Atmospheres absolute is usually abbreviated “ata”
or “ATA” to distinguish it clearly from “atm” which
could also mean gauge pressure. Many short-handedly
use “ata” as a word in itself–usually in the plural form
“atas.” Don’t be taken aback if someone asks: “What’s
the Pee Oh Two of Eee A En thirty-two at four atas?” As
you’ll discover shortly, the answer is 1.28 atmospheres.
CONVERTING BETWEEN DEPTH AND
PRESSURE BY FORMULA
Converting depth to pressure uses a simple equation
in which you first find the water (hydrostatic) pressure
in atmospheres and then add one atmosphere for the
sea-level air pressure to convert to the absolute pressure.
Written as an equation using the U.S./Imperial system:
Pata = D fsw +
1
where P = the pressure in atmospheres absolute
D = the depth in feet of seawater
For example: What is the absolute pressure at a depth of
108 fsw?
Pata = 108 fsw + 1 =
4.27
Using the S.I./metric system:
Pbar Dmsw absolute = +
1
where P = the pressure in bars absolute
D = the depth in meters of seawater
For example: What is the absolute pressure at 27 msw?
Pbar = 27 msw + 1 =
3.7
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:
Pata D fsw fsw
or:
= + 33
fsw atm
33 /
Chapter 2-Gases & Gas Mixtures
Pbar Dmsw msw absolute
= +10
msw bar
10 /
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:
D fsw = (P ata −1atm)×33 fsw/ atm
if you choose to convert from atmospheres absolute to
gauge pressure first. Or it can be written:
D fsw = (P ata×33 fsw/ atm)− 33 fsw
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?
D fsw = (3.5ata −1atm)×33 fsw/ atm = 82.5 fsw
In S.I./metric the equation becomes:
Dmsw (Pbar bar) msw bar = absolute −1 ×10 /
or:
Dmsw (Pbar msw bar ) msw = absolute ×10 / −10
For example: At what depth is the absolute pressure
equal to 4.6 bars?
absolute absolute bar bars
msw bar
10 /
bar
msw bar
10 /
atm ata
fsw atm
33 /
atm
fsw atm
33 /
Dmsw ( bar bar) msw bar msw = 4.6 absolute −1 ×10 / = 36
Gases & Gas Mixtures
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