Isobaric counterdiffusion

Posted 23 June 2026 · Field Note #15

ICD: the gas switch that can bite back

First the name in plain words. Isobaric counterdiffusion (ICD) means two gases moving in opposite directions through your tissues at the same time, while the pressure around you stays the same — "isobaric" simply means "at constant pressure." The danger is that this can happen without you changing depth at all.

Here is how it bites. Suppose you have been breathing a helium-rich trimix and then switch to a deco mix with much more nitrogen, at depth. Helium is a small, nimble molecule, so it leaves your tissues quickly. Nitrogen is larger and slower, so it floods in more gradually. For a window of time, helium is rushing out while nitrogen is still pouring in faster than the helium leaves — and the total dissolved gas in some tissues briefly climbs, even though your depth never changed. If the tissue that spikes is the inner ear, the result can be inner-ear decompression sickness: sudden vertigo, nausea, hearing changes.

Why does this catch people out? Because it violates the intuition that DCS comes from going up too fast. Here the depth is constant; the culprit is the mismatch in how fast the two gases move. It is a planning problem, not an ascent problem.

The guard rails most technical training teaches: be conservative about how big a nitrogen jump you make in a single switch — the often-cited "rule of fifths" says the rise in nitrogen fraction should be no more than one-fifth of the drop in helium fraction — and arrange your switches so the nitrogen-rich deco mixes come shallower, where they are needed, rather than swapping into them deep. The point is to avoid a large, abrupt nitrogen jump while a lot of helium is still on board.

The practical takeaway: plan every gas switch deliberately, on the surface, as part of your decompression schedule. Do not improvise switches because a cylinder is handy. And if you feel vertigo after a deep switch, treat it as a serious warning sign and follow your emergency procedures — not as a passing quirk of the depth.

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