Best mix: the best nitrox
The richest nitrox you can breathe down to the deepest point of your dive without exceeding your max ppO2.
Your dive
The deepest point you will reach.
Max ppO2
The oxygen limit you allow yourself on the bottom.
Your best mix
Nx3535 % O2
at most, for 30 m at 1.4 bar
No-stop time at 30 m+10 min
Indicative (Bühlmann ZH-L16C, GF 85 %). Your computer has the final say.
With the club mixes
You do not always have the exact mix: take the richest one that stays below your best mix.
Quick reference
Best mix by depth and ppO2. Tap a cell to use it.
| Depth | 1.2 bar | 1.4 bar | 1.6 bar |
|---|---|---|---|
| 10 m | |||
| 15 m | |||
| 20 m | |||
| 25 m | |||
| 30 m | |||
| 35 m | |||
| 40 m | |||
| 45 m | |||
| 50 m | |||
| 55 m | |||
| 60 m |
— : even air exceeds the ppO2 at this depth.
How it is calculated
1. Bottom pressure (sea water, 10 m = 1 bar, 1 bar at the surface)
P = depth ÷ 10 + 1
2. Best mix: Dalton’s law, ppO2 = FO2 × P, so
FO2 = max ppO2 ÷ P
The percentage is rounded down to the unit so it never exceeds the chosen ppO2. It is capped at 100 % (pure oxygen).
3. Actual ppO2 of the rounded mix on the bottom
ppO2 = FO2 × P
4. MOD of the mix (rounded down to the metre)
MOD = (max ppO2 ÷ FO2 − 1) × 10
5. EAD, equivalent air depth for nitrogen (rounded up to the metre)
EAD = (depth + 10) × (1 − FO2) ÷ 0.79 − 10
6. CNS limit: maximum single exposure time at the bottom ppO2, from the NOAA table (1.4 bar → 150 min, 1.6 bar → 45 min), interpolated between rows.
7. Gas density on the bottom
density = P ÷ 1.013 × (FO2 × 1.429 + FN2 × 1.251) g/L
Above 5.2 g/L, breathing takes more effort and the risk of breathlessness rises (Anthony and Mitchell, 2016). Nitrox changes almost nothing: only helium makes it lighter.
8. No-stop time: Bühlmann ZH-L16C model (nitrogen), high gradient factor 85 %, descent at 20 m/min, time counted from the surface. It is a ballpark figure to compare with air: every computer has its own settings.
Example: 30 m at 1.4 bar → P = 4 bar → 1.4 ÷ 4 = 0.35 → EAN35, ppO2 1.40 bar, MOD 30 m, EAD 23 m.
Good to know
- 1.4 bar is the usual bottom value. 1.6 bar is kept for stops and deco, at rest.
- A real mix varies by ± 1 point: always analyse your cylinder and work out your MOD from the analysis.
- Below 40 m, narcosis matters as much as oxygen: check the EAD, or switch to trimix.
- This tool helps you plan a dive. It replaces neither your nitrox training nor your computer.
Understanding best mix
What is the best mix?
The best mix is the most oxygen-rich nitrox you can breathe down to your dive’s maximum depth without exceeding your ppO2 limit. More oxygen means less nitrogen: you gain no-stop time and off-gas better. But beyond the chosen ppO2, oxygen becomes toxic. The best mix is the right balance between the two.
The calculation in one line
From Dalton’s law, ppO2 = FO2 × absolute pressure, so FO2 = max ppO2 ÷ absolute pressure. At 30 m in sea water the pressure is 4 bar: 1.4 ÷ 4 = 0.35, i.e. EAN35. The tool always rounds the percentage down, so it never exceeds the chosen ppO2. For the maximum depth of an existing mix, use the MOD tool.
With the club mixes
In practice you rarely get a custom EAN35: you take the richest club mix that stays below the best mix. At 30 m and 1.4 bar, that is EAN32. The tool points it out and gives its MOD and EAD. Before the dive, analyse your cylinder and record the result with the nitrox label tool; to fill a specific mix, use the gas blender.
Safety benchmarks
Below 40 m, narcosis matters as much as oxygen: check the EAD, detailed in the EAD tool. The CNS limit (NOAA table) gives the maximum exposure time at the bottom ppO2, and gas density warns you when breathing becomes hard. The no-stop time shown is a ballpark figure: your computer has the final say.
Frequently asked questions
Which nitrox for a 30 m dive?
At 1.4 bar, the best mix is EAN35: 1.4 ÷ 4 bar = 35 %. With the usual club mixes, EAN32 is the one to use.
Which nitrox for 40 m?
At 1.4 bar, the best mix is EAN28: 1.4 ÷ 5 bar = 28 %. With the club mixes, air (21 %) remains the choice, as EAN32 would exceed 1.4 bar.
Why round the best mix down?
A higher percentage would exceed the chosen ppO2 on the bottom. Rounding down to the unit guarantees the mix stays under your limit.
Is the no-stop time shown reliable?
It is a ballpark figure (Bühlmann ZH-L16C, gradient factor 85 %) to compare the best mix with air. Every computer has its own settings: it has the final say.