---
title: "ICD safety"
description: "The last section of the Plan step: three pairs of counterdiffusion checks run at every gas switch, what each pair measures, and the error each one raises."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# ICD safety

import { IcdChecks } from "@/components/react/PlannerSettingsScreens";

The **ICD Safety** section closes the Plan step. It sets what the planner should
object to when you change what you are breathing. With no helium in the plan it
carries a single note and no controls; add a trimix or heliox mix and the **ICD
Safety Checks** card appears, holding three pairs of checks.

Every check is **Enabled** or **Disabled**, and a disabled check is not
evaluated. Each pair works the same way: a control that turns the check on, and
a number that says how much is too much. Raising a number widens what passes.

## Composition

- **N₂ Fraction Increase** with **Max N₂ Increase (% pts)**, starting at 20: how
  far the nitrogen fraction rises across the switch.
- **He Fraction Decrease** with **Max He Decrease (% pts)**, starting at 15: how
  far the helium fraction falls across it.

This pair reads the mixes alone, so it catches a large change of composition
wherever in the water column you make it. Switching 18/45 for 32 % raises
nitrogen by 31 points and drops helium by 45:

```text
ICD: gas switch 18/45 → 32% at 30m — N₂ fraction increase 31.0% exceeds limit 20.0%
ICD: gas switch 18/45 → 32% at 30m — He fraction decrease 45.0% exceeds limit 15.0%
```

## Partial pressure

- **ppN₂ Increase** with **Max ppN₂ Increase (bar)**, starting at 0.5: the same
  nitrogen rise, measured as a pressure at the depth you switch.
- **ppHe Decrease** with **Max ppHe Decrease (bar)**, starting at 0.5: the same
  helium fall, at that depth.

This pair is the same two events weighed by depth, which is why both pairs
exist: a change that is modest by fraction can still be severe at 30 m.

```text
ICD: gas switch 18/45 → 32% at 30m — ppN₂ increase 1.24 bar exceeds limit 0.50 bar
ICD: gas switch 18/45 → 32% at 30m — ppHe decrease 1.80 bar exceeds limit 0.50 bar
```

## Switch conditions

- **Switch at Ceiling**, set to **Block** or **Allow**: whether a switch made at
  or above the decompression ceiling is objected to. It reports the ceiling it
  found, as `switch at deco ceiling 21.0m`.
- **EAD Consistency** with **EAD Tolerance (m)**, starting at 2: how far apart
  the two gases' narcotic depths may be.

Narcosis is the one you can check by eye. At 30 m, 18/45 narcoses like 8.8 m and
32 % like 24.5 m, a gap of nearly 16 m against a 2 m tolerance:

```text
ICD: gas switch 18/45 → 32% at 30m — EAD mismatch: from 8.8m to 24.5m (diff 15.7m)
```

## Next

Those five errors are one gas change on one dive.
[ICD violations](/gases/icd) covers what the planner does with the schedule
afterwards, and where the failures are listed.

Source: https://docs.divelogic.ai/planning/icd-safety/index.mdx