How Nitrox Is Blended: Partial Pressure, Sticks and Membranes

The four ways a dive shop makes nitrox

TDI's nitrox course standard lists three common mixing procedures its students must be taught: partial pressure blending, continuous blending and membrane separation. A fourth route is not really a mixing method at all, it is a storage bank the shop already filled with a standard mix. DAN's Alert Diver piece on production methods adds pressure swing adsorption as the industrial process behind bulk oxygen supply.

Which one your shop uses decides what happens to your cylinder. Membrane and stick systems make the mix before or inside the compressor, so the cylinder only ever sees nitrox. Partial pressure blending decants pure oxygen into the cylinder, and that is why the cylinder and valve get pulled into oxygen service. Our nitrox blending calculator plans the partial pressure version and also prints the same fill as a continuous feed ratio.

Partial pressure blending, worked out in bar

Two amounts decide the mix: the bar of pure oxygen that goes in first, and the bar of top-up gas that follows. DAN describes the method as a calculated amount of pure oxygen added first, then topped off with compressed oxygen compatible air. The oxygen itself has to be breathing grade; NOAA accepts medical grade (U.S.P.), technical diving grade or aviator grade.

Take an empty 12 L cylinder going to 232 bar (3,365 psi) of EAN32. Work the nitrogen side first, because the top-up gas is the only thing carrying nitrogen in:

• Air to add = (232 × 0.68 − 0) ÷ 0.79 = 199.7 bar.
• Oxygen the finished cylinder must hold = 232 × 0.32 = 74.24 bar.
• Oxygen to add = 74.24 − 0.21 × 199.7 = 32.3 bar, because the air brings oxygen of its own.
• Fill order: oxygen first, stop the gauge at 32.3 bar, then air until the gauge reads 232 bar.
• Predicted result with both amounts rounded to 0.1 bar: 32.00% oxygen, MOD 33.7 m at a ppO₂ of 1.4 bar.

When the cylinder is not empty, and when gas has to be dumped

Leftover gas counts, and it rarely helps as much as divers expect. A cylinder holding 40 bar of EAN32 topped straight to 232 bar with air alone finishes at 22.90%, barely richer than air. The plan still needs 26.7 bar of oxygen first, so the gauge stops at 66.7 bar, then 165.3 bar of air brings it to 32 percent.

The awkward case is leftover gas that is too rich. A cylinder back from a cancelled dive with 180 bar of EAN36 already holds 64.8 bar of oxygen, and 200 bar of EAN32 is only allowed to hold 64.0 bar in total. Nothing can be added to fix that, because oxygen cannot be taken out, so gas goes overboard. Draining to 146.6 bar leaves 52.776 bar of oxygen, and 53.4 bar of air finishes the job with no oxygen decanted at all. The exact oxygen figure at that point is 0.0127 bar, which rounds to zero.

The drain target does not depend on how full the cylinder was. It is the highest starting pressure at which the oxygen to add stops being negative, so 160, 180 or a full 200 bar of EAN36 all drain to the same 146.6 bar for this fill.

Continuous blending: the same fill as a flow ratio

A blending stick sits at the compressor intake and meters oxygen into the air before it is compressed, so the same mix can be made without calculating each cylinder. DAN calls this a safer and more controlled environment than partial pressure blending, and warns that the risk climbs sharply if the gas reaching the compressor goes above 40 percent oxygen.

The arithmetic is the same fill seen as a flow. For 232 bar of EAN36 from an empty cylinder, the gas being added is EAN36, oxygen makes up 18.99% of the feed, and the ratio is 1 part oxygen to 4.27 parts air. That is an ideal mixing ratio rather than a compressor setting, and every real system has its own oxygen limit and procedure.

Why oxygen service matters, and what cleaning involves

Oxygen does not burn, it makes other things burn faster. DAN's production methods article puts a number on it: cotton burns twice as fast in 40 percent nitrox at ambient pressure as it does in 25 percent oxygen, and burn rates rise steeply once the gas is compressed to 207 bar (3,000 psi). That is why decanting is slow, and why NOAA requires slow-opening shut-off valves on oxygen systems above 8.6 bar (125 psig).

Oxygen clean is a defined condition, not a habit. CGA G-4.1, Cleaning Equipment for Oxygen Service, covers cleaning of equipment used with liquid and gaseous oxygen. Its third edition, the copy carried in the US federal incorporation-by-reference library, is marked obsolete, so check the current edition number before quoting it to anyone. That edition names the contaminants to remove: solvents, acids, moisture, corrosion products, non-compatible thread lubricants, filings, scale, weld splatter, organic material such as oil, grease, crayon and paint, and lint. It sets an acceptable residual contamination level of about 500 mg/m2 (47.5 mg per square foot), and requires that ultraviolet inspection show cleaned surfaces free of any hydrocarbon fluorescence.

