Why your fills come up short

Posted 16 June 2026 · Field Note #9

Real gas ≠ ideal gas

Ever topped a cylinder by the book and found the finished mix landed a point or two off? You probably didn't make a mistake. The simple formula made an assumption that quietly breaks down at high pressure.

Most blending arithmetic treats gases as "ideal," meaning it assumes the molecules have no size and don't interact, so pressures add up in tidy, linear proportions. That assumption is excellent at everyday pressures. But the gases in a scuba cylinder aren't sitting at everyday pressures. At 200 bar and beyond, molecules are crammed close enough that their real size and the forces between them start to matter, and different gases respond differently. Oxygen and nitrogen drift modestly from the ideal model; helium drifts a lot. The upshot: a perfectly executed partial-pressure blend, calculated the simple way, can finish off-target by a meaningful amount.

A Van der Waals calculation, a real-gas equation that corrects for molecular volume and intermolecular attraction, accounts for this non-ideal behaviour. Blend with the real-gas maths and you land closer to your target on the first attempt, which means fewer frustrating cycles of top-up, analyse, bleed off, and try again. The discrepancy is largest exactly where precision matters most: high-pressure fills and helium-containing trimix, where helium's strongly non-ideal behaviour and the bigger pressures compound.

Why should a working diver care about the equation behind the curtain? Because it explains a real, repeatable phenomenon, your fills coming up off-target, that has nothing to do with sloppy technique, and it points to the fix. Using a tool that does real-gas calculation saves gas, time, and irritation, and gets safety-critical helium blends right with less fiddling.

Practical takeaway: for high-pressure or helium mixes, blend using a calculator that applies real-gas (Van der Waals) corrections rather than the basic ideal formula. But hold the same hard rule as always: whatever the maths predicts, the calibrated analyser on the finished mix is the source of truth, not the plan.

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⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.