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ICD — Isobaric Counter Diffusion

The ICD Calculator checks a planned gas switch against the Doolette 1:5 rule to prevent Isobaric Counter Diffusion bubble formation. The classic risk: replacing a helium-rich bottom mix with a nitrogen-rich deco gas at depth — helium leaves tissues faster than nitrogen enters, producing a net inert-gas pressure spike even though ambient pressure is unchanged.

Free Tool

The ICD Calculator is free for all users with no usage limits.

Inputs​

  • Current gas — your bottom gas (O₂ % and He %)
  • Next gas — the gas you plan to switch to (O₂ % and He %)
  • Switch depth — the depth at which the change happens
  • Water type and altitude — affect the partial-pressure display only (the verdict itself is depth-independent)

Calculation​

The calculator looks at the change in inert-gas fractions:

ΔHe = fHe_next  − fHe_current     (negative when He decreases)
ΔN₂ = fN₂_next  − fN₂_current     (positive when N₂ increases)
ratio = |ΔHe| / ΔN₂

The verdict is decided from this single ratio:

  • ≥ 5 : 1 (safe) — satisfies the Doolette / Steve Burton 1:5 rule, the current planning standard
  • 3 : 1 – 5 : 1 (caution) — meets the older Hamilton 1:3 rule but not Doolette
  • < 3 : 1 (high risk) — change the gas or the switch depth

The ratio is depth-independent: at the switch depth, both gases share the same ambient pressure, so the ambient term cancels out. Depth still appears in the result to show the absolute partial pressures (ppHe and ppN₂ before / after), which is the physical quantity tissues actually see.

Minimum safe O₂ suggestion​

When the verdict is caution or danger, the calculator computes the lowest O₂ % a helium-free deco gas would need to satisfy the 1:5 rule:

O₂_min = O₂_current + 0.8 × He_current

Example: bottom gas Tx 18/45 → minimum nitrox deco = 18 + 0.8 × 45 = 54 % O₂. EAN50 (the classic deco choice after Tx 18/45) falls short and lands in the caution zone — the calculator surfaces this immediately.

Direction matters​

ICD requires helium to be decreasing while nitrogen increases. Switching to a richer-helium mix (e.g. a deeper trimix during a multi-segment dive) carries no ICD risk — the calculator displays "No ICD risk on this direction" in that case.

important

The 1:5 and 1:3 thresholds are community guidelines, not regulatory limits. Tissue saturation, switch order, and time at depth all influence real-world risk. Treat 1:5 as the planning target and never approach 1:3 on high-stakes deco profiles.