15-8 UBA GAS USAGE

UBA GAS USAGE

Gas usage can be the controlling factor in the planning for a mission and determining appropriate excursions. However, gas usage is UBA- and platformspecific.

Emergency Gas Supply Duration

The gas computation in paragraph 15-8.1 is used to determine excursion limits based on diver’s gas storage. The diver’s emergency gas supply (EGS) duration should also be calculated using the following formulas:

mmp = (D × .445) + psi (obp)

psi available for use = psi (cylinder) - mmp

scf gas available   =   psi   ( Available ) + 14.7

scfm = acfm × ata

duration in minutes =scfm ------------

D = depth of diver

psi (obp) = over-bottom pressure required for specific UBA

mmp = minimum manifold pressure

fv = floodable volume of cylinder

acfm = actual cubic feet per minute at excursion depth required by specific UBA being used

scfm = standard cubic feet per minute required to deliver acfm

Example. Using an 80-cubic-foot aluminum cylinder (floodable volume = .399 cu. ft.) filled to 3,000 psig, calculate the diver’s EGS duration at 300 fsw..

1. Calculate the psi available for use:
185.0 overbottom psi MK21 MOD0
 133.5 300 fsw converted to psi
----------------------------------
  318.5 psi round upto 319 psi

2.Calculate the psig available for use:

3,000-319 psig=2,681 psig available for use

3. Calculate the scf of gas available:

2681 + 14.7        
------------ * 0.399 = 73.2  scf of gas available
    14.7
´

4. Calculate the total gas usage requirement:

1.4  acfm × 10.09 ata = 14.13 scfm

5.Calculate the duration of the gas supply:

73.2 scf        
---------   =   5.18 minutes
14.13scfm        

The duration of the emergency gas supply is very short, especially at greater depths.

Gas Composition

The percentage of oxygen in the mix depends on diver depth and can be calculated as follows:

1. % decimal equivalent = ppO2 desired ata

2. % decimal equivalent × 100 = % of O2 required to maintain desired ppO2

Example. Calculate the minimum and maximum percentage of O2 required to sustain a .44 to 1.25 ppO2 range at 300 fsw.

1. Calculate the minimum percentage of O2 required to sustain the lower value of the range:

0.44 ata 10.09 ata ---------------------- =0.0436 ´ 100=4.36%

4.36% O2 in He provides the minimum ppO2.

2. Calculate the maximum percentage of O2 required to sustain the lower value of the range:

1.25 ata 10.09 ata ----------------------= 0.1239 ´ 100=12.39%

12.39% O2 in He provides the maximum ppO2.

Specific Dives

For a specific dive, storage of gas to support the mission may be the controlling parameter. The following formulas may be used to calculate gas usage by divers:

        D+33
ata   =   ----------
        33

scfm (for one diver at depth) = ata × acfm

total scfm = scfm × number of divers

scf required = scfm × minutes

D = depth of diver

ata = atmosphere absolute

acfm = actual cubic feet per minute required by specific UBA being used (refer to the tech manual)

number of divers = total number of divers making excursion

minutes = duration of excursion

scf required = standard cubic feet of gas required to support the divers

Example. Two divers and one standby diver using the MK 21 MOD 0 and MK 22 MOD 0 UBAs at 300 fsw are deployed for a 15-minute excursion. Determine the gas usage.

1. Convert the depth to atmospheres:

  300 fsw + 33 fsw
  --------------- = 10.09 ata
  33 fsw

2.Calculate gas usage for 1 diver:

10.09 ata
x 1.4 acfm for MK21MOD 0
14.13 scfm for 1 diver at 300 fsw

3. Calculate gas usage for 3 divers:

14.13 scfm for 1 diver at 300 fsw
x 3 divers 2 and standby 1
42.39 scfm for 3 diver at 300 fsw

4. Calculate the total gas usage requirement:

42.39 scfm
x 15 minutes excursion time
635.85 scf ( round up  to 635 scf )

A gas usage requirement of 636 Standard Cubic Feet of helium-oxygen can be expected for this two-diver excursion.

NOTE Usage for three divers is computed even though the standby would not normally be using gas for the entire 15 minutes.

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