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