---
title: "Mixes and roles"
description: "How to add a gas to a plan, and what each role decides about a mix's oxygen ceiling and when it may be breathed."
---

> 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.

# Mixes and roles

import { GasWizard } from "@/components/react/ModalScreens";

Every gas carries a composition, a role, a cylinder and a set of limits. The
role matters most: it decides the ppO₂ ceiling the gas is held to and whether
the planner may breathe it at all. The gas builder asks for them in five stages.

1. **Step 1**

   **Tank Configuration.** Tank size, quantity, working pressure and the cylinder
   configuration. See
   [Cylinder configurations](/gases/cylinder-configurations). A drop tank also asks
   for a **Drop Depth (m)**.

2. **Step 2**

   **Gas Composition.** **Manual**, where you type `O₂ %` and `He %`, or
   **Calculator**. See [Best mix](/gases/best-mix).

3. **Step 3**

   **Gas Limits.** The ppO₂ ceiling and, optionally, a switch depth. See
   [Switch depths](/gases/switch-depths).

4. **Step 4**

   **Label & Customization.** The gas's name and its colour. See
   [Gas names and labels](/gases/names-and-labels) and
   [Colour coding](/gases/colour-coding).

5. **Step 5**

   **Confirm Gas Mix.** A read-back of the cylinder and composition, with any
   warnings against the mix.

You enter oxygen and helium only. Nitrogen is the remainder, a read-only
`N₂ % (auto)` field, and a mix whose oxygen and helium exceed 100 together is
rejected.

## The six roles

| Role | Default ceiling | When the planner may use it |
|---|---|---|
| Bottom gas | 1.4 bar | The primary gas, breathed from the surface on descent, when it has no switch depth. Selectable on the ascent. |
| Travel gas | 1.5 bar | Also a candidate primary gas. Selectable on the ascent. |
| Deco gas | 1.6 bar | Selectable on the ascent, but never primary unless nothing else qualifies. |
| OC bailout | 1.6 bar | Breathable open circuit, and counted as bailout-capable. |
| Onboard / offboard diluent | 1.1 bar | Loop only. Never breathed directly. |
| Diluent + bailout | 1.6 bar | Both: a loop diluent, and breathable open circuit. |

The default applies when you leave the gas's own ppO₂ limit blank. The wizard
pre-fills that field at 1.4 bar for the two bailout roles rather than 1.6, so a
bailout gas built here carries 1.4 bar unless you change it.

## The cylinder

- **Tank size (L)**: internal volume. Starts at 11.1 L.
- **Working pressure (bar)**: the fill pressure the plan assumes. Starts at
  200 bar.
- **Quantity**: how many cylinders the gas is carried in. See
  [Cylinder configurations](/gases/cylinder-configurations).

## Next

[Consumption](/gases/consumption) turns the cylinder you gave each role into
litres, bar and a turn pressure.

Source: https://docs.divelogic.ai/gases/mixes-and-roles/index.mdx