ICD Calculator

ICD Calculator — Key Features

• Verdict (safe / caution / danger) for any gas switch at depth
• Color-coded thresholds: Doolette 1:5 (safe), Hamilton 1:3 (caution)
• Live |ΔHe| / ΔN₂ ratio plus partial-pressure changes at the switch depth
• Auto-recommends the minimum nitrox O₂% that makes the switch safe
• Detects the safe direction (helium-increasing switches carry no ICD risk)
• Works for any salt or fresh water environment, at altitude

ICD Calculator — How It Works

Enter your current bottom gas, the next gas you plan to switch to, and the switch depth. The calculator computes the absolute change in helium and nitrogen fractions and derives the ratio |ΔHe| / ΔN₂. It compares the ratio against two thresholds in common use: the Doolette 1:5 rule (used by GUE and most modern tech-diving curricula) and the older Hamilton 1:3 rule (more permissive). If a switch fails the rule, the calculator returns the minimum O₂ fraction a helium-free deco gas would need to satisfy the chosen threshold. Isobaric counter-diffusion is the inner-ear vestibular DCS mechanism that hits when helium leaves the body faster than nitrogen enters at the moment of a switch — the two gases move in opposite directions through the same tissue and can summate locally even when ambient pressure stays flat. The 1:5 rule is conservative; many real-world tech dives push the boundary, but the calculator lets you know exactly how much you're betting before you bet it.

Why use an ICD Calculator?

• ICD-induced DCS can occur at the gas switch itself, before any pressure change
• Trimix-to-nitrox switches concentrate the risk — especially deeper ones
• The 1:5 rule turns a complex diffusion problem into a single number you can verify
• Knowing the safe O₂% lets you re-plan your gas without guesswork

ICD Calculator — Frequently Asked Questions

What is Isobaric Counter Diffusion (ICD)?

Isobaric Counter Diffusion (ICD) is a form of decompression sickness that affects the inner ear (vestibular DCS) and occurs at the moment of a gas switch when helium leaves the body faster than nitrogen enters. The two gases move in opposite directions through the same tissue at different rates; if helium clearance is much faster than nitrogen uptake, gas tension can locally summate and form bubbles even though ambient pressure stays constant — hence 'isobaric'. The classic trigger is a switch from a high-helium trimix to a low-helium nitrox at depth.

What is the Doolette 1:5 rule?

The Doolette 1:5 rule, established by David Doolette at the US Navy Experimental Diving Unit, states that the increase in nitrogen partial pressure at a gas switch should not exceed one-fifth of the decrease in helium partial pressure. In practical terms, for every 1 percentage-point drop in helium fraction during a switch, the nitrogen fraction can increase by at most 5 percentage points. GUE adopted the rule in the early 2010s and most modern tech-diving curricula follow it. The ICD calculator checks every switch against this threshold.

How does it differ from Hamilton's 1:3 rule?

Hamilton's 1:3 rule is the older, more permissive predecessor of the Doolette 1:5 rule. It allows a nitrogen increase of one-third of the helium decrease — significantly more aggressive at the gas-switch boundary. Many real-world tech dives ran on the 1:3 rule for years without obvious incident, but vestibular DCS cases at gas switches motivated the move to the more conservative 1:5 in modern training. DiveToolbox checks both rules and reports the result for each so you can see which threshold your switch crosses.

When is the switch risk highest?

Switch risk is highest at the moment of switching at depth, when the tissue is still saturated from the previous gas and the new gas's partial pressures are immediately imposed. Risk scales with the magnitude of the helium drop and the nitrogen rise, not strongly with depth itself. Switching from a 21/35 trimix to an EAN50 at 20 m drops helium from 35% to 0% while nitrogen jumps from 44% to 50% — a |ΔHe|/ΔN₂ ratio of 35/6 ≈ 5.8, which violates the 1:5 rule and is a known vestibular-DCS trigger.

Does the rule depend on the switch depth?

The Doolette 1:5 rule is depth-independent because it operates on gas fractions (percentage points), not partial pressures — the ratio of helium decrease to nitrogen increase is the same whether the switch happens at 20 m or 50 m. What does change with depth is the absolute partial-pressure delta, which is larger at depth and produces a faster real-world counter-diffusion. The calculator flags the rule violation by ratio, but you should also avoid switches at depths where the absolute change would be unusually large.

Switching from a leaner to a richer helium mix — is there risk?

Switching from a leaner-helium gas to a richer-helium gas creates the opposite gradient: helium enters the body faster than nitrogen leaves. The Doolette/Hamilton rules formally apply to both directions, but the symmetric risk is less well-documented because most operational switches reduce helium for decompression on richer-oxygen blends. Still, the ICD calculator checks both directions and flags any switch that exceeds the 1:5 ratio in either sense. The safest pattern is to step gas changes in small increments rather than one large transition.

→ How to Plan a Trimix Dive — A 6-step workflow connecting every calculator in the right order

→ Diving Glossary — Essential acronyms and concepts every diver should know