As a S.I./metric problem:
P = 1.4 bars =
absolute 6.67
bars Then convert to depth:
Dmsw = (6.67bars −1bar)×10msw/ bar = 56.7msw
As a U.S./Imperial problem:
ata ata
P
= gas
= 1.4 =
absolute 6.67
F
Then convert to depth:
( )
( ata atm) fsw atm fsw
D fsw = P − atm ×
fsw atm absolute
6.67 1 33 / 187
For the mathematically inclined, this two-step procedure
can also be written as a single formula:
or
⎞
⎟ ⎟
atm fsw atm
P
gas
F
D fsw
gas
1 × 33 /
⎠
⎛
⎡
=
⎜ ⎜
⎝
−
⎤
⎥ ⎥⎦
⎢ ⎢⎣
In a similar way, the partial pressure for any component
gas in any breathing mixture (nitrox or trimix) can
be determined for any depth. For example, the fraction of
nitrogen in EAN32 is 0.68 (the nitrox mix is 68% nitrogen).
At a depth of 30 meters (99 feet), the absolute pressure
is 4 atmospheres. So, the partial pressure of the
nitrogen (PN2) is 2.72 atmospheres (0.68 x 4 = 2.72).
Written out as a metric calculation:
First find the absolute pressure at 30 msw.
P Dmsw absolute 1 4
bar bars
bar 30
msw
= + = + =
msw bar
msw bar
10 /
1
10 /
Then find the partial pressure of the nitrogen in
EAN32 at that absolute pressure.
PN FN P ata ata absolute 0.68 4 2.72 2 2 = × = × =
As a U.S./Imperial calculation:
First find the absolute pressure at 99 fsw.
P D fsw absolute 1 4
Then:
atm 99
fsw
= + = + =
fsw atm
fsw atm
33 /
1
33 /
PN FN P ata ata absolute 0.68 4 2.72 2 2 = × = × =
atm ata
The two separate calculations could also be combined
into a single formula:
⎞
⎛
P F Dmsw gas
gas gas
or
( )
msw bar
F × Dmsw +
msw
10 /
10
( )
fsw atm
F D fsw fsw
= ⎟ ⎟⎠
⎞
⎜ ⎜⎝
= × +
⎛
bar
msw bar
1
10 /
P = F × D fsw +
atm
gas
gas gas
fsw atm
× +
33 /
33
1
33 /
= ⎟ ⎟⎠
⎜ ⎜⎝
It is straightforward to calculate oxygen or nitrogen
partial pressures at depth for enriched air nitrox. Of
course, to save the effort of calculating, one can employ
an oxygen partial pressure table, which can quickly show
the partial pressure of oxygen for various mixes over a
range of depths.
⎞
⎤
gas / 10 1 × ⎟ ⎟
bar msw bar
P
F
Dmsw
gas
⎠
⎛
⎡
=
⎜ ⎜
⎝
−
⎥ ⎥⎦
⎢ ⎢⎣
1 33 /
= − × =
P
gas
0.21
0.21
Chapter 2-Gases & Gas Mixtures
Gases & Gas Mixtures
25
FIGURE 2-8: MANY TABLES ARE AVAILABLE TO SIMPLIFY NITROX
DIVE PLANNING