---
title: "Gas names and labels"
description: "How the planner names a mix from its composition, the role labels a gas can carry, and what makes one of them the primary gas."
---

> 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.

# Gas names and labels

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

A gas card carries three labels: the name, the role and the cylinder
configuration. You type at most one of them.

## The name

**Mix Name**, on the wizard's **Label & Customization** step, is optional. Left
empty, the planner names the gas from its composition:

- **Any helium at all**: `TMX 21/35`, oxygen then helium.
- **21 % O₂ and no helium**: `Air`.
- **100 % O₂**: `O₂`.
- **Anything else**: `Nitrox 32`, the oxygen percentage alone.

A name you type replaces that at the top of the card, and the generated name
moves to the start of the composition line so the mix stays readable. Typing the
name the planner would have written counts as no name at all.

## The role

On a single, backmount double or sidemount cylinder the role is fixed to Bottom
Gas and labelled **Back Gas**. [Mixes and roles](/gases/mixes-and-roles) gives
each role's ceiling.

Change **Cylinder Configuration** and the role follows it, so the two are never
left disagreeing:

- **A stage cylinder** on a rebreather becomes Offboard Diluent, a drop cylinder
  Bailout.
- **A back-gas configuration** becomes Bottom Gas.
- **Leaving a back-gas configuration** falls back to Travel Gas, so a gas moved
  out of the doubles is not left claiming to be the bottom mix.

Set the cylinder first, then the role, or the cylinder will overwrite it.

## The primary gas

A gas is the diver's primary when its role is Bottom Gas, or when it sits in a
single, backmount double or sidemount cylinder. The card chips it **Primary**.
A primary gas has no depth ceiling of its own.

## Next

[Colour coding](/gases/colour-coding) is the last of the labels a gas carries,
and the one that identifies it on the profile.

Source: https://docs.divelogic.ai/gases/names-and-labels/index.mdx