Partial-pressure blending basics

Posted 15 June 2026 · Field Note #8

Blending by partial pressure

Making nitrox or trimix isn't magic, it's stacking pressures. The key insight is that in a mixture of gases, each gas contributes its own share of the total pressure, called its partial pressure, in proportion to how much of it is present. Add a known amount of one gas, then top off with another, and you've controlled the final recipe by controlling how much pressure each component contributed.

A worked example makes it concrete. Say you want EAN32, that's 32% oxygen, in an empty cylinder you'll fill to 200 bar. You first add a measured slug of pure oxygen, then top up with air to the final 200 bar. The amount of pure O₂ you added at the start, plus the ~21% oxygen already riding along in the air you top with, together set the final oxygen fraction. Get the initial oxygen pressure right and the blend lands where you want it.

Now the catches, because this is where blends drift off target, or worse.

First, oxygen is the silent hazard. Decanting pure O₂ under pressure demands oxygen-clean cylinders and valves, oxygen-compatible lubricants, and a slow, controlled fill, because rushing it risks an adiabatic fire. If your kit isn't O₂-clean and you aren't trained, don't partial-pressure blend.

Second, temperature. Compressing gas heats the cylinder, and a hot tank reads higher pressure than it will once it cools to room temperature. Fill to your target on a hot cylinder and the pressure, and therefore your carefully set fraction, drops as it cools. Patient blenders let cylinders cool, or compensate for the temperature swing.

Third, the starting state. The clean example above assumes an empty tank. The moment you start from a cylinder that already holds some gas, of some unknown or partly-known mix, the whole calculation changes, because you're now stacking your additions on top of whatever was already in there. "Just top it off" without accounting for the existing contents is how blends go wrong.

Practical takeaway, and this is non-negotiable: the calculation tells you what to aim for, but it is never the final word. Always re-analyse the finished mix with a calibrated analyser before you breathe it, then mark the actual oxygen percentage and MOD on the cylinder. The analyser is the source of truth, not the plan.

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