2
32 NAUI Nitrox Diver
CALCULATING PARTIAL
PRESSURES
To determine the partial pressure of a gas in a mixture,
multiply the gas fraction (the gas percentage expressed as
a fraction of one) by the absolute pressure. You have seen
the formula earlier in this chapter:
Pg = Fg x Ptotal
or summed up as: “The part is a fraction of the whole.”
This equation can be rearranged to find the gas
fraction/percentage if you know the partial pressure and
the total pressure (Fg = Pg / Ptotal) or to find the absolute
pressure if you know the partial pressure and fraction/
percentage of the gas (Ptotal = Pg / Fg).
Some people find it easier to use a graphic memory
picture rather than a mathematical formula or a
mnemonic phrase. Figure 2-8 shows the mathematical
relationship of partial pressure, gas fraction, and total
pressure.
To use the figure, cover the item you want to know
or solve for. The mathematical expression will be shown
by the two exposed items. If “partial pressure” is covered,
the side-by-side terms indicate that they are to be
multiplied. Similarly, if you want to know what fraction
will provide a desired partial pressure on the dive, cover
“fraction.” The uncovered terms (“partial pressure” over
“total pressure”) tell you to divide the desired partial
pressure by the absolute pressure at depth.
For example: At what depth does the partial pressure
of oxygen in the air (21% O2) reach a partial pressure of
1.4 atmospheres?
First find the absolute pressure at which the PO2
reaches 1.4 atmospheres. Then convert that absolute
pressure to its equivalent depth.
As an S.I./metric problem:
Then convert to depth:
As a U.S./Imperial problem:
Then convert to depth:
For the mathematically inclined, this two-step
procedure can also be written as a single formula:
÷ ÷
X
Figure 2-8 Mathematical relationship of partial
or:
Partial
Pressure
Total
Pressure
Gas
Fraction
pressure, gas fraction, and total pressure.
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 × 4 = 2.72).
Written out as a metric calculation:
First find the absolute pressure at 30 msw.
Then find the partial pressure of the nitrogen in
EAN32 at that absolute pressure.
As a U.S./Imperial calculation:
First find the absolute pressure at 99 fsw.
Then:
Pabsolute = 1 . 4 b a r = 6.67 bar
0.21
D msw = (6.67 bar −1 bar) × 10 msw / bar = 56.7 msw
Pabsolute = P g a s = 1 .4 a t a = 6.67 ata
Fgas 0.21
D fsw = (Pabsolute - 1 atm) × 33 fsw / atm
= (6.67 ata - 1 atm) × 33 fsw / atm = 187 fsw
( Pgas
D msw = Fgas
− 1 ba r ) × 10 msw / bar
( Pgas
D fsw = Fgas
− 1 a tm) × 33 fsw / atm
Pabsolute = D m s w + 1 b a r = 3 0 m s w +1 bar = 4 bar
10 msw / bar 10 msw / bar
PN2 = FN2 × Pabsolute = 0.68 × 4 ata = 2.72 ata
Pabsolute = D f s w + 1 atm= +1 atm = 4 ata
33 fsw / atm
99 fsw
33 fsw / atm
PN2 = FN2 × Pabsolute = 0.68 × 4 ata = 2.72 ata