Nitrox Blending: Oxygen, Air, and Residual Gas

A Nitrox blend calculation conserves pressure-weighted oxygen: the oxygen already in the cylinder plus the oxygen in each added gas must equal the target. It can calculate an ideal plan, but a trained blender still has to use the correct equipment and analyze the cooled, mixed cylinder before anyone dives it.

What the calculation solves

For a binary oxygen-and-nitrogen fill made from pure oxygen and air, two balances determine the additions. Pressure is used as a proxy for gas quantity under the ideal-gas, constant-cylinder-volume model.

Here $P_s$ and $P_t$ are starting and target gauge pressures in one consistent unit; $F_s$ and $F_t$ are oxygen fractions such as 0.32, not percentages; $P_{O_2}$ is the pure-oxygen addition; and $P_{air}$ is the air addition. The model fixes air at 0.21 oxygen.

The same engine powers the interactive example below and the standalone Nitrox blending calculator. It returns an explicit impossible result when oxygen and air cannot make the requested top-up.

Empty-cylinder EAN32 example

Suppose an empty cylinder is to finish at EAN32 and 200 bar2900.75 psi. With no residual pressure, the starting mix has no pressure contribution. The inputs are therefore $P_s=0$, $F_t=0.32$, and $P_t=200\,\text{bar}$ in the metric case.

The unrounded engine result is 27.84810 bar oxygen and 172.15190 bar air403.90 psi oxygen and 2496.86 psi air. For a hand-check only, rounding the oxygen addition down to the shown gauge increment gives 27.8 bar400 psi, then air fills the remaining pressure. Rounding oxygen down makes the theoretical result slightly leaner, but it does not replace final analysis; temperature and instrument effects are larger than this paper rounding rule.

Residual gas and continuous blending

Residual gas matters twice: its pressure says how much gas remains, and its analyzed FO₂ says how much of that gas is oxygen. Starting with EAN32 at 50 bar and targeting EAN36 at 200 bar gives an unrounded ideal addition of 31.01266 bar pure oxygen and 118.98734 bar air. Substituting those additions back into the oxygen balance predicts 36.00% oxygen.

A continuous-blending system asks a related question: what composition must all 150 bar of added gas have? In this example it is 37.333% oxygen. If that stream is itself made from pure oxygen and air, the ideal feed is 20.675% pure-oxygen stream and 79.325% air stream—about 1 part oxygen flow to 3.84 parts air flow.

When a target is impossible

The oxygen fraction required in the added gas is the remaining oxygen quantity divided by the pressure increase. Oxygen plus ordinary air can only create an added stream between 21% and 100% oxygen. The calculator does not silently clamp values outside that interval.

• A rich residual mix and a much leaner target may need added gas below 21% oxygen. That requires a different diluent and is outside this Nitrox tool.
• A nearly full cylinder with a low starting FO₂ may not leave enough pressure room: the added gas would have to exceed 100% oxygen.
• A target pressure at or below the starting pressure is not an additive fill and needs a different operation, not a zeroed answer.

Oxygen service and final analysis

Partial-pressure blending exposes part of the fill path and cylinder to pure oxygen even when the final Nitrox is much leaner. The applicable oxygen-clean and oxygen-compatible requirements therefore depend on the method and every exposed component—not only the final FO₂ printed on a label.

The 40% figure appears in some regulations and industry practices. For example, the cited U.S. commercial-diving equipment rule specifies oxygen-service design above 40% by volume. That makes it an important jurisdiction-specific rule, not a universal physical transition at which all lower percentages become automatically safe. Follow the equipment manufacturer, fill station, training standard, and applicable law.

Model limits and mistakes

This tool conserves ideal pressure-weighted gas quantities at one cylinder volume. It does not model heating during a fast fill, cooling afterward, real-gas compressibility at high pressure, gauge calibration, incomplete mixing, bank pressure, compressor suitability, or the operator's required procedure. Those effects explain why the analyzed result—not extra displayed decimals—is operationally authoritative.

• Do not assume a partly full cylinder contains air; analyze it.
• Do not mix bar and psi in one calculation. Changing the app preference converts both pressure inputs together.
• Do not round a required oxygen addition upward just to reach a convenient gauge mark.
• Do not carry the requested mix directly into a dive plan. The blending tool intentionally stops at fill math; analyze the finished gas and use that measured FO₂ in the dedicated MOD tool.

The practical next action is simple: use the calculation to understand the intended gas balance, then have a qualified blender perform the fill and personally verify the final analysis before accepting the cylinder. Use the measured FO₂—not the requested blend—when you next calculate and apply MOD.