# Gas Blending Calculator (Dive Kit): Free Partial Pressure Blending Calculator for Nitrox and Trimix

_Last updated: 2026-10-30_

> A free web calculator at https://divekit.app/tools/gas-blending-calculator/ that plans a nitrox or trimix fill from what is already in the cylinder: the partial pressure steps in order (bleed to, add helium to, add oxygen to, top with air to, let it cool to, then analyze), or the air, oxygen and helium flow ratios and analyzer readings for a continuous blending panel. It runs the same @divekit/core code as the Blend Planner in the Dive Kit app, with a real-gas compressibility model. Free, no account.

## What does the gas blending calculator do?

- **Partial pressure mode:** enter the cylinder, its current pressure and mix, and the target pressure and mix. The page returns each stop as a gauge pressure, the mix after each step, the mix the plan reaches, and the free gas drawn from each bank. Banks are unlimited air, oxygen and helium (the app's defaults).
- **Continuous mode:** the panel feeds are fixed (air, oxygen and helium into the compressor intake). The page returns the blend the fill line must carry, each feed's share of the flow, the O₂ readings to expect on the in-line analyzers for He-first or O₂-first injection, the gas drawn from each feed, and a bleed pressure when the current contents are in the way. A cylinder that already holds the target mix at or above the target pressure reads "Already at target".
- **Notes it raises:** a target the banks cannot make (with the app's reason, such as the O₂ range reachable at that helium fraction), a plan that vents the whole cylinder, a step that takes the cylinder above 40% O₂ (oxygen service), a continuous O₂-first intake where analyzer 1 reads above 40% O₂ before helium dilutes it, a target or hot fill above the cylinder's working pressure, and a final row to analyze the cylinder (O₂ and He for trimix) before diving.

## Who is the gas blending calculator best for?

- **Trained gas blenders** at a shop or club fill station who want the stop pressures for a partial pressure fill.
- **Technical divers** who blend their own trimix and want to see the fill before opening a valve.
- **Shops with a continuous blending panel** that need flow ratios and analyzer targets.
- **Instructors** teaching blending: the page shows the hand formulas next to the real-gas plan.

Blending with oxygen needs training and oxygen-clean equipment. The calculator plans numbers; it does not check equipment or procedure.

## Formula

```latex
\Delta P_\text{He} = P_t f_{\text{He},t} - P_n f_{\text{He},n}, \qquad
P_\text{air} = \frac{P_t f_{\text{N}_2,t} - P_n f_{\text{N}_2,n}}{0.79}, \qquad
\Delta P_{\text{O}_2} = P_t - P_n - \Delta P_\text{He} - P_\text{air}
```

The ideal-gas hand method in gauge pressure: P_t and P_n are the target and current pressures, f the gas fractions. Air is the only nitrogen source, so the nitrogen balance sets the air top-up. A negative result means the cylinder must be bled first.

```latex
f_\text{blend} = \frac{M_t f_t - M_n f_n}{M_t - M_n}, \qquad M = \frac{P_\text{abs}}{Z}
```

Continuous blending: the mix the panel must feed, with M the real-gas molar pressure (absolute pressure over the compressibility factor Z).

## How does Dive Kit compute it?

- Stops are solved on absolute pressures (gauge plus the atmospheric pressure at the entered altitude) with a real-gas compressibility model for oxygen, nitrogen and helium, not 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 bar; for EANx50 it is 73.4 against 71 (computed by the page at build time).
- Each gas is added in whole bar; a bleed appears only when there is at least a bar to vent. With more than one valid plan, the options are ranked: plans within tolerance of the target first, then the fewest gases, the fewest steps and the closest mix.
- A fill temperature above ambient adds a settle step: the hot pressure and the pressure it cools to.
- Continuous mode floors the bleed pressure to 5 bar (100 psi in imperial), as the app does. The injection order defaults to O₂-first, as in the app; in the default 21/35 fill analyzer 1 then reads about 40.2% O₂, and the page warns about it.
- Worked example (computed by the page at build time): 50 bar of air in an AL80 to Trimix 21/35 at 200 bar plans helium to 122 bar, oxygen to 140 bar, air to 200 bar. The same fill on a continuous panel sets about 39.5% air, 12.7% oxygen and 47.8% helium by flow.

## How does it compare?

| | Dive Kit web calculator | Dive Kit app (Blend Planner) | Typical online blending calculator |
|---|---|---|---|
| Partial pressure steps | Yes: bleed, add, settle | Yes | Yes |
| Continuous blending | Flow ratios and analyzer readings | Yes | Rarely |
| Decant through a whip | No (see the top-up calculator) | Yes, with the reachable range | Rarely |
| Banks | Unlimited air, O₂, He | Unlimited or limited, any mix | Usually fixed |
| Gas model | Real gas | Real gas | Usually ideal gas |
| Price | Free, no account | Free tool in the app | Free |

MultiDeco includes a gas mixer alongside its deco planner (paid); Dive Kit's Blend Planner is a free tool in the app, and this page runs its partial pressure and continuous modes in a browser.

## Facts at a glance

- **Page:** https://divekit.app/tools/gas-blending-calculator/
- **Inputs:** cylinder (38 from the Dive Kit open cylinder dataset), current pressure, O₂ % and He %, target pressure, O₂ % and He %, method (partial pressure or continuous), gas injection order, altitude, ambient and fill temperature
- **Outputs (partial):** plan status, achieved O₂ and He, final pressure, ordered steps with the mix after each, gas used per bank
- **Outputs (continuous):** blend in the line, air, O₂ and He flow ratios, analyzer readings, bleed pressure, gas used per feed
- **Units:** metric and imperial toggle (bar or psi, litres or ft³, °C or °F)
- **Works without an account:** yes; the default result and a table of fill recipes for common mixes are in the page HTML
- **Same code as the app:** yes, @divekit/core `calculateGasBlenderResult`, `calculateBlendConsumption` and `calculateContinuousBlend`
- **In the app:** the Blend Planner adds limited banks, saved cylinders and a Decant mode; free on iOS and Android, offline
- **Guide:** https://divekit.app/docs/tools/gas-blender/
- **Limits:** a planning aid for trained gas blenders, not a substitute for analysis. Analyze every blended cylinder (O₂, and He for trimix) and dive the analysed mix.
- **Last updated:** 2026-10-30

## Sources

- [Gas blending for scuba diving (Wikipedia)](https://en.wikipedia.org/wiki/Gas_blending_for_scuba_diving): gases added in sequence, allowing for temperature; mixes of 40% oxygen or more need equipment suited for oxygen service; continuous blending feeds the compressor intake; a mix must be analysed before use
- [NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems](https://webbook.nist.gov/chemistry/fluid/): compressibility data for oxygen, nitrogen and helium
- [Ortiz Vega et al. (2023), An Equation of State for the Thermodynamic Properties of Helium, NIST IR 8474](https://doi.org/10.6028/NIST.IR.8474): helium equation of state

## Related

- [Nitrox and trimix top-up calculator](https://divekit.app/tools/trimix-top-up-calculator/): the mix you get from a fill you already made, and decanting
- [Trimix best mix calculator](https://divekit.app/tools/trimix-best-mix-calculator/): pick the target mix for a depth before you blend it
- [Nitrox MOD calculator](https://divekit.app/tools/nitrox-mod-calculator/): MOD, END and density of the analysed mix