Calculator
Gas blending calculator — Partial-pressure top-off
Mix gases at home or at the fill station with the partial-pressure recipe: drain the cylinder if needed, add helium, add oxygen, top with air. The calculator finds the largest drain pressure that still lets the recipe land on your target mix and final pressure — wasting as little starting gas as possible.
The four-step recipe
Each step adds one gas. Starting from a cylinder containing some gas at some pressure, the algorithm picks a drain target so that — after adding pure helium, pure oxygen, then air — the final partial pressures match the target mix.
Step 1. Drain to P_drain (preserve helium fraction) Step 2. Add helium to P_He = P_drain + (FHe_target × P_final − FHe_start × P_drain) Step 3. Add oxygen to P_O₂ (so air top-off lands at target FO₂) Step 4. Top with air to P_final
Worked example: empty 12 L cylinder → 232 bar Trimix 21/35. Add helium to ~81 bar (35% × 232), no oxygen needed, top with air to 232 bar. Air contributes 21% × 151 bar ≈ 32 bar of O₂, which lands on 21% O₂ overall.
Practical notes
Real fills account for temperature (cylinders heat as they fill, then cool), partial-pressure errors from compressibility at high cylinder pressures (the calculator uses a real-gas EOS for cylinder volume), and analyzer drift. Always verify the final mix with a calibrated analyzer before diving — the recipe is a planning tool, not a substitute for analysis.
Frequently asked questions
- Why do I drain before adding gas?
- If the starting gas already contains helium, you preserve its proportion by scaling the drain pressure. Draining lets you discard old gas that can't fit the target mix — for example, when topping a leftover fill of richer Nitrox into a leaner mix.
- Can I add oxygen first?
- You can, but the standard recipe adds He first, O₂ second, air third. This order avoids high O₂ partial pressures during the fill (a fire risk) and makes the math clean — air's 21% O₂ contributes the last increment.
- Why does the calculator sometimes drain to 0 bar?
- When the start gas is incompatible with the target (e.g., already too rich in helium for the target mix), the only way to land on target is to discard the start gas entirely. The recipe surfaces this so you don't dilute a perfectly good fill into something off-spec.
- Is partial-pressure blending accurate?
- It's accurate to about 1% under normal conditions — good enough for sport / tech diving with a final analyzer check. Continuous-blending and pre-mixed fills are more accurate but require more equipment.