Gas Blending Calculator

This partial pressure blending calculator gives the fill steps for nitrox blending and trimix (how far to bleed, then which gas to add up to which gauge pressure), or the flow ratios for a continuous blending panel.

Cylinder
Now
%
%
bar
→ Target
%
%
bar
Environment Sea level · 20 °C
m
°C
°C
TMX 21/35 at 200 bar Fill plan · achieved 21.2/35.0
  1. 1Add He toThen O₂ 9.1% · He 56.7%122bar
  2. 2Add O₂ toThen O₂ 21.3% · He 49.1%140bar
  3. 3Add Air toThen O₂ 21.2% · He 35.0%200bar
  4. 4Analyze O₂ and HeExpect He 35.0%; the plan is a target, the analyzer is the truth21.2% O₂
Total gas usedpressure added · free gas
He +72 bar741 L
Air +60 bar605 L
O₂ +18 bar202 L

Planning aid for trained divers, not a dive computer. Check results against a second source. Warning and disclaimer

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Partial pressure blending fills a cylinder one gas at a time, stopping each at a set gauge pressure so the finished mix lands on the target. Enter what is in the cylinder now and the mix you want: this calculator lists the stops in order (bleed, helium, oxygen, air), or the flow ratios for a continuous blending panel. For trained gas blenders; analyse every cylinder before diving.

The formula

ΔPHe=Pt⁢fHe,t−Pn⁢fHe,n
Helium to add, in gauge pressure. Pt and Pn are the target and current pressures, f the gas fractions.
Pair=Pt⁢fN₂,t−Pn⁢fN₂,n0.79ΔPO₂=Pt−Pn−ΔPHe−Pair
Air is the only nitrogen source, so the nitrogen balance sets the air top-up, and oxygen fills the rest. This is the ideal-gas hand method; a negative result means the cylinder must be bled first.
fblend=Mt⁢ft−Mn⁢fnMt−MnM=PabsZ
Continuous blending: the mix the panel must feed, for O₂ and for He. M is the real-gas molar pressure, absolute pressure divided by the compressibility factor Z.

Worked example

The Dive Kit app opens on this fill: a stage cylinder (AL80, 11.1 L) holds 150 bar of leftover EANx32, and the next dive needs Trimix 18/45 at 200 bar. With unlimited air, oxygen and helium banks, Dive Kit plans it in 3 steps, then analyze:

  1. 1Bleed toVent before adding any gas101bar
  2. 2Add He toThen O₂ 17.5% · He 45.4%199bar
  3. 3Add O₂ toThen O₂ 17.9% · He 45.1%200bar
  4. 4Analyze O₂ and HeExpect He 45.1%; the plan is a target, the analyzer is the truth17.9% O₂

The cylinder ends at O₂ 17.9% and He 45.1% at 200 bar. The bleed comes first: 150 bar of EANx32 already carries about 48 bar of oxygen, more than the 36 bar that 200 bar of 18/45 holds.

In imperial: an AL80 with 500 psi of air, filled to EANx32 at 3000 psi, reads O₂ to 897 · Air to 3000 psi. Each gas is added in whole bar, so the stops in psi are not round hundreds.

The same EANx32 into an empty cylinder on a continuous panel: set the intake to 86.1% air and 13.9% oxygen by flow, and the analyzer in the line should read 32.0% O₂ before you fill to 200 bar.

Fill recipes from an empty cylinder

Each row is the plan Dive Kit gives for an empty cylinder at 20 °C, with unlimited air, oxygen and helium banks. The pressures are where to stop each gas on the cylinder gauge.

