---
title: "Gas Switching and Isobaric Counterdiffusion"
description: "Isobaric counterdiffusion explained step by step: the rule of fifths, the 0.5 bar rule, Doolette's timing rule, phased switching, and a numbered gas-switch checklist."
canonical: "https://donarundas.com/depth-notes/decompression/gas-switching-and-isobaric-counterdiffusion"
author: "Donarun Das — TDI Trimix Instructor, KISS Sidewinder CCR Instructor"
category: "decompression"
published: "2026-09-23"
tags: ["decompression", "isobaric-counterdiffusion", "trimix", "gas-switching", "inner-ear-dcs"]
disclaimer: "Dive theory published by a working instructor for education. It is not medical advice and not a substitute for training. Decompression, gas, and depth decisions belong with your own training, your dive computer, and a dive medical officer."
license: "© Donarun Das. Quote with attribution and a link to the canonical URL."
---

# Gas Switching and Isobaric Counterdiffusion

## The switch you make on every trimix dive

You are hanging at 18 m after a dive to 60 m on Tx 18/45. The plan says EAN50 from here. You unclip the regulator, confirm the label, confirm your depth, and switch.

Nothing about the water changed. The pressure around you is the same as it was a breath ago. But the gas going into your lungs just changed completely: helium went from 45 percent to zero, and nitrogen went from 37 percent to 50.

Almost every trimix diver makes this exact switch, and almost every one of them has been told, at some point, that it can hurt them. The injury has a name, inner-ear decompression sickness, and a mechanism with a longer name, isobaric counterdiffusion, or ICD. There are at least six rules in circulation for avoiding it, and they disagree with each other about the very switch you just made.

So here it is from the bottom up, one layer at a time, ending with what I do at 18 m and why.

If you have not read [how decompression models work](https://donarundas.com/depth-notes/decompression/how-decompression-models-work), start there. Everything below assumes you know what a tissue compartment and a half-time are.

## Layer 1: gas goes into you in proportion to its partial pressure

Partial pressure is fraction times ambient pressure. At 60 m the ambient pressure is 7 bar, and 18/45 splits like this.

| Gas in Tx 18/45 | Fraction | Partial pressure at 60 m |
|---|---|---|
| Oxygen | 0.18 | 1.26 bar |
| Helium | 0.45 | 3.15 bar |
| Nitrogen | 0.37 | 2.59 bar |

Every tissue slowly fills with helium and nitrogen until its dissolved gas tension matches those inspired numbers. Fast tissues match in minutes, slow ones in hours. When you ascend, ambient pressure falls faster than tissues can unload, the tissue tension ends up above ambient, and that gap is supersaturation. Too much of it and bubbles form. All of decompression is about keeping that gap small enough.

## Layer 2: a switch changes the gas without changing the depth

Now the switch itself. At 18 m the ambient pressure is 2.8 bar. Before and after look like this.

| Inspired | On Tx 18/45 at 18 m | On EAN50 at 18 m |
|---|---|---|
| Helium | 1.26 bar | 0 |
| Nitrogen | 1.04 bar | 1.40 bar |

Helium in your tissues now sees zero helium in your blood, so it leaves. Nitrogen in your blood is now higher than in your tissues, so it enters. Two gases crossing in opposite directions at the same pressure. Iso-baric, same pressure. Counter-diffusion, opposite directions.

For most tissues this is fine or even helpful. Helium leaves quickly, nitrogen arrives slowly, and the tissue's total gas tension dips for a while. That dip is why people once believed a nitrogen switch accelerated decompression.

## Layer 3: why crossing gases can make a tissue fuller

Two properties of the gases can flip that dip into a rise.

Helium diffuses about 2.65 times faster than nitrogen through tissue. And nitrogen dissolves about 4.5 times more readily in fat and in blood than helium does.

Whether a tissue's total goes up or down after a switch depends on its geometry and on which of those two effects wins there. Each rule picks one of the two to worry about, which is why the rules exist and why they disagree.

## Layer 10: at the stop

If vertigo arrives a few minutes after a switch at a constant depth, it is inner-ear DCS until proven otherwise. The diver cannot hold depth or read a gauge. The team holds them. Do not ascend to get them out. The remaining obligation is real and surfacing does nothing for the vertigo. Keep the highest safe oxygen going, complete the decompression with hands-on support, and get to a chamber. Doolette and Mitchell call this the technical diver's "Catch 22", and it is the strongest argument for switching shallow and late: if it happens at 18 m the remaining deco is short and warm.

## What is settled, and what is not

**Settled:** the inner ear is the target tissue; it is supersaturated by the ascent before any switch; a helium-to-nitrogen switch at 30 m or shallower carries low measured risk; a right-to-left shunt is a major factor; switching does not speed up decompression on a bounce dive.

**Contested:** how much a switch contributes at all, which Rebreather Forum 4 leaves as "uncertain"; whether any of the numeric rules predict anything; whether the oxygen window explains any of the benefit of a rich deco gas, or whether the inert-gas gradient explains all of it.

**Not consensus, but convergent:** the same two researchers, Doolette and Mitchell, have said the same thing in five venues over twelve years, and no peer-reviewed work contradicts them. The field rules are agency conventions with experience behind them and no trial.

