---
title: "Switch depths"
description: "A switch depth is a ceiling on where a gas may be used, not an instruction to switch there."
---

> 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.

# Switch depths

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

A switch depth is the **deepest depth at which a gas may be breathed**, a
ceiling on availability rather than a command to change gas there. Set 21 m on a
50 % mix and the gas exists from 21 m upwards; where you reach for it depends on
what else is in the water.

The **Switch** row is that ceiling, and MOD, ppO₂, EAD and density are all
evaluated at it. A switch depth deeper than the gas's MOD produces a schedule
with an oxygen warning attached, not a refusal.

## How the planner picks a gas

At every depth of an open-circuit ascent, in this order:

1. **A gas you gave a switch depth is available at or shallower than it.** A
   manual switch depth is authoritative: if the switch is hot, the planner warns
   you and schedules it anyway.
2. **A gas with no switch depth turns on at its own MOD**, judged against its
   ppO₂ limit or the plan's decompression ppO₂ limit, whichever is lower.
3. **Among the qualifying gases, the richest wins.**
4. **The gas in use is a floor.** The planner never hands back a leaner gas, so
   a switch is one-way.
5. **A candidate carrying more helium than the gas in use is rejected.**

With nothing qualifying, the planner falls back to the first gas with no switch
depth.

Rules 4 and 5 mean **a leaner-but-heliox gas configured shallow will never be
selected on the way up** if a helium-free gas of equal or greater oxygen is in
use. Nothing warns you; it simply never appears in the schedule.

## The limits one gas can carry

Optional overrides at the wizard's **Gas Limits** stage; leave one blank and the
plan's limit applies.

- **ppO₂ limit**: this gas's ceiling in place of the role default, itself capped
  by the plan's decompression ppO₂ limit.
- **OC ppO₂ limit** and **OC depth limit**: open-circuit ceilings, deciding
  whether the gas can carry a bailout.
- **CCR ppO₂ limit** and **CCR depth limit**: the loop-side pair, deciding
  diluent eligibility.

On a bailout ascent the gas becomes available at the shallowest of its
open-circuit MOD, its open-circuit depth limit, and the switch depth you set.

## Diluents are chosen differently

A diluent's switch depth is a ceiling in the same sense, but **the ranking flips
with direction of travel**: descending and on the bottom the planner takes the
deepest MOD, breaking ties on least helium; ascending it takes least helium
first, then most oxygen. Before the schedule runs, each diluent's switch depth
is reduced to its MOD at the loop ppO₂ limit.

## Next

- [Mixes and roles](/gases/mixes-and-roles) gives each role's default ceiling.
- [Diluents](/ccr/diluents) covers the rebreather side.

Source: https://docs.divelogic.ai/gases/switch-depths/index.mdx