Nitrox Top-Off Calculator: Trimix Top-Up and Decant

A top-up adds gas to a cylinder that already holds some, and the mix changes with it. Enter what is in the cylinder and what you add: this calculator gives the resulting O₂ and He fractions and the pressure once the gas settles, and its Decant mode does the same for two cylinders joined by a transfill whip. For trained gas blenders; analyse every cylinder before diving.

The formula

f_{end} = \frac{P_{n,abs} f_{n} + ΔP f_{add}}{P_{n,abs} + ΔP}

Dalton's law for one gas (O₂ or He). P_n,abs is the cylinder's absolute pressure before the top-up, ΔP the gauge rise from the added gas, f the fractions.

P_{settled} = P_{hot} \times \frac{T_{ambient}}{T_{fill}}

Gay-Lussac's law, temperatures in kelvin. The law holds for absolute pressures (gauge plus atmospheric); Dive Kit applies it to the gauge reading, which differs by less than a bar. The O₂ and He fractions do not change as the gas cools.

P_{eq} \approx \frac{P_{A} V_{A} + P_{B} V_{B}}{V_{A} + V_{B}}

Decant: the ideal volume-weighted pressure two joined cylinders settle at. Dive Kit conserves the real-gas surface litres instead; the worked example shows the difference.

Worked example

A cylinder has 70 bar of EANx32 left and is topped up with 130 bar of air. It ends at EANx25, O₂ 25.0% at 200 bar. The textbook check, Dalton's law on absolute pressures with ideal gas, gives 24.9%. Dive Kit's real-gas figure is 0.14 percentage points away from it.

A trimix top-up: 100 bar of TMX 21/35 plus 100 bar of air gives TMX 21/18 (O₂ 21.0%, He 18.1%) at 200 bar. Air brings no helium, so the helium fraction drops with every bar added, and only a helium analyser shows where it landed.

In imperial: 1000 psi of EANx32 topped with 2000 psi of air ends at EANx25 (O₂ 24.9%) at 3000 psi.

Decanting: a 12 L cylinder at 50 bar of air is joined by a whip to a 12 L cylinder at 200 bar of EANx32. Both settle at 122 bar, against 125 bar from the ideal volume-weighted average. Cylinder A becomes EANx27; cylinder B keeps its EANx32 at the lower pressure.

Topping up with air to 200 bar

The mix you get when a part-used cylinder is topped up with air to 200 bar, at 20 °C and sea level. Nitrox rows show O₂; trimix rows show O₂/He.

Left in the cylinder	50 bar left	100 bar left	150 bar left
EANx32	23.9%	26.7%	29.5%
EANx36	24.9%	28.8%	32.6%
EANx40	26.0%	30.9%	35.6%
TMX 21/35	21.0/9.2	21.0/18.1	21.0/26.8
TMX 18/45	20.2/11.8	19.4/23.2	18.7/34.4

How Dive Kit computes this

Fill mode mixes on real gas: it conserves each component's partial molar pressure (fraction times absolute pressure, divided by the compressibility factor Z), which reduces to Dalton's law above when Z is 1. The pressure you add is the rise on the cylinder's own gauge, and absolute pressure uses the atmospheric pressure at your altitude.

When the fill temperature is above ambient, the result is the pressure after the gas cools, and the steps show the hot pressure it starts from.

Decant mode needs both cylinder sizes, because the settled pressure is shared by volume. Gas flows from the higher-pressure cylinder to the lower one; the donor keeps its mix and only loses pressure, and the receiver's mix moves toward the donor's. A whip equalises pressure, not composition. The figure assumes the transfer stays at one temperature: a freshly decanted cylinder is warm and reads a little lower once it cools.

This page takes one addition. The Mix Result screen in the app chains several (oxygen, then helium, then air, in the order you filled) and lists the mix after each. To work backwards from the mix you want, use the gas blending calculator. Topping up with oxygen, or anything that takes the cylinder above 40% O₂, is for trained gas blenders with oxygen-clean equipment, and the page flags it. Either way, analyse the cylinder before you dive it.

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

What do I get if I top off (top up) nitrox with air?

A leaner nitrox: the oxygen fraction falls toward 21% as air makes up more of the cylinder. For example, 70 bar of EANx32 topped off with 130 bar of air gives EANx25 (O₂ 25.0%) at 200 bar. The table above lists other common cases at 200 bar, and the calculator gives any other. Analyse the cylinder and set your computer and MOD to the new mix.

How do you calculate the mix after a top-up?

Multiply each gas's fraction by its pressure, add them, and divide by the final pressure, using absolute pressures (gauge plus atmospheric). That is Dalton's law, shown in the formula above. At fill pressures real gases pack differently from that ideal sum, so Dive Kit applies compressibility; for nitrox the difference is a fraction of a percent.

What happens when you decant one cylinder into another?

Gas flows from the fuller cylinder into the emptier one until the pressures match. Only the receiving cylinder's mix changes, toward the donor's; the donor keeps its mix at the lower pressure. Decant mode gives the settled pressure and both mixes. To stop part way toward a target mix, the app's Blend Planner plans a partial decant.

Why does the pressure drop after a fill?

Filling heats the gas, so the gauge reads high while the cylinder is warm, and the pressure falls as it cools to the room. Set the fill and ambient temperatures under Environment to see the pressure it settles to. The mix does not change as it cools.

Sources

Gas blending for scuba diving (Wikipedia) : an air top-up fills a cylinder holding a blend with air; the pressure falls as a hot cylinder cools; mixes of 40% oxygen or more need equipment suited for oxygen service; a mix must be analysed before use
NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems : the compressibility data for oxygen, nitrogen and helium behind the real-gas mixing

Last updated 30 October 2026

Nitrox and Trimix Top-Up Calculator