Gas blending calculator — nitrox & trimix

Blend nitrox and trimix by partial pressure, with real-gas correction and cost. DecoLog's blending calculator tells you exactly how much helium and oxygen to add and what pressure to top off to — for an empty tank or one with gas already in it.

Partial-pressure blending

The standard shop method: add helium to a target partial pressure, add oxygen, then top off with air. DecoLog computes the fill pressure at each step and — crucially — accounts for the oxygen in your top-off gas, so a trimix fill lands on the mix you actually wanted instead of running hot.

Custom top-off from any bank

Topping off with air is the default, but you can choose any analysed bank — a nitrox bank, for example — and the calculator solves the helium, oxygen and top-off amounts simultaneously. If a target can't be reached by adding gas, it tells you to drain and start over rather than giving you a wrong sequence.

Real gas vs ideal (Van der Waals)

At cylinder pressures of 200–300 bar, gases no longer behave ideally. DecoLog's real-gas mode blends in molar-density space and converts back through the Van der Waals equation, so the fill pressures are compressibility-corrected. The effect is largest for helium — exactly where ideal calculators drift.

Cost and continuous blending

The calculator estimates the cost of a fill from your per-litre gas prices, and for nitrox it shows the oxygen flow fraction for continuous (nitrox-stick) blending.

⚠ Always analyse every cylinder yourself before breathing it. DecoLog is a planning aid; blending requires gas-blender training. Verify mixes and labelling independently.