---
title: "Partial-pressure fill"
description: "Turning what is in a cylinder now and the mix you want into an ordered list of oxygen, helium and air additions in bar."
---

> 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.

# Partial-pressure fill

import { PartialPressureFillScreen } from "@/components/react/ToolScreens";

Partial-pressure fill answers *how do I fill this cylinder to the mix I want?*
It takes what the cylinder holds now and the mix and pressure you want, and
returns the fill as numbered additions in bar. It is the only tool in the
**Blending** group.

## What you enter

- **In the cylinder now**: **Oxygen** (%), **Helium** (%) and **Pressure**
  (bar) of the gas already in it. Enter 0 bar for an empty cylinder.
- **Target**: **Oxygen** (%), **Helium** (%) and **Pressure** (bar) of the
  finished fill.

## Reading the result

- **Fill to**: the finished mix by name, *EAN32* or *Trimix 21/35*, with its
  pressure and composition underneath.
- **The steps**: one numbered row per addition, in the order it goes in:
  oxygen, then helium, then a top-up with air. Each row gives the action, the
  amount to add in bar, a line of instruction, and the mix and pressure the
  cylinder holds after it. The helium step is left out when the target has no
  helium.

The verdict counts the steps and always ends *Analyse the mix before you dive
it.* When the cylinder already holds the target, the steps read *The cylinder
already holds this mix. Nothing to add.*

The method note carries two conditions of its own: the figures are ideal-gas
partial pressures, and mixes above 40 % oxygen need oxygen-clean equipment.

## When the fill cannot work

- **The cylinder is already at or above the target pressure.** The verdict
  turns amber and asks you to lower the start pressure or raise the target.
- **The cylinder is richer than the target.** If it holds more oxygen or helium
  than the finished mix needs, the steps do not produce the target. Drain the
  cylinder and start again.
- **Real gas and temperature.** Oxygen and helium depart from ideal behaviour
  at fill pressures, and a cylinder filled fast reads high until it cools. Put
  the analysed mix into a plan, not the target.

The steps assume an unlimited bank behind each addition. To fill by decanting
from your own cylinders, use the blending calculator in the planner.

## Next

- [Blending](/gases/blending) covers the planner's calculator, including
  decanting from source cylinders.
- [Best mix](/tools/best-mix) finds a target mix to fill to.

Source: https://docs.divelogic.ai/tools/partial-pressure-fill/index.mdx