---
title: "Isobaric counterdiffusion"
description: "Why a helium-rich to nitrogen-rich gas switch can raise tissue loading at constant depth, and what the planner's six ICD checks test."
---

> 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.

# Isobaric counterdiffusion

import { WarningsDialog } from "@/components/react/ReviewScreens";
import { trimixDeco } from "@/demo/plans";

Isobaric counterdiffusion is what happens when two inert gases move in opposite
directions through the same tissue at the same time, without any change in
depth. Helium leaves, nitrogen arrives, and because they do not move at the same
rate the total inert-gas loading in some tissues can rise for a period after the
switch, with the diver doing nothing wrong. It is the one decompression hazard
in this handbook driven by a change of gas rather than a change of depth, so the
depth gauge shows nothing.

## Why one direction is the risky one

Switching from a helium-rich mix to a nitrogen-rich one is the direction that
causes trouble: nitrogen diffuses into the tissue faster than the resident
helium diffuses out, and while that imbalance lasts a tissue that was
comfortably clear can transiently exceed its limit. The reverse direction is not
symmetric, because there the fast gas is the one arriving and the total tends to
fall.

The tissue this is best documented in is the inner ear, which is why deep
trimix-to-nitrox switches are associated with isolated vestibular decompression
sickness: vertigo and nausea, sometimes with no other symptom, in a diver whose
schedule was otherwise clean.

## The six checks

The planner runs six independent tests on every planned gas switch, including
the loop-to-open-circuit change on a rebreather bailout. All six are on by
default, and each limit is editable.

| Check | Default limit |
|---|---|
| **N₂ fraction increase** | 20 percentage points |
| **He fraction decrease** | 15 percentage points |
| **ppN₂ increase** | 0.5 bar |
| **ppHe decrease** | 0.5 bar |
| **Switch at ceiling** | on or off |
| **EAD consistency** | 2 m |

**Switch at ceiling** flags a switch made at or shallower than the current
decompression ceiling. **EAD consistency** flags two mixes whose equivalent air
depths differ at the switch depth.

The fraction checks and the partial-pressure checks look redundant and are not.
Fractions describe the mix; partial pressures describe what the tissue sees, so
a switch that passes on fractions at 21 m can fail on pressures at 45 m.

Each violation becomes a plan message at **Error** severity, naming both gases,
the switch depth and the test that failed. Bailout switches are labelled
`bailout switch` rather than `gas switch`.

```text
ICD: gas switch 21/35 → 50% at 21m — He fraction decrease 35.0% exceeds limit 15.0%
```

**The checks never change the schedule.** They do not move a switch depth,
refuse a gas or lengthen a stop. The plan is the one the model computed, with
the violations attached.

The handbook's 45 m dive switches from 21/35 to 50 % at 21 m, and that
conventional switch fails three of the six: helium fraction, helium partial
pressure and EAD consistency. It passes both nitrogen checks, because a third of
the bottom mix was helium rather than nitrogen.

## What the checks do not tell you

They do not model counterdiffusion. None of the six simulates a two-gas tissue;
they are threshold tests on the shape of a switch, chosen because switches of
that shape are the ones associated with reported incidents. A switch that passes
all six is a switch of an unremarkable shape, not one shown to be safe.

They say nothing about timing or rate either. Descending before switching,
switching gradually, or waiting at a stop first are all mitigations divers use,
and none changes any of the six numbers. The limits themselves are conventions
rather than measured boundaries, which is why they are editable.

## Next

- [ICD safety](/planning/icd-safety) covers turning the checks on and editing
  the limits.
- [Plan warnings](/planning/warnings) shows how these messages appear on a plan.

Source: https://docs.divelogic.ai/theory/isobaric-counterdiffusion/index.mdx