---
title: "CNS and OTU"
description: "Reading the CNS clock and oxygen tolerance units for one exposure at a constant ppO₂, against the planner's advisories and limits."
---

> 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.

# CNS and OTU

import { CnsOtuScreen } from "@/components/react/ToolScreens";

CNS and OTU answers *how much oxygen exposure is this?* Give it a ppO₂ and a
time and it returns the two oxygen clocks the planner tracks on a dive: the CNS
percentage and oxygen tolerance units (OTU).

## What you enter

- **From**: **ppO₂**, to type the partial pressure directly, or **Mix and
  depth**, to have the tool work it out.
- **ppO₂** (bar): the oxygen partial pressure breathed, for example a
  rebreather setpoint.
- **Oxygen** (%) and **Depth** (m): shown instead with Mix and depth. The ppO₂
  assumes salt water at sea level.
- **Time** (min): how long the exposure lasts.

## Reading the result

- **CNS** (%): the main figure, a share of the NOAA single-exposure limit at
  this ppO₂. Its rail has a dashed mark at the **80** advisory and a solid mark
  at the **100 %** limit. Amber above 80 %, red above 100 %.
- **OTU**: pulmonary oxygen dose. Dashed mark at **200**, solid mark at **300**,
  the daily limit. Amber above 200, red above 300.
- **NOAA limit** (min): the longest single exposure the table allows at this
  ppO₂, or **None** below 0.5 bar.
- **ppO₂** (bar): the partial pressure used, useful when it came from a mix.

The advisories and limits are the ones the planner raises warnings at on a
plan. The verdict takes the worse of the two clocks:

- *Over the oxygen limit. Shorten the exposure or lower the ppO₂.*
- *Past the advisory. Allow a long surface interval before the next dive.*
- *Within both the CNS and OTU limits*, with the time and ppO₂.
- Below 0.5 bar, *oxygen exposure does not accumulate.*

CNS follows the NOAA table, read between its rows. OTU follows the standard
UPTD formula, about one unit a minute at 1.0 bar.

## What it does not count

- **Anything but this one exposure.** Earlier dives the same day, the surface
  interval, and the descent and ascent are not included. The planner tracks
  the whole dive.
- **ppO₂ above 1.6 bar.** The NOAA table stops at 1.6 bar. Above it the tool
  holds the 1.6 bar figures, 45 minutes to 100 % CNS, so it understates the
  exposure.
- **You.** The limits are population figures. Exertion, cold and CO₂ retention
  change your tolerance and do not enter the numbers.

## Next

- [Oxygen time limit](/tools/oxygen-time-limit) gives the time allowed at one
  ppO₂.
- [Oxygen exposure](/theory/oxygen-exposure) explains both clocks on a plan.

Source: https://docs.divelogic.ai/tools/cns-and-otu/index.mdx