2
Chapter 2 Gases and Gas Mixtures 31
For example: What is the absolute pressure at a depth of
108 fsw?
Using the S.I./metric system:
where P = the pressure in bars absolute
D = the depth in meters of seawater
For example: What is the absolute pressure at 27 msw?
27 msw
10 msw / bar
Pbarabsolute = + 1 bar = 3.7 barabsolute
These equations can be written another way, by
adding the one atmosphere or bar of pressure, expressed
as depth, to the actual depth and then performing the
division:
or:
Once you have converted the depth to absolute
pressure in atmospheres/bars, it is simple to determine
the partial pressure of a gas at that depth. Just multiply
the absolute pressure by the gas fraction to find the
partial pressure of the gas at depth.
You may also need to convert an absolute pressure
into its equivalent depth in fsw or msw. The above
equation works equally well if rewritten to solve for
depth:
U.S./Imperial:
If you choose to convert from atmospheres absolute to
gauge pressure first. Or it can be written:
If you choose to convert from atmospheres absolute to
fsw absolute first and then subtract the air pressure (in
fsw) to find the depth.
For example: At what depth is the absolute pressure equal
to 3.5 atm?
In S.I./metric the equation becomes:
or:
For example: At what depth is the absolute pressure equal
to 4.6 bar?
Converting Between Depth and Pressure
With a Table
You can also use a table for conversions between depth
and pressure (Figure 2-7). Interpolating is simple if using
metric depth – each meter adds 0.1 ata. In U.S./Imperial
measure, each three feet adds approximately 0.1 ata.
Pata = D f s w + 3 3 f s w
33 fsw / atm
Pbarabsolute = D m s w + 1 0 m s w
10 msw / bar
D fsw = (Pata − 1 atm) × 33 fsw / atm
D fsw = (Pata × 33 fsw / atm) − 33 fsw / atm
D fsw = (3.5 ata − 1 atm) × 33 fsw / atm = 82.5 fsw
D msw = (Pbarabsolute × 10 msw / bar) − 10 msw
D msw = (4.6 barabsolute − 1 bar) × 10 msw/bar = 36 msw
Pata = 1 0 8 f s w + 1 atm = 4.27 ata
33 fsw / atm
Pbarabsolute = D m s w + 1 bar
10 msw / bar
D msw = (Pbarabsolute − 1 bar) × 10 msw / bar
Absolute Pressure at Depth
Depth
(msw)
Depth
(fsw)
Pressure
(ata)
0 0 1
3 10 1.3
6 20 1.6
9 30 1.9
12 40 2.2
15 50 2.5
18 60 2.8
21 70 3.1
24 80 3.4
27 90 3.7
30 100 4.0
34 110 4.4
37 120 4.7
40 130 5.0
43 140 5.3
46 150 5.6
Figure 2-7 This table can be used to convert either
from depth to absolute pressure or
absolute pressure to equivalent depth.