Nitrox Blending Calculator
Work out how much oxygen and air go into a tank for a nitrox fill, including when the tank still holds gas.
Quick examples
Load a full example, then change any value.
Results update as you type.
Ideal partial-pressure blend
Oxygen to add
Within limitsAdd 27.8 bar of oxygen, so the gauge reads 27.8 bar. Then top up with air to 200 bar.
Top-up gas to add
172.2bar
Air goes in after the oxygen, until the gauge reads 200 bar.
Predicted final mix
EAN32
31.98% O₂ at 200 bar, with both amounts rounded to 0.1 bar.
- MOD at ppO₂ 1.4 bar
- 33.7 m
- MOD at ppO₂ 1.6 bar
- 40 m
Fill order
Oxygen goes in first, then the top-up gas. Stop each step at the gauge reading shown.
Continuous-blending equivalent
Continuous blending mixes oxygen into the top-up gas as it flows, instead of adding it to the tank as a separate step. The same fill needs this feed.
- Added gas
- EAN32
- Oxygen : air
- 1 : 6.18
- Oxygen in the feed
- 13.92%
This is the ideal mixing ratio, not a compressor setting. Use it only within the oxygen limit and procedure set for the blending system you actually use.
A calculation is not a fill procedure
Pure oxygen under pressure can start a fire. Partial-pressure blending is for trained gas blenders with oxygen-clean tanks, valves and fill equipment. These numbers assume ideal gas and the tank heats up during the fill, so the finished mix can differ from the plan. Analyse the cooled tank and label it with the oxygen percentage and MOD before anyone dives it.
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About partial-pressure blending
Partial-pressure blending makes nitrox inside the tank. Pure oxygen goes in first, then air or another top-up gas fills it to the target pressure. Each gas adds its own share of the pressure, so the two amounts set the final oxygen percentage.
The calculator counts gas already in the tank, says when part of it has to be drained, and shows the same fill as a continuous-blending feed ratio. All numbers assume ideal gas, so every fill is analysed before it is dived.
With your numbers
Enter valid values to see the calculation worked through with your numbers.
Good to know
- A warm tank reads high. Let it cool after the fill and top up again if the pressure has dropped.
- Oxygen added first needs time to mix with the top-up gas. Analyse after the tank has cooled and the gas has had time to mix, not straight off the fill whip.
- If the analysis is off, enter the analysed oxygen and the current pressure as the gas in the tank now. The calculator then plans the correction, including a drain if one is needed.
- Topping up from a nitrox bank needs less pure oxygen for a rich mix. If the bank already holds the target mix, no oxygen is needed at all.
Formula
Top-up gas
Ptop = (Pt × (1 − ft) − Ps × (1 − fs)) ÷ (1 − fu)
Oxygen to add
PO₂ = Pt × ft − Ps × fs − fu × Ptop
Continuous blending: added gas
fadd = (Pt × ft − Ps × fs) ÷ (Pt − Ps)
Oxygen share and feed ratio
x = (fadd − fu) ÷ (1 − fu) O₂ : top-up = 1 : (1 − x) ÷ (x)
Where
- Ps, fs: pressure (bar) and O₂ fraction in the tank now
- Pt, ft: target pressure (bar) and O₂ fraction
- fu: O₂ fraction of the top-up gas (air is 0.21)
- fadd: O₂ fraction of all the gas added; x: oxygen's share of the feed
How it's calculated
- S = pressure in the tank now, fS = its oxygen fraction, P = target pressure, fT = target oxygen fraction, fU = top-up gas oxygen fraction.
- Top-up gas (bar) = (P × (1 − fT) − S × (1 − fS)) ÷ (1 − fU).
- Oxygen (bar) = P × fT − S × fS − fU × top-up gas. Oxygen is rounded to 0.1 bar and the top-up gas takes the rest, so the total always equals P.
- Gauge after oxygen = S + oxygen. Gauge after top-up = P.
- If either amount comes out below zero, the tank must be drained: the drain pressure is the highest start pressure at which both amounts are zero or more, rounded down to 0.1 bar, and the plan is recalculated from it. If even that does not work, the mix can't be made with these gases.
- Predicted final mix = (S × fS + oxygen + fU × top-up gas) ÷ P, using the rounded amounts, shown to 0.01%.
- Continuous blending, from the pressure the plan starts at: added gas fraction fA = (P × fT − S × fS) ÷ (P − S), oxygen share of the feed x = (fA − fU) ÷ (1 − fU), feed ratio oxygen : top-up = 1 : (1 − x) ÷ x. Shown to 0.01 and only when both oxygen and top-up gas are added.
- MOD (m) = (ppO₂ ÷ fT − 1) × 10 at 1.4 and 1.6 bar, rounded down to 0.1 m. Ideal gas behaviour is assumed throughout.
These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification.
How to use the nitrox blending calculator
- 1
Enter what is in the tank now: the gauge pressure and the oxygen percentage of that gas. For an empty tank, set the pressure to 0.
- 2
Enter the pressure you want to fill to, the nitrox mix you want and the top-up gas. Most fills top up with air (21%).
- 3
Add the oxygen first, then the top-up gas, stopping at the gauge readings shown. Analyse the tank once it has cooled.
What the results mean
Partial-pressure blending builds a nitrox mix inside the tank. Pure oxygen goes in first, then air or another top-up gas fills it to the target pressure. Each gas adds its share of the pressure, so the amounts of oxygen and top-up gas set the final oxygen percentage. For 200 bar of EAN32 from an empty tank, that is about 27.8 bar of oxygen and 172.2 bar of air.
If the tank already holds gas, the calculator counts the oxygen and nitrogen already in it. When it holds too much of the wrong gas, for example rich nitrox left over for a leaner fill, it has to be drained first. The calculator shows the pressure to drain to and the fill plan from there. A mix with less oxygen than the top-up gas usually can't be made this way.
The continuous-blending figures describe the same fill made by mixing oxygen into the top-up gas as it flows. For 200 bar of EAN32 from an empty tank with air, the added gas is EAN32 and the feed is 1 part oxygen to 6.18 parts air (13.92% of the feed is oxygen).
The MOD rows show how deep the finished mix can go at a ppO₂ of 1.4 bar for the dive and 1.6 bar for deco stops. Write the analysed oxygen percentage and MOD on the tank before it is used.