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.
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.
Dive Warnings
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 covers turning the checks on and editing the limits.
- Plan warnings shows how these messages appear on a plan.