## The honest summary

The profile is what hurts the ear, and the switch only trims the risk. Decompress the ear first, switch shallow and late, breathe the richest gas the depth allows, and check that the nitrogen step is under half a bar. Keep the rule of fifths as the reason helium-bearing deco gases exist. Keep Exley's one-two-two-one because it is good history and because I happen to like it.

## References

1. Doolette DJ, Mitchell SJ. *Biophysical basis for inner ear decompression sickness.* J Appl Physiol. 2003;94(6):2145–2150.
2. Mitchell SJ, Doolette DJ. *Pathophysiology of inner ear decompression sickness: current concepts.* Diving Hyperb Med. 2015;45(2):105–110.
3. Doolette DJ, Mitchell SJ. *Recreational technical diving part 2: decompression from deep technical dives.* Diving Hyperb Med. 2013;43(2):96–104. Citing Doolette DJ, Gerth WA. *Safe inner ear gas tensions for switch from helium to air breathing during decompression.* NEDU TR 12-04, 2013.
4. Kas S. *How two tech agencies address isobaric counterdiffusion.* InDEPTH, May 2020.
5. Guenzani S, Mereu D, Messersmith M, Olivari D, Arena M, Spanò A. *Inner-ear decompression sickness in nine trimix recreational divers.* Diving Hyperb Med. 2016;46(2):111–116.
6. Mitchell SJ, Pollock NW, eds. *Rebreather Forum 4 Proceedings.* 2024. Decompression theory and practice, inner-ear DCS.
7. Burton S. *Isobaric counter diffusion.* scubaengineer.com, December 2004.
8. British Sub-Aqua Club. *BSAC recommended diving gas mixtures for open circuit diving.* bsac.com.
9. HHS Software, *MultiDeco FAQ*; Subsurface source, core/gas.cpp and core/deco.cpp; Heinrichs Weikamp forum, IBCD calculations, 2021.
10. Helling R. *Isobaric counter diffusion criteria* and *More confusion from isobaric counter diffusion.* The Theoretical Diver, 2018.
11. Exley S. *Caverns Measureless to Man.* Cave Books, 1994. p. 259. Quoted in Burton S, *Sheck Exley Mexico dive*, scubaengineer.com.
12. Mitchell SJ. *Decompression science: critical gas exchange.* In: Rebreathers and Scientific Diving Proceedings, 2016. pp. 163–174.
13. Doolette DJ. *Gradient factors in a post-deep stops world.* InDEPTH, 2019.

*Gas switching under a ceiling is the skill every technical course I teach is built around. [Enquire about training →](https://donarundas.com/contact)*

## Common questions

### What is isobaric counterdiffusion?

Isobaric counterdiffusion (ICD) is what happens when you change the inert gas you breathe without changing depth, classically switching from helium-rich trimix to nitrogen-rich deco gas at a stop. Helium leaves the tissues while nitrogen enters, and in one tissue, the inner ear, the total can briefly rise even though the pressure around you never moved.

### Does the rule of fifths work?

It reliably points you to a helium-bearing deco gas, which is a good outcome. As a pass or fail gate it fails the most common switch in technical diving, Tx 18/45 to EAN50, which thousands of divers make without incident and which the one human trial at 30 m found low risk. Treat it as the reason gases like 35/25 exist, not as a rule that predicts injury.

### What is the 0.5 bar rule for gas switches?

A BSAC recommendation, also the default alarm in MultiDeco and V-Planner: the nitrogen partial pressure you inhale should not rise by more than about 0.5 bar at the switch depth. It is depth-aware, unlike the rule of fifths, and its author calls it advisory. No trial produced the number.

### How deep can I switch from trimix to nitrox?

The only human trial switched heliox to air at 30 m after an hour at 46 to 67 m and found low inner-ear DCS risk, so 30 m is the evidence-backed limit for a helium-to-nitrogen switch. In practice the oxygen ceiling puts EAN50 at 18 m at PO₂ 1.4, which is shallower still and better on every rule.

### What actually causes inner-ear DCS on deep dives?

Mostly an inner ear that was under-decompressed before the switch, plus a right-to-left shunt such as a PFO in most cases. Doolette and Mitchell's model shows the inner ear already heavily supersaturated during the early ascent before any switch. The switch adds a small transient on top of that base.

---

**Source:** [https://donarundas.com/depth-notes/decompression/gas-switching-and-isobaric-counterdiffusion](https://donarundas.com/depth-notes/decompression/gas-switching-and-isobaric-counterdiffusion)

**About the author:** Donarun Das is a TDI Trimix and KISS Sidewinder CCR instructor teaching technical and rebreather diving in the Maldives, Sri Lanka, and India. He holds the deepest verified civilian dive in India (103 m, open-circuit trimix, Lakshadweep, March 2026). Courses: https://donarundas.com/courses — enquiries: https://donarundas.com/contact

> Dive theory published by a working instructor for education. It is not medical advice and not a substitute for training. Decompression, gas, and depth decisions belong with your own training, your dive computer, and a dive medical officer.

## Sitemap

See the full [sitemap](https://donarundas.com/sitemap.md) for all pages available as Markdown.