Trimix Best Mix & Blending Calculator
Enter the planned depth, END limit, cylinder pressure, and any residual gas to get a trimix target and an additive partial-pressure fill plan.
Calculated best mixTx20/35 (20.0% O2, 34.9% He, 45.1% N2)
- Fill feasibility
- Additive fill is feasible with pure oxygen, pure helium, and 20.9% oxygen air.
- Oxygen pressure to add (bar)
- 16.14
- Helium pressure to add (bar)
- 69.71
- Air pressure to add (bar)
- 114.14
- Surface oxygen required (surface L)
- 193.73
- Surface helium required (surface L)
- 836.57
- Surface air required (surface L)
- 1,369.69
- Maximum operating depth (m)
- 60
- Achieved equivalent narcotic depth (m)
- 30
- Surface hypoxic warning
- Hypoxic at the surface compared with air; use only within the planned depth range and training.
Fill pressure checkpoints
| Step | Action | Stop pressure (bar) | Increment (bar) |
|---|---|---|---|
| 1 | Add oxygen | 16.14 | 16.14 |
| 2 | Add helium | 85.86 | 69.71 |
| 3 | Top with air | 200 | 114.14 |
Assumes ideal-gas behavior, settled temperatures, gauge-pressure blending, and 20.9% oxygen compressor air.
END uses the nitrogen-only narcotic model.
How to use this calculator
- Choose metric or imperial units before entering depths, pressures, and cylinder water volume.
- Enter the maximum planned depth, target END, oxygen partial-pressure limit, and narcotic-gas model.
- Enter the final pressure, cylinder water volume, and any starting pressure with analyzed oxygen and helium.
- Choose the fill order allowed by the station, then read the gas additions and stop-pressure checkpoints.
How the trimix target is calculated
The calculator first reverse-solves the gas, then checks whether that gas can be blended by adding pure oxygen, pure helium, and air to the cylinder you described. It uses metres and bar internally for metric entries. With imperial entries, depth uses 33 feet per atmosphere and the pressure and gas-volume results are converted for display.
Let D be maximum depth, K be 10 metres or 33 feet per atmosphere, A = 1 + D / K, E = 1 + END / K, and q be the selected maximum oxygen partial pressure. The oxygen fraction is:
FO2 = q / A
For the nitrogen-only END model, nitrogen is set from the air-equivalent nitrogen pressure:
FN2 = 0.79 × E / A
For the oxygen-plus-nitrogen model, helium is set by removing enough narcotic gas to match the selected END:
FHe = 1 − E / A
The remaining fraction is assigned to the third gas. A negative oxygen, helium, or nitrogen fraction means the requested depth, END, and oxygen limit cannot all be true at once.
How the fill plan is calculated
The blending section is a Dalton's-law component balance. Let Pt be final pressure, P0 be starting pressure, O0 and H0 be the analyzed starting fractions, and a = 0.209 be oxygen in air. The pure helium addition is:
xH = Pt × FHe − P0 × H0
The oxygen addition accounts for residual oxygen and oxygen supplied by the air top-up:
xO = (Pt × FO2 − P0 × O0 − a × (Pt − P0 − xH)) / (1 − a)
The air addition is whatever pressure remains: xA = Pt − P0 − xH − xO. All three increments must be zero or positive. If one is negative, the target mix may still be real, but it is not reachable as a simple additive fill from the starting gas.
What the calculator leaves out
This is a planning and blending worksheet, not a dive plan. It does not calculate decompression, gas reserves, oxygen exposure, work of breathing, density, or whether the gas is breathable at every point of the dive. It also assumes ideal gas behavior, settled temperatures, gauge-pressure blending, and 20.9% oxygen compressor air. Real cylinders warm during filling and real gases deviate from ideal behavior, especially at high pressure, so the finished cylinder must still be analyzed.
Worked example
For a 60 m dive, 30 m target END, 1.4 ata maximum ppO2, and the nitrogen-only model, depth pressure is 1 + 60 / 10 = 7 ata. Oxygen is 1.4 / 7 = 0.200, nitrogen is 0.79 × 4 / 7 = 0.451, and helium is the remaining 0.349. The displayed mix is Tx20/35.
With an empty 12 L cylinder filled to 200 bar, the additive fill is about 16.14 bar oxygen, 69.71 bar helium, and 114.14 bar air. In oxygen-first order, stop at 16.14 bar after oxygen, 85.86 bar after helium, and 200 bar after the air top-up. The surface gas estimates are about 194 L oxygen, 837 L helium, and 1,370 L air.
Common questions
How do I calculate the best trimix for a planned depth?
Choose an oxygen partial-pressure limit for the working depth, then divide that limit by the absolute pressure at maximum depth to get oxygen fraction. Choose a target END and narcotic model to set either nitrogen or helium. The remaining fraction becomes the third gas.
What does Tx20/35 mean?
Tx20/35 means a trimix with about 20% oxygen and 35% helium. The balance is nitrogen, so Tx20/35 has about 45% nitrogen before rounding. Rounded mix names are labels; blending and END calculations should use the unrounded fractions.
Should oxygen be treated as narcotic when calculating END?
Training agencies and divers use different conventions. The nitrogen-only model treats nitrogen as the narcotic gas. The oxygen-plus-nitrogen model treats both oxygen and nitrogen as narcotic, which usually calls for more helium at the same depth and END.
Why can a top-up calculation produce a negative gas amount?
A negative increment means the residual gas already contains more of that component than the final target allows after accounting for the planned top-up. You cannot remove that gas by only adding oxygen, helium, and air. The practical choices are venting, using another top-up gas, changing the target, or starting with a different cylinder.
Why must the finished cylinder be analyzed after blending?
Partial-pressure blending is sensitive to gauge accuracy, temperature change, compressor air composition, and real-gas behavior. The calculator estimates the fill pressures, but the actual cylinder must be checked with calibrated oxygen and helium analyzers before it is marked or breathed.