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# 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.