Dive Planning

CNS & OTU Exposure Planner

Build an ordered diving-day profile and track retained CNS percentage and cumulative pulmonary oxygen loading together.

Exposure profile

Add constant-depth segments in chronological order. Enter a surface interval before a later segment only when the diver is out of the water.

Selected maximum PPO₂ check

This marks segments above the selected PPO₂/MOD constraint. It does not change CNS or OTU calculations at the actual PPO₂.

Exposure

Segment 1

Constant depth

Analyzed oxygen

Duration

min

Segment result

EAN32 · 32% O₂

PPO₂: 1.28 ATA
Interval / retained CNS: Start of profile
CNS loading: +13.44%
OTU loading: +36.2 OTU

Exposure

Segment 2

Surface interval before this segment

min

Enter 0 for another segment in the same dive. Surface time reduces retained CNS; this single-day model keeps accumulated OTU unchanged.

Constant depth

Analyzed oxygen

Duration

min

Segment result

EAN32 · 32% O₂

PPO₂: 1.28 ATA
Interval / retained CNS: 90 min before segment
CNS loading: +13.44%
OTU loading: +36.2 OTU

Frequently Asked Questions

What is the difference between CNS percentage and OTU?

CNS percentage tracks a table-based budget for acute central-nervous-system oxygen exposure, which is strongly affected by PPO2. OTU tracks cumulative pulmonary oxygen loading, which is more strongly affected by exposure duration. They are separate planning measures and neither predicts exactly when toxicity will occur.

Does a surface interval reduce both CNS and OTU?

This planner applies the common 90-minute half-life approximation to retained CNS. It does not reduce the single-day OTU total during ordinary surface intervals, because its OTU model simply sums the day's exposure segments. It does not model pulmonary recovery across days.

How does the planner calculate OTU?

For PPO2 above 0.5 ATA, it applies the NOAA/REPEX oxygen-tolerance-unit equation to each constant-depth segment and sums the unrounded results. A segment at or below 0.5 ATA adds zero OTU under this equation.

What does more than 100% CNS or 850 OTU mean?

It means the entered profile has exceeded the corresponding reference budget: the NOAA CNS table allowance or the isolated-day 850 OTU reference. It is not a prediction that symptoms begin at the next percentage or unit. Multi-day OTU guidance can be lower and is outside this planner.

Related Guides

Nitrox & oxygen

CNS Percentage and OTU: Tracking Oxygen Exposure

Learn how the CNS clock and OTU loading track oxygen exposure across a dive and a diving day, and what the limits mean.

Nitrox & oxygen

Diving Safety: Essential PPO2 Limits Every Diver Should Know

Understand partial pressure of oxygen limits, oxygen toxicity risks, and how to stay safe at depth.

Decompression & time limits

Surface Intervals and Repetitive Dives

Understand residual nitrogen between dives, how surface intervals restore no-stop time, and the rules for flying after diving.

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Calculate the depth where a Nitrox mix reaches a selected PPO2 threshold.

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