4
Chapter 4 Choosing the Best Nitrox Mix 53
separate steps, or the two steps can be blended into a
single formula.
Example: What is the best mix for a dive to 30 msw
(99 fsw) if the oxygen partial pressure is not to exceed 1.4
atmospheres?
Using the U.S./Imperial depth (99 fsw):
Step 1: Calculate the absolute pressure at 99 fsw.
Divide the depth by 33 fsw/atm to determine the
hydrostatic pressure; then add one atmosphere for the air
pressure at the surface to find the absolute pressure. Use
either of the formulas in Chapter 2:
and enter the known values:
Step 2: Find what fraction of oxygen will produce
the target PO2 at 4 bars/atmospheres absolute. The basic
formula (the part is a fraction of the whole) is:
Pg = Fg × Ptotal
Or, since you know the target partial pressure and
the absolute pressure:
Fg = Pg / Ptotal or FO2 = PO2 limit / Ptotal
Enter the known values:
FO2 = 1.4 ata / 4 ata = 0.35
EAN35 is the best mix for the dive. The procedure is
identical in an S.I./metric calculation.
The two-step process can be combined into a single
formula by replacing the absolute pressure in Step 2 with
its formulaic equivalent from Step 1:
FO2 = PO2 limit ÷ ( f s 33 D fsw /w atm
+ 1) =
PO2 limit × 33 fsw / atm
D fsw + 33 fsw
Or, in the example:
Oxygen Percentages for Best Mix
PO2 1.2 1.3 1.4 1.5 1.6
msw fsw
12 40 40% 40% 40% 40% 40%
14 45 40% 40% 40% 40% 40%
15 50 40% 40% 40% 40% 40%
17 55 40% 40% 40% 40% 40%
18 60 40% 40% 40% 40% 40%
20 65 40% 40% 40% 40% 40%
21 70 40% 40% 40% 40% 40%
23 75 37% 40% 40% 40% 40%
24 80 35% 38% 40% 40% 40%
26 85 34% 36% 39% 40% 40%
27 90 32% 35% 38% 40% 40%
29 95 31% 34% 36% 39% 40%
30 100 30% 32% 35% 37% 40%
32 105 29% 31% 33% 36% 38%
33 110 28% 30% 32% 35% 37%
35 115 27% 29% 31% 33% 36%
36 120 26% 28% 30% 32% 35%
38 125 25% 27% 29% 31% 33%
40 130 24% 26% 28% 30% 32%
Figure 4-4 By selecting the planned maximum
depth and the maximum oxygen partial
pressure (usually 1.4 ata), the best oxygen
percentage for the nitrox mix is easily
established. Although percentages higher
than 40% are obtained by calculation, this
table shows EAN40 as the highest oxygen
mix used in recreational nitrox diving.
Using the NAUI MobileTM App to
Determine Best Mix
The NAUI MobileTM Application Dive Tools feature
allows divers to quickly calculate the best nitrox mixes
for PO2 between 1.2 and 1.6 ata and depths of up to 40
meters (130 feet). From the NAUI MobileTM App, open
the Dive Tools menu. Use the slide-bars to select the
Pata = D f s w + 1 atm or
33 fsw / atm
D fsw + 33 fsw
33 fsw / atm
Pata =
Pata = 9 9 f s w + 1 atm = 4 ata
33 fsw / atm
FO2 = 1 . 4 a t m × 3 3 f s w / a t m = 0.35
99 fsw + 33 fsw