---
title: "Gas roles"
description: "What each closed-circuit gas role decides, and which of them change the dive you are planning rather than only a contingency."
---

> 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 roles

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

A closed-circuit gas role decides two things: which circuit the planner may
breathe the gas on, and whether it belongs to the dive you are planning or only
to a contingency. The role is on the gas wizard's **Tank** step, in the **Gas
Role** dropdown. A stage cylinder chooses among the stage roles, the
**Onboard** cylinder chooses **Onboard Diluent** or **Onboard Oxygen**, and a
drop tank implies its own.

## The five roles

- **Onboard Diluent**: the unit's own cylinder. It supplies the loop from the
  surface down, and a CCR plan will not run without one.
- **Onboard Oxygen**: the unit's oxygen cylinder. Never breathed and never a
  diluent; its size and pressure set the oxygen supply the plan is graded
  against. Its ppO₂ is fixed at 1.6 bar and it takes no switch depth.
- **Offboard Diluent**: a stage diluent the loop switches onto during the
  planned dive, at or shallower than its switch depth.
- **Bailout**: open circuit only. The loop never sees it.
- **Diluent + Bailout**: a stage cylinder plumbed to both. The planner may fly
  the loop on it *and* carries it in every bailout scenario.

## What each role changes

- **Bailout** changes nothing about the planned dive. The schedule, the runtime
  and the time to surface are unchanged; the gas is used only when you ask what
  happens if you leave the loop.
- **Diluent + Bailout** changes the loop. On a 45 m dive with a 21/35 diluent,
  an 80 % in this role becomes the loop diluent at 9 m on the way up: a switch
  step in the schedule, and a slightly different time to surface.

Descending and on the bottom a dedicated diluent always outranks it, so the
deep part of the dive stays on your back diluent either way.

## Deciding where the loop may go

The cylinder's **ppO₂ Limit** sets how deep the loop may run on it. Left at the
bailout default, that 80 % is loop-eligible from about 10 m up. Give it the
1.1 bar limit a diluent carries and it never becomes eligible on a 45 m dive,
and the plan matches Bailout exactly.

Choose **Bailout** for a cylinder you carry only to leave the loop on, and
**Diluent + Bailout** for one you would also plug into the loop.

## Next

- [Diluents](/ccr/diluents) covers the loop limit and how the planner chooses
  between eligible diluents.
- [Bailout scenarios](/ccr/bailout) covers what the bailout roles are for.

Source: https://docs.divelogic.ai/ccr/gas-roles/index.mdx