Gas Blender · Nitrox blending
Enter your cylinder, the mix you want and the gases you have: the tool gives you the partial pressure blending steps, read on the gauge to the nearest 10 bar.
Your cylinders
12 L · 50 bar at 21%A single cylinder or a manifold of identical cylinders filled together. Remaining pressure: that of each cylinder, manifold open. Empty cylinder: enter 0 bar.
Target mix
Nx32 · 200 bar · 33 m maxPick a common nitrox or set the % by hand. Under each mix: its MOD at ppO2 1.4.
Maximum depth: 1 bar at the surface, +1 bar every 10 m. With 32% O2, ppO2 reaches 1.4 at 4.38 bar, i.e. 33 m.
Available gases
3 gasesYour storage cylinders and your compressor. Number: identical cylinders (e.g. 3 × 50 L of EAN36). The volume is used to check there is enough gas left; leave it empty for a compressor.
Blending plan
0/2 steps doneFollow the steps in order while reading the cylinder's gauge, then analyse the mix.
Recommended plan
Pure O2 + Compressor
32.9 %
200 bar · target 32 % ✓
MOD 32 m at ppO2 1.4
Final composition of the cylinder
Tap a step number once it is done.
Pure O2 100 %
+30 bar · cylinder at 50.6 % · storage left: 193 bar
fill up to80 barCompressor 21 %
+120 bar · cylinder at 32.9 %
fill up to200 bar
Pure O2: oxygen-clean, O2-compatible equipment.
Fill slowly: the cylinder heats up and its pressure drops as it cools. Let it cool, top up if needed, then analyse.
Dalton's law: the pressure of a mix is the sum of the pressures of its gases: each gas you add raises the gauge by its share.
Your stock
Enough gasDoes each gas have enough pressure and volume for this fill?
Pure O2100 %
Enough
Gives +30 bar to the cylinder (up to 80 bar)
Storage 200 → 193 bar
15 more identical cylinders
Nitrox bank36 %
Not used
Storage at 220 bar · 50 L
Compressor21 %
Unlimited
Gives +120 bar to the cylinder · max 300 bar
Final analysis
After fillingOnce the cylinder has cooled, analyse it and enter the measured %: that is the value that counts for the MOD.
Tick the plan steps as you go: the analysis unlocks once the cylinder is filled.
✓ Within spec
Off by +0.9 point of % O2 from the target 32 %
Label
32.9 %
MOD 32 m at ppO2 1.4 · 38 m at ppO2 1.6
At a glance
RecapThe gas to put in the cylinder, including pure O2, and the MOD of the resulting mix.
To add
+150barincl. +30 bar of pure O2
MOD
32mat ppO2 1.4 · 38 m at ppO2 1.6 · for the expected 32.9 %
Rejected combinations2
Understand nitrox blending
How it is calculated
- What needs to be added
- It starts from two needs: the bar to add (final pressure − remaining pressure) and the oxygen that added gas must contain (target % × final pressure − remaining % × remaining pressure). That oxygen is in all the added gas, air and nitrox included: it is not the amount of pure O2 to put in. The cylinder volume does not change the mix, only what the banks can supply.
- Two gases, two unknowns
- With two gases (for example pure O2 and air), the tool looks for how many bar of each give both the right pressure and the right amount of oxygen: the original spreadsheet method, generalised. The answer is only valid if both amounts are positive. The tool tries every pair of your gases, and each gas on its own.
- Rounding to the gauge
- The intermediate pressure is rounded to 10 bar, up or down depending on which gives the mix closest to the target, then the final mix is recalculated. The plan is kept if it stays within 2 percentage points of O2 of the target.
- Banks and equalisation
- A bank equalises with the cylinder: the common final pressure is the average of the two pressures, weighted by their volumes. The bank loses as many litres as the cylinder gains, so the bigger it is, the less its pressure drops. A step is marked "tight" if equalisation stops less than 10 bar from the target pressure.
- Cascade or all together
- Several identical cylinders opened in cascade, one after the other from the lowest to the highest pressure, each equalise in turn with the cylinder: this draws the most gas. Opened all together, they behave like a single large bank.
- Manifold of cylinders
- Several identical connected cylinders are calculated as a single cylinder of the total volume: gauge pressures do not change, only what the banks must supply is multiplied by the number of cylinders.
- Choosing the recommended plan
- First within tolerance, then without pure O2, then with the least pure O2, then closest to the target. If nothing works, the tool suggests bleeding the cylinder down in 10 bar steps, otherwise the maximum pressure reachable at the right mix.
- Limits
- Ideal gases, no temperature correction: the real mix can differ from the calculation. Always analyse the cooled cylinder before diving.
