---
title: "Diluents"
description: "Add onboard and offboard diluents, and see what the 1.1 bar loop limit and the Diluent + Bailout role change."
---

> 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.

# Diluents

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

The diluent supplies the inert fraction of the loop: the setpoint fixes the
oxygen, the diluent the ratio of nitrogen to helium in what is left. The role is
chosen in the gas wizard's **Tank** step, from the **Gas Role** dropdown; an
onboard diluent has none, because the role follows the cylinder.

## The four CCR roles

- **Onboard Diluent** supplies the loop from the unit's own cylinder. Mandatory:
  the plan will not run without one.
- **Offboard Diluent** is a second loop diluent in a stage tank, with a switch
  depth.
- **Bailout** is open circuit only, enumerated in
  [bailout scenarios](/ccr/bailout).
- **Diluent + Bailout** is one stage cylinder doing both jobs. Because it may be
  breathed directly, a ppO₂ ceiling always applies in both roles.

## The 1.1 bar loop limit

Both diluent roles default their **ppO₂ Limit** to 1.1 bar, and from it the
planner derives a maximum operating depth. For the 21/35 diluent that is 42.3 m,
the depth at which 21 % oxygen reaches 1.1 bar. The MOD on the gas card is that
figure, not the 1.4 bar one a bottom gas is held to.

The dive goes to 55 m and the planner does not object: a pure diluent is never
inhaled directly, so an onboard diluent has no ppO₂ gate unless you set a CCR
limit on it.

A switch depth is a ceiling: the diluent is usable at or shallower than it,
never deeper ([Switch depths](/gases/switch-depths)). Leave an offboard
diluent's empty and the planner uses that maximum operating depth, 42.3 m here;
enter a deeper one and the MOD is planned instead, silently. An onboard diluent
with no switch depth stays available at every depth.

## Which diluent the loop is on

When more than one diluent is eligible, the planner picks by direction of
travel: descending and on the bottom, the most helium; ascending, the least;
ties on the most oxygen. Because switch depth and depth limit are ceilings, the
eligible set only grows on the way up, so switches are one-way.

> **Caution**
>
> Changing diluent changes the nitrogen-to-helium ratio at constant inert
> pressure, the condition isobaric counterdiffusion describes. The planner
> checks planned switches against the ICD rules and reports what it finds as
> warnings; it does not block the switch. See [ICD violations](/gases/icd).

## Next

- [Bailout scenarios](/ccr/bailout) covers the open-circuit-capable gases;
  [Switch depths](/gases/switch-depths) gives the ceiling convention.

Source: https://docs.divelogic.ai/ccr/diluents/index.mdx