MixTo 200 barTo 3000 psi
EANx32O₂ to 27 · Air to 200 barO₂ to 406 · Air to 3000 psi
EANx36O₂ to 37 · Air to 200 barO₂ to 551 · Air to 3000 psi
EANx50O₂ to 71 · Air to 200 barO₂ to 1059 · Air to 3000 psi
TMX 25/25He to 50 · O₂ to 72 · Air to 200 barHe to 752 · O₂ to 1086 · Air to 3000 psi
TMX 21/35He to 70 · O₂ to 88 · Air to 200 barHe to 1044 · O₂ to 1305 · Air to 3000 psi
TMX 18/45He to 90 · O₂ to 105 · Air to 200 barHe to 1347 · O₂ to 1579 · Air to 3000 psi
TMX 15/55He to 110 · O₂ to 123 · Air to 200 barHe to 1651 · O₂ to 1854 · Air to 3000 psi
TMX 10/70He to 141 · O₂ to 150 · Air to 200 barHe to 2103 · O₂ to 2234 · Air to 3000 psi

How Dive Kit computes this

Dive Kit works backwards from the target. It solves for each stop pressure on absolute pressures (gauge plus the atmospheric pressure at your altitude) with a real-gas compressibility model for oxygen, nitrogen and helium, rather than the ideal-gas sums above. Filling an empty cylinder to 200 bar, the hand method stops the oxygen at 27.8 bar for EANx32 and Dive Kit at 27; for EANx50 it is 73.4 against 71. On trimix an oxygen analyser cannot see a helium error.

Each gas is added in whole bar, so every stop can be read on a gauge, and a bleed only appears when there is at least a bar to vent. When there is more than one valid plan, the options come in the order Dive Kit ranks them: plans that land within tolerance of the target first, then the fewest gases, the fewest steps and the closest mix. A fill temperature above ambient adds a final step: the hot pressure, and the lower pressure it settles to once the gas cools.

Continuous mode treats the panel as fixed feeds of air, oxygen and helium into the compressor intake. It returns the blend the line must carry, each feed's share of the flow, what the in-line O₂ analyzers should read for the chosen injection order, and a bleed pressure (rounded down to 5 bar, or 100 psi) when the cylinder's contents are in the way.

This page uses the app's default banks, unlimited air, oxygen and helium. The app also takes limited banks (a part-full bottle that drains as you draw from it) and plans decants through a transfill whip. Blending with oxygen is for trained gas blenders with oxygen-clean equipment. The page flags a step that takes the cylinder above 40% O₂, a continuous intake above 40% O₂ before helium dilutes it, and a fill above the cylinder's working pressure.

The app screen for this tool is documented in the user guide: Blend Planner in the Dive Kit guide.

How do you calculate a nitrox fill by partial pressure?

Work out how much oxygen the finished cylinder must hold, subtract what is already in it, and top up with air for the rest. Because air brings 21% oxygen with it, the oxygen to add is smaller than the simple difference: the formulas above solve the nitrogen balance first. The calculator does this on real-gas absolute pressures and gives the gauge pressure to stop each gas at.

What is the difference between partial pressure and continuous blending?

Partial pressure blending adds pure gases one after another into the cylinder, so the cylinder and valve see pure oxygen. Continuous blending meters oxygen (and helium) into the compressor intake and fills the cylinder with the finished mix in one go, checked by in-line analyzers. Switch the calculator's method to compare the two for the same fill.

Why does the plan start by bleeding the cylinder?

Gas can only be added, so when the cylinder already holds more oxygen or helium than the finished fill should contain, some of it has to be vented first. The plan gives the pressure to bleed down to, and warns when that means emptying the cylinder.

Do I need oxygen-clean equipment to blend nitrox?

For partial pressure blending, usually yes: pure oxygen goes into the cylinder even when the finished mix is EANx32. A common convention is that anything exposed to more than 40% oxygen needs oxygen service, and the calculator flags any step that goes past it. Follow your training agency and fill station rules. The top-up calculator works the other way round: it gives the mix you get from a fill you have already made.

Sources

  1. Gas blending for scuba diving (Wikipedia) : partial pressure blending adds the gases in sequence and allows for temperature; mixes of 40% oxygen or more need equipment suited for oxygen service; continuous blending feeds the gases to the compressor intake; a mix must be analysed before use
  2. NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems : the compressibility data for oxygen, nitrogen and helium behind the real-gas model
  3. Ortiz Vega et al. (2023), An Equation of State for the Thermodynamic Properties of Helium (NIST IR 8474) : the helium equation of state behind the helium compressibility

Last updated 30 October 2026

The same calculation in your pocket, offline.

The Blend Planner adds limited banks, saved cylinders and decant planning, offline on iOS and Android. Free.

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