The principle: partial pressures add up
In a cylinder, each gas contributes its share of the total pressure: this is Dalton's law. A cylinder of air at 200 bar holds 21% oxygen, which is 42 bar of O2 and 158 bar of nitrogen. Partial pressure blending means adding gases one after the other while watching the gauge: you put in just enough oxygen, then top up with air to reach the right percentage. Example: a 12 L cylinder still holding 50 bar of air, + 30 bar of pure O2 (up to 80 bar), then air up to 200 bar, gives about 32.9% O2. The tool tries every combination of your gases and gives you the recommended plan, using as little pure O2 as possible.
Why pure O2 goes in first, and slowly
Pure oxygen under pressure is a powerful oxidiser: a trace of grease or a pressure rise that is too fast can be enough to start a fire. So it goes in first, while the cylinder is still at low pressure, and you then top up with air, which also helps the mix even out. A cylinder that receives pure O2 must be oxygen clean and O2 compatible, valve included, even if the final mix is only EAN32; above 40% O2, the regulator too. Open the valves slowly: the gas heats up as it is compressed, and the pressure drops as the cylinder cools. Let it rest, then top up if needed.
Banks, cascade and compressor
A storage bank (a large cylinder of nitrox, air or O2) doesn't push gas: it equalises with your cylinder until both are at the same pressure. The bigger and fuller the bank, the higher it can take your cylinder. With several banks of the same gas, you open them in cascade, from the lowest to the highest pressure: each one equalises in turn and you draw far more gas than by opening them all together. A compressor can go up to its maximum pressure. The tool checks that every bank has enough pressure and volume, and tells you how many more identical cylinders you can fill.
Reading to 10 bar, analysis and MOD
A diving pressure gauge can't be read more precisely than the nearest 10 bar: the tool rounds every step to 10 bar and works out the mix you actually get, with a tolerance of 2 percentage points of O2. The calculation assumes ideal gases and a constant temperature: it does not replace analysis. Once the cylinder has cooled, always check it with an oxygen analyser and write the measured % and the maximum operating depth on the label. The MOD comes from the ppO2: 1.4 bar for the dive, 1.6 bar at most. With EAN32, ppO2 1.4 gives 33 m. For your mix's MOD, EAD and CNS toxicity, use the nitrox calculator.
Frequently asked questions
What is a gas blender?
In diving, a gas blender is both the person who mixes breathing gases at the fill station and the calculator that tells them which pressures to add. This gas blender works out an air or nitrox fill by partial pressure blending, from what is left in your cylinder and the gases you have: pure O2, nitrox or air banks, a compressor. Handling pure oxygen requires specific gas blender training.
How do you calculate a partial pressure nitrox blend?
Work out the pressure to add (final pressure − remaining pressure) and the oxygen that added gas must contain (target % × final pressure − remaining % × remaining pressure). With two gases, for example pure O2 and air, you then get how many bar of each to put in. The tool does this for every combination of your gases, rounds to 10 bar for the gauge and checks the resulting mix.
How much pure O2 do you need to blend EAN32?
It depends on what is left in the cylinder. For a 12 L cylinder holding 50 bar of air, filled to 200 bar of EAN32: about 30 bar of pure O2 (up to 80 bar), then air up to 200 bar, giving about 32.9% O2. From an empty cylinder, you need about 28 bar of pure O2, rounded to 30 bar on the gauge, then air up to 200 bar.
Why add pure oxygen before air?
For safety: pure O2 goes in at low pressure, which limits heating and the risk of fire in the valve and cylinder. Topping up with air afterwards stirs the gas and helps the mix even out. The cylinder must still be oxygen clean and O2 compatible, and the fill must stay slow.
What is cascade filling?
When you have several banks of the same gas, you open them one after the other, from the lowest to the highest pressure. Each one equalises with your cylinder and takes it a little higher. You recover more gas than by opening all the banks together, and keep a full bank for the end.
Do you need to analyse a nitrox cylinder after filling?
Yes, always. The calculation assumes ideal gases and a gauge read to the nearest 10 bar: the real mix can be one or two points off. Let the cylinder cool, check it with an oxygen analyser and write the measured % and the MOD at ppO2 1.4 on the label. If the gap is too large, the tool suggests a correction: bleed down to a given pressure, then top up again.
What is the maximum depth for my nitrox?
Divide the maximum ppO2 by the oxygen fraction to get the limiting absolute pressure, then subtract 1 bar and count 10 m per bar. For EAN32 at ppO2 1.4: 1.4 ÷ 0.32 = 4.375 bar, so 33 m rounding down; at ppO2 1.6: 40 m. The tool shows it for the mix you will actually get, then for the analysed %.