---
title: "Blending"
description: "Turning a target mix and pressure into an ordered partial-pressure fill sequence."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Blending

import { BlendingCalculator } from "@/components/react/PlannerScreens";

The blending calculator takes what is in the cylinder now, the mix you want and
the pressure you want it at, and returns the fill as an ordered list of
additions in bar. It opens from a gas already in the plan, so the target
composition is known before you start.

1. **Step 1**

   **Current & Target Conditions.** Enter the **Current Gas in Tank** as `O₂ %` and
   `He %`, with the **Current Pressure (bar)**. Nitrogen shows as `N₂ % (auto)`.
   The target composition is read-only; you set only the **Target Pressure**.

2. **Step 2**

   When the inputs are workable the panel reads *Ready to calculate blending
   requirements*. Continue.

3. **Step 3**

   **Blending Instructions.** Step through the additions one at a time. Each card
   shows the action, the amount in bar, and the mix and pressure before and after
   it.

## The sequence

Three additions, always in this order: **Add Oxygen**, **Add Helium**, **Add
Air**. Each is a partial-pressure shortfall, how much of that component the
finished mix needs less how much the cylinder already holds, with air as the
top-up to the final pressure. The helium step is skipped when the target carries
no helium.

Amounts are shown as whole bar, the resolution a fill whip and a gauge work at,
but the pressure after each step is not rounded, so errors do not accumulate.
Any addition of **0.5 bar or less is left out entirely**.

## What it cannot see

The blend is refused when the **current pressure is not lower than the target
pressure**. There is nothing to add to a cylinder already at or above where you
want it.

> **A cylinder already richer than the target is not handled**
>
> That refusal looks at pressure only. If the cylinder holds *more* oxygen or
> helium than the finished mix needs (50 % in the cylinder against a 21 % target,
> say), no step is printed for that component and the rest of the sequence quietly
> assumes gas was removed. The instructions you are given will not produce the
> target mix. Drain the cylinder and start again.

Partial-pressure blending assumes ideal gas behaviour, and real oxygen and
helium deviate from that at fill pressures. It does not model temperature: a
cylinder filled fast is hot and reads high until it settles. The third step is
**air**, not nitrox and not a banked mix.

The calculator says so itself: *always follow proper gas blending safety
procedures and verify final mix with analyzer*. Put what your analyser read into
the gas, not what you asked the blender for.

## Next

- [Breathing rates](/planning/breathing-rates) is the next step of the plan.
- [Depth limits](/gases/analysing) checks the gas that came out of the cylinder.

Source: https://docs.divelogic.ai/gases/blending/index.mdx