations and Maintenance Instruction Manual is the technical manual for the MK 20 MOD 0. To ensure safe and reliable service, the MK 20 MOD 0 system must be maintained and repaired in accordance with PMS procedures and the MK 20 MOD 0 operation and maintenance manual.
8-3.2 Air Supply. Air for the MK 20 MOD 0 system is supplied from the surface by either an air compressor or a bank of high-pressure flasks as described in paragraph 8-6.2.3.
8-3.2.1 EGS Requirements for MK 20 MOD 0 Enclosed-Space Diving. In order to ensure a positive emergency air supply to the diver when working in a ballast tank, mud tank, or confined space, an Emergency Gas Supply (EGS) assembly must be used. As a minimum, the EGS assembly consists of:
Single scuba cylinder steel 72 (minimum 64.7 cubic feet) with either a K- or J-valve, charged to a minimum of 1,800 psi.
■ An approved scuba regulator set at manufacturer's recommended pressure, but not lower than 135 psi, with an extended EGS whip 50 to 150 feet in length. If the diving scenario dictates leaving the EGS topside, adjust the first-stage regulator to 150 psig.
The scuba cylinder may be left on the surface and the EGS whip may be married to the diver's umbilical, or it may be secured at the opening of the enclosed space being entered. The diver may then enter the work space with the extended EGS whip trailing. The second-stage regulator of the EGS is securely attached to the diver's harness before entering the work space so that the diver has immediate access to the EGS regulator in an emergency.
8-3.2.2 Flow Requirements. The MK 20 MOD 0 requires a breathing gas flow of 1.4 acfm and an overbottom pressure of 90 psig. Flow and pressure requirement calculations are identical to those for the MK 21 MOD 1 (see paragraph 8-2.2.3).
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