Trimix blend: the right bottom mix
The ideal bottom trimix for your dive: little enough O₂ to stay under your ppO₂, enough helium to keep a clear head, and a gas that is not too dense.
Your dive
The planned maximum depth.
Your limits
The oxygen you accept at depth and the narcosis you tolerate.
Is oxygen narcotic?
Minimum ppO₂ (hypoxic floor)
Gas density at depth
Reference table
Recommended mix at each depth with ppO₂ 1.40, END 30 m, density ≤ 6.2 g/L. Tap a row to use it.
| Depth | Mix | Floor | Density |
|---|---|---|---|
| 30 m | Nx 35 | surface | 5.3 |
| 40 m | Tx 28/9 | surface | 6.0 |
| 50 m | Tx 23/25 | surface | 6.1 |
| 60 m | Tx 20/38 | surface | 6.2 |
| 70 m | Tx 17/48 | surface | 6.1 |
| 80 m | Tx 15/55 | 1 m | 6.2 |
| 90 m | Tx 14/62 | 2 m | 6.1 |
| 100 m | Tx 12/67 | 4 m | 6.1 |
| 110 m | Tx 11/71 | 5 m | 6.1 |
| 120 m | Tx 10/74 | 6 m | 6.2 |
Floor: depth from which the mix provides enough O₂. Density at depth: blue ≤ 5.2 g/L, orange up to 6.2 g/L, red above.
Mix to order
Tx20/38
At depth, 60 m (7.0 bar)
Fairly dense gas
6.2 g/L: breathing is harder under exertion. Below 5.2 g/L is more comfortable.
Helium added for density
Your END alone gave Tx 20/35; it takes 38 % helium to stay under 6.2 g/L. The actual END drops to 27 m.
ppO₂ 1.26 · END 23 m · 5.6 g/L · floor surface
A little more helium than needed, but it is probably the gas your club or fill station already has.
Usable range of this mix
Usable from 0 m to 60 m (ppO₂ 0.16 to 1.40). MOD at 1.4: 60 m · contingency at 1.6: 70 m.
How it is calculated
1. Oxygen sets the ceiling
At maximum depth, we put the most O₂ that respects your ppO₂:
P abs = depth ÷ 10 + 1; FO₂ = max ppO₂ ÷ P abs
2. Nitrogen sets the narcosis
We keep as much nitrogen as air contains at your END depth, no more:
FN₂ = 0.79 × (END ÷ 10 + 1) ÷ P abs
If O₂ counts as narcotic, it is the O₂ + N₂ sum that is limited: FO₂ + FN₂ = (END ÷ 10 + 1) ÷ P abs.
3. Helium fills the rest
FHe = 1 − FO₂ − FN₂. If nothing is left to fill, nitrox or air is enough.
4. Rounding on the safe side
Whole percentages are ordered: O₂ is rounded down (ppO₂ never exceeded), nitrogen too (END never exceeded), helium takes the rest. Exact values are shown in small under each gas.
5. Floor, MOD and density
Floor = (min ppO₂ ÷ FO₂ − 1) × 10, rounded up to the metre
If FO₂ is under the minimum ppO₂ (under 16 % O₂ with 0.16 bar), the mix is hypoxic: it cannot be breathed at the surface and needs a travel gas down to the floor.
MOD = (ppO₂ ÷ FO₂ − 1) × 10, rounded down to the metre
Density = P abs × (FO₂ × 1.429 + FN₂ × 1.251 + FHe × 0.179) g/L
Above 5.2 g/L the work of breathing rises sharply; 6.2 g/L is the generally accepted limit (Anthony and Mitchell, 2016).
6. Limiting density
By default, density is limited to 6.2 g/L: the tool removes nitrogen and adds helium until it holds ("Free" keeps only the warning). The actual END then becomes lower than your target.
FN₂ ≤ (max density ÷ P abs − FO₂ × 1.429 − (1 − FO₂) × 0.179) ÷ (1.251 − 0.179)
7. Standard gases
Many teams use a handful of common trimixes (21/35, 18/45, 15/55, 12/65, 10/70). The tool suggests the one with the least helium that meets all your limits.
Assumptions: sea level, sea water (10 m = 1 bar), air = 21 % O₂ + 79 % N₂, helium non-narcotic, gas densities at 0 °C (slightly conservative). A calculation aid: it replaces neither trimix training nor analysing the cylinder before the dive.
Understanding trimix blending
How do you choose a bottom trimix?
Start from the maximum depth. Oxygen is set by the ppO₂ accepted at depth (often 1.2 to 1.4 bar), nitrogen by the narcosis tolerated (the target END, often 30 m), and helium fills the rest. The result is rounded on the safe side: never above the requested ppO₂, END or density. To check the END of an existing mix, use the END tool.
Why limit gas density?
A dense gas takes more effort to breathe: CO₂ builds up, breathlessness looms and narcosis worsens. The Anthony and Mitchell guidelines (5.2 g/L recommended, 6.2 g/L maximum) are now used by several agencies, which plan first on ppO₂ and density. When the density limit decides, the tool adds helium and says so.
Hypoxic trimix and travel gas
Under 16 % oxygen (with a 0.16 bar floor), a trimix cannot be breathed at the surface: you descend on a travel gas down to the floor. The tool shows that floor, the usable range and the contingency MOD at 1.6 bar. For the maximum depth of a gas, see the MOD tool.
Frequently asked questions
Which trimix for 60 m?
With a ppO₂ of 1.4, an END of 30 m (oxygen not narcotic) and density limited to 6.2 g/L: Tx 20/38, at 6.15 g/L at depth. The nearest standard gas that meets these limits is 18/45.
How do you calculate the helium fraction?
FO₂ = max ppO₂ ÷ P abs, FN₂ = 0.79 × (END ÷ 10 + 1) ÷ P abs, and helium takes the rest: FHe = 1 − FO₂ − FN₂. Oxygen and nitrogen are rounded down.
What is a normoxic trimix?
A trimix with enough oxygen to be breathed at the surface (at least 16 % with a 0.16 bar floor, or 18 % for a cautious floor). Below that it is hypoxic and needs a travel gas.
Why 18/45 rather than the exact mix?
Standard trimixes (21/35, 18/45, 15/55, 12/65, 10/70) simplify filling and gas sharing within the team. They contain a little more helium than needed, but stay within all your limits.