The air is part of the system too. NOAA's standard for breathing air used in partial pressure blending with an enriching gas above 40 percent oxygen tightens oil mist and particulate to 0.1 mg/m3 each and carbon monoxide to 10 ppmv, against 20 ppmv for membrane systems. DAN gives the same idea for oxygen compatible air as hydrocarbons below 0.1 mg/m³, ideally none. Ordinary breathing air that passes the normal purity table can still be wrong for this job.

The 40 percent rule, and why it is not settled

The number comes from the US commercial diving rule. OSHA 29 CFR 1910.430(i) says equipment used with oxygen or mixtures containing over forty percent by volume oxygen shall be designed for oxygen service, and that components except umbilicals shall be cleaned of flammable materials before use. NOAA repeats it and adds a pressure qualifier: equipment exposed to more than 40 percent oxygen at pressures above 14 bar (200 psi) shall be cleaned and maintained for oxygen clean service. TDI trains recreational divers on EAN22 to EAN40 and splits its equipment syllabus at the same line.

DAN's 2026 article calls the rule a misconception. It points out that any mixture above 23.5 percent oxygen is legally classified as an oxidizer, that industry cleanliness standards sit around 23.5 to 25 percent, and that any component normally exposed to more than 25 percent oxygen should be oxygen compatible and oxygen clean. So the standards and the safety literature disagree, and a shop can be inside the regulation while a specialist thinks the line is drawn too high.

At the counter the distinction that matters is how the gas was made, not what it says on the label. East Coast Divers fills cylinders with pre-mixed nitrox up to 40 percent without oxygen cleaning the cylinder or valve, but requires cylinders certified and stamped for over 40 percent whenever pure oxygen is used in the blend. Partial pressure blending pulls your cylinder into oxygen service even when the finished mix is only EAN32.

What the analyser reading actually tells you

The blend is the shop's work, the number you dive is yours. NOAA makes each diver analyse their own cylinder and acknowledge five things in writing: oxygen fraction, MOD, cylinder pressure, date of analysis and user's name. Cylinders are marked NITROX, EANx or Enriched Air with a 10 cm (4 inch) green band, and labelled to show whether they are prepared for gases above 40 percent oxygen. TDI has students complete and sign the fill station's log with the mix and the MOD on it.

A reading is a measurement with a tolerance, and the tolerance is wider than the display suggests. The Analox O2 EII reads 0.1 to 100 percent oxygen to 0.1 percent resolution, with an accuracy of ±1% of reading plus ±0.2% oxygen. On a 32 percent display that spans 31.48 to 32.52 percent, and the 1.4 bar MOD moves between 34.4 m and 33.0 m. Our MOD and EAD worked examples run those sums out in full.

The manual's procedure matters as much as the spec. Calibration must be done at the same atmospheric pressure as the measurement, air calibration is needed before every use with two minutes in clean air, and the pillar valve is opened very slowly until the gas is just heard hissing. Read while the flow is on, because the display starts drifting back towards 20.9 percent within seconds of the flow stopping, and very high flows can pressurise the sensor and damage it.

Hot fills: why the gauge drops overnight

Compressing gas heats it, and a warm cylinder reads high. Bauer, which builds the compressors many shops run, states the problem plainly: filling SCBA or SCUBA cylinders too fast will decrease the stored air in the cylinder when the cylinder cools down after charging. Its charge rate controller exists to slow the fill down, and it offers the operator three rates: Normal, Hot Fill and Custom.

Heat is a cylinder problem as well as a pressure problem. Luxfer tells owners never to expose an aluminium cylinder above 130°C (265°F); anything suspected of 130 to 175°C must be hydrostatically retested, and above 175°C (350°F) the cylinder is condemned under CGA C-6.1. Overfilling to leave room for cooling is not the answer either: Luxfer says filling above the stamped pressure is not recommended, and that the 10 percent overfill allowance does not apply to 3AL aluminium cylinders under 49 CFR 173.302(c).

For the diver it means two things. Let the cylinder cool and check the gauge again before you pay for 232 bar, and analyse the cooled gas rather than the fill whip, both to let the oxygen and air mix and because the analyser itself shifts about 0.1 percent oxygen per degree Celsius. The Analox manual warns against moving the unit between temperatures immediately before use. Steel and aluminium cylinders behave the same way here; the sizes and pressures differ, and our cylinder sizes guide covers those.

What a nitrox fill costs against air

Banked nitrox costs a little more than air; a custom blend costs more again, and the gap is the blender's time. East Coast Divers lists $14 per tank for air and $16 for EAN32, with ten-fill cards at $120 and $140, and prices oxygen at $0.75 per cubic foot for custom blends. Eight Diving lists $10 for a single air fill, $15 for a single EAN32 fill and $25 for EAN32 in doubles, plus a $15 custom gas blending fee for non-banked fills or gas switches.

So the standard mix off the bank is a couple of dollars over air, while anything the shop has to mix for you carries a separate fee. These are the prices those two shops list. Fills vary a lot by country, by how the shop makes its gas and by what oxygen costs locally. The depth you dive decides whether that extra money buys you anything, which our nitrox bottom time comparison works through, and richer technical mixes get expensive fast once helium is involved.