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Gas Management

Gas Top-Up Calculator

Find the final oxygen, helium and nitrogen percentages after topping up a cylinder with any known gas.

Top-up inputs
Enter the analyzed gas already in the cylinder, the gas you will add, and the final pressure. Percentages are of the whole gas; nitrogen is the remainder.

Gas already in the cylinder

bar

Allowed range: 0 to 350 bar.

%

Allowed range: 0 to 100 %.

%

Allowed range: 0 to 100 %.

Gas you add

Added gas presets
%

Allowed range: 0 to 100 %.

%

Allowed range: 0 to 100 %.

bar

Allowed range: 1 to 350 bar.

Oxygen
23.8%
Helium
0.0%
Nitrogen
76.2%

Sea-level atmosphere of 1.01325 bar is added to both gauge readings. At zero gauge, the cylinder still contains the analyzed residual gas. Ideal-gas, constant-temperature, complete-mixing model; results are rounded for display. Real-gas behavior, fill heating, and gauge error shift the true mix.

A calculation is not a fill procedure

Mixing gases, especially anything above air in oxygen or any helium, needs trained blenders and suitable equipment. Let the cylinder mix and cool, analyze it with calibrated analyzers, label it, and plan the dive from the measured mix.
How it works
Ffinal=P1F1+(P2−P1) FaddP2F_{\text{final}} = \frac{P_1 F_1 + (P_2 - P_1)\,F_{\text{add}}}{P_2}

Where

FfinalF_{\text{final}}
final fraction of oxygen or helium
P1P_1
residual absolute pressure: gauge + atmosphere (bar)
P2P_2
final absolute pressure: gauge + atmosphere (bar)
F1F_1
fraction in the residual gas
FaddF_{\text{add}}
fraction in the added gas

Worked example at sea level: 50 gauge bar of EAN32 topped to 200 gauge bar with air gives (51.01325 × 0.32 + 150 × 0.21) ÷ 201.01325 ≈ 23.79% oxygen. Nitrogen is the remainder.

Result

+150.0 bar to reach 200.0 bar

Frequently Asked Questions

How is the final mix calculated?
The ideal-gas calculation weights the residual gas by absolute pressure, adding the assumed sea-level atmosphere of 1.01325 bar to gauge readings. For example, 50 gauge bar of EAN32 topped to 200 gauge bar with air gives (51.01325 × 32 + 150 × 21) ÷ 201.01325 ≈ 23.79% oxygen. A cylinder at zero gauge still contains residual gas; it is not evacuated.
Does it work for trimix?
Yes. Enter the analyzed oxygen and helium in the residual gas and in the gas you add. Nitrogen is shown as the remainder. It reports the result of a known addition; it does not plan a helium fill sequence.
How accurate is the result?
It is an ideal-gas, constant-temperature, complete-mixing estimate. Fill heating, real-gas behavior at high pressure and gauge error all shift the true mix, so the cooled, mixed cylinder must be analyzed.
Can I use the result to dive?
Plan from the analyzed mix, not the prediction. Handling oxygen or helium also needs trained blenders and suitable equipment.

Related Guides

Nitrox & oxygen

Nitrox Blending: Oxygen, Air, and Residual Gas

Nitrox & oxygen

Best Nitrox Mix for Any Depth

Gas planning & consumption

Diving Gases Explained: Air, Nitrox, Trimix, Heliox, Oxygen, and Argon

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Reference Data

Reference

Scuba Cylinder Specifications

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