---
title: "Gas switch"
description: "Checking one gas switch against the planner's isobaric counterdiffusion rules before it goes into a plan."
---

> 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 switch

import { GasSwitchScreen } from "@/components/react/ToolScreens";

Gas switch checks one switch, from the gas you are breathing to the next, at
one depth, against the planner's isobaric counterdiffusion (ICD) rules. It is
the quick way to test a switch before you build it into a plan.

## What you enter

- **Breathing now**: the **Oxygen** (%) and **Helium** (%) of the gas you are
  switching from.
- **Switching to**: the **Oxygen** and **Helium** of the gas you switch to.
- **Depth** (m): where the switch happens.

## Reading the result

The main figure names the switch, for example *18/45 to 50% at 21 m*, and
reads **Clear** in green or **At risk** in amber. Below it, one row per check:

- **N₂ fraction rise**: how far the nitrogen share of the mix rises.
- **He fraction drop**: how far the helium share falls.
- **ppN₂ rise** and **ppHe drop**: the same two changes as partial pressures at
  the switch depth, which is what the tissue sees.
- **EAD step**: how far the equivalent air depth jumps between the two mixes.

A passing check reads **Pass**. A failing one shows the amount in amber with
the limit it broke underneath, such as *Limit 15 %*. The verdict counts the
failures and suggests the two usual remedies: a gas with less nitrogen, or a
different switch depth.

The limits are the planner's starting limits for these checks. If you have
changed the ICD limits on a plan, this tool does not know; it always checks
against the defaults.

## A caution, not a stop

A failed check is amber, never red. It does not stop anything in a plan either:
the planner reports each failed check but still computes the schedule and
leaves the switch where you put it. The tool also checks every switch you give
it, whatever the depth, while the planner skips switches on dives shallower
than its **Minimum Dive Depth**.

- **Clear is not a guarantee.** The checks are threshold tests on the shape of
  a switch. They do not model the tissues, and a switch that passes has an
  unremarkable shape, not a proven one.
- **The switch-at-ceiling check needs a plan.** With no schedule there is no
  ceiling, so that check is not run here.
- **Oxygen is not checked.** A deco gas taken too deep passes every row here.
  Read its ppO₂ at the switch depth in [Mix at depth](/tools/mix-at-depth).

## Next

- [Isobaric counterdiffusion](/theory/isobaric-counterdiffusion) explains the
  checks and their default limits.
- [ICD in a plan](/gases/icd) covers what the planner does with a failed switch.

Source: https://docs.divelogic.ai/tools/gas-switch/index.mdx