Gas Blending Calculator

Gas Blending — Key Features

• Nitrox blending with partial pressure method
• Trimix blending — O₂, He, and air calculations
• Step-by-step fill instructions with exact pressures
• Support for starting with partially filled tanks
• Clear warnings for safety limits and equipment compatibility
• Works with both metric (bar) and imperial (PSI) units

Gas Blending — How It Works

Enter your starting tank pressure and mix, then your target pressure and desired mix. The blender computes a partial-pressure recipe: how much pure oxygen (and helium for trimix) to bleed into the cylinder first, then how much air to top up to reach the final pressure. Calculations assume the ideal-gas law, which is accurate enough below ~200 bar for standard recreational and technical blends; for high-pressure fills (>230 bar) or hyperoxic mixes the gas's compressibility starts to matter and Z-factor corrections become useful. Each step shows the exact pressure to fill to, so you can pause, sanity-check with an analyser, and continue. Always blend cold-fill in an oxygen-clean, well-ventilated area, analyse the final mix with a calibrated O₂ analyser (and a helium analyser for trimix), and label the cylinder with the analysed values, MOD, and date. The blender is a planning aid — never replace gas analysis with calculated values, even on cylinders you filled yourself.

Why Use Our Blending Calculator?

• Precise partial pressure calculations every time
• Clear step-by-step instructions reduce blending errors
• Supports both nitrox and trimix blending
• Accounts for existing gas in partially filled tanks

Gas Blending — Frequently Asked Questions

What blending method does this calculator use?

DiveToolbox uses the partial-pressure blending method: starting from a known pressure and mix, the calculator computes how much pure oxygen (and helium for trimix) to bleed into the cylinder, followed by an air top-up to reach the target pressure. Computations assume the ideal-gas law, which is accurate enough below ~200 bar for standard mixes; for higher pressures or hyperoxic blends the Z-factor compressibility correction starts to matter. The method is the industry standard for shop and self-blending up to 36% nitrox and trimix mixes.

Can I blend starting from a partially filled tank?

Yes — that is exactly the use case the calculator is built for. Enter the current pressure and analysed mix in the cylinder, then the target pressure and desired mix; the planner computes the exact additions of O₂, He, and air needed to reach the target. Re-blending from partial fills is the normal case in busy fill stations and on multi-day trips. Always re-analyse after blending, because the residual gas may not match the label exactly and small mix errors compound.

Is blending training required?

Absolutely — gas blending involves oxygen-clean equipment, high-pressure cylinders, and partial-pressure handling of pure O₂, which is a non-trivial fire risk if mishandled. Most agencies (TDI Gas Blender, IANTD Blender, SSI Equipment Tech) offer dedicated certifications. The calculator is a planning aid for trained blenders; it does not teach the procedural and safety knowledge needed to actually fill a cylinder safely. If you are not trained, have a qualified shop do your fills and use the calculator to check their math.

What gas mixes can I blend?

The blender supports air top-up, nitrox (EAN21 to 100% O₂), trimix (any O₂/He/N₂ combination), heliox, and pure helium. For trimix the calculator handles the standard fill order: helium first (low-pressure injection), oxygen second, air last. Mixes with helium fractions above ~50% or oxygen above ~40% have practical fill challenges (heat from compression, oxygen-clean requirements) that the calculator flags as warnings. Always blend within your training and your shop's equipment certification.

How do I verify the final mix?

Always analyse the final mix with a calibrated O₂ analyser (and a helium analyser for trimix) before breathing it. Allow the cylinder to settle and cool for at least 15 minutes after fill so the gases mix homogeneously and density stabilises. Tilt the cylinder horizontally and gently roll it before analysing for the best mixing. Calculated values are planning targets, never a substitute for verified analysis — small errors in pressure gauges, partial-pressure injection, or temperature compensation can shift the final mix by 1-2 percentage points.

Does temperature affect blending calculations?

Yes — blending is fundamentally a partial-pressure problem and temperature affects the actual pressure your gauge reads. Filling generates significant heat (a fresh nitrox 32% fill can warm a cylinder by 20-30°C); the cylinder reads high while hot, then settles low as it cools to ambient. Always let cylinders cool to ambient before final-trim adjustments, ideally 15-30 minutes. Wet-fill (cooled in a water bath) is the professional standard for accurate trimix because it eliminates the cooling correction.

→ How to Plan a Trimix Dive — A 6-step workflow connecting every calculator in the right order

→ Diving Glossary — Essential acronyms and concepts every diver should know

→ Bottom Gas vs Deco Gas: Why Two Different Mixes? — The role each gas plays in a technical dive and how to choose them