ICD Gas Switch Checker
Check a deco gas switch for isobaric counterdiffusion risk. The tool applies Burton's rule of fifths and shows the dissolved-gas load (pressure × solubility) before and after the switch — and if the switch fails, it tells you how much helium the deco gas needs.
Inputs
O₂ He N₂
Current gas (before the switch)
—
Usually the bottom mix, e.g. Tx 18/45. N₂ is the remainder.
Switch gas (after the switch)
—
The deco gas you switch onto, e.g. EAN50 = 50 / 0.
Switch gas PO₂ at this depth is — bar — above 1.4. Switch shallower or check the mix.
Verdict
He drop
— pts
N₂ rise allowed
— pts
N₂ rise actual
— pts
Dissolved gas load
Helium Nitrogen
Pressure × lipid solubility (He 0.015, N₂ 0.067) at — m: — → —
To satisfy the rule at —% O₂, keep at least —% helium in the switch gas — blend it as —. Alternatives: an intermediate mix, or feathering the switch.
Burton's solubility model and rule of fifths (scubaengineer.com, 2004): the load bars are Σ(inert-gas partial pressure × lipid solubility), the field heuristic behind helium in deco gases. It is not a guarantee — inner-ear DCS can occur without a gas switch (Mitchell). Cross-check against your training and dive planning software.
Common questions
What is isobaric counterdiffusion (ICD)?
ICD happens when you change the inert-gas composition you are breathing at constant depth — classically switching from helium-rich trimix to nitrogen-rich deco gas. Two inert gases move through tissues in opposite directions at once, and the total dissolved gas load can rise without any depth change. The characteristic hit is inner-ear (vestibular) DCS: vertigo, nausea, hearing changes right after a deep gas switch.
What is the rule of fifths for gas switching?
Steve Burton's field rule: on any gas switch, the nitrogen percentage should rise by no more than one fifth of the drop in helium percentage. It comes from solubility — nitrogen is roughly 4.5 times more soluble than helium in lipid tissue, so one point of nitrogen carries about five points of helium worth of dissolved gas. This tool checks the rule live and computes the dissolved-gas load before and after the switch.
Is ICD caused by helium leaving tissues faster than nitrogen enters?
That popular explanation is backwards. Helium is actually the faster gas — it diffuses about 2.65 times faster than nitrogen. The real driver is solubility: nitrogen holds about 4.5 times more dissolved gas per bar of partial pressure in lipid tissue, so swapping helium for nitrogen raises the dissolved load even though depth never changes.
How do I fix a gas switch that breaks the rule of fifths?
Keep some helium in the deco gas — this tool computes the minimum helium fraction that satisfies the rule for your oxygen choice. Alternatives: use intermediate mixes so no single switch is too big, or feather the switch (alternate breaths of old and new gas, Sheck Exley's original technique).
Does following the rule of fifths guarantee no inner-ear DCS?
No. It is a field heuristic, validated informally over many deep trimix dives, not a proven mechanism or a guarantee. As Simon Mitchell notes, inner-ear DCS can occur without any gas switch at all — some of it is ordinary DCS in a poorly perfused, vulnerable tissue. Treat the rule as one planning constraint among many.