Diving guide
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.
Maximum operating depth answers “how deep”. It says nothing about how long. Oxygen exposure is tracked on two separate clocks, because acute nervous-system toxicity and cumulative lung irritation respond differently to partial pressure and time.
Calculate a complete diving day
Use the CNS & OTU Exposure Planner to add ordered constant-depth segments, surface intervals, and analyzed gas mixes. The guide below explains exactly what its two totals mean.
Two clocks, two injuries
- CNS percentage tracks central nervous system oxygen toxicity. It is acute and dominated by partial pressure. Its worst outcome is a convulsion, which underwater can be fatal regardless of how healthy the diver is.
- OTU, the oxygen tolerance unit, tracks pulmonary oxygen toxicity. It is cumulative and dominated by duration. Its outcome is lung irritation — burning on inhalation, coughing, reduced vital capacity — which is unpleasant and reversible rather than immediately dangerous.
A short exposure to a high partial pressure loads CNS heavily and OTU barely. A long exposure at a moderate partial pressure builds OTU steadily, but under the NOAA table it also uses CNS budget, and within one continuous exposure the CNS allowance usually runs out first. OTU becomes the binding limit across repeated dives and consecutive days: retained CNS falls during each surface interval, while the day's OTU total keeps growing and the multi-day allowance shrinks. They are separate budgets that bind in different situations, which is why serious planning carries both.
How the CNS clock works
Reference tables — most familiarly the NOAA oxygen exposure limits — give a maximum single-exposure duration for each oxygen partial pressure. Time spent at a given partial pressure is expressed as a percentage of that allowance, and the percentages from each part of the dive add up.
For one constant-depth segment, the planner uses:
Here, t is the segment duration in minutes and T(PPO₂) is the number of minutes representing 100% at that oxygen partial pressure. PPO₂ is absolute pressure in ATA and the oxygen input is a fraction, so 32% is used as 0.32. The result is a percentage of the table allowance, not a probability of toxicity.
The allowance falls steeply as partial pressure rises, and that steepness is the point. This is why the difference between a 1.4 and a 1.6 ATA ceiling is not a small one, and why 1.6 is generally treated as contingency rather than a working level. The mechanics of the ceiling itself are covered in PPO₂ safety limits.
NOAA CNS Oxygen Exposure Limits
Normal single-exposure planning limit at each PPO2, shown as 100% CNS. Values come from the NOAA Diving Manual, 6th edition; no calculator-specific substitute values are used.
| PPO2 (ATA) | 100% allowance |
|---|---|
| 1.6 | 45 min |
| 1.5 | 120 min |
| 1.4 | 150 min |
| 1.3 | 180 min |
| 1.2 | 210 min |
| 1.1 | 240 min |
| 1.0 | 300 min |
| 0.9 | 360 min |
| 0.8 | 450 min |
| 0.7 | 570 min |
| 0.6 | 720 min |
The table is a planning convention, not a guaranteed toxicity threshold. It supplies exposure durations, not labels such as “working,” “recreational,” or “decompression.” Those operating choices come from the applicable training and procedure.
Exact table rows are used unchanged. Between rows, DiveCalc linearly interpolates the allowed duration from the two surrounding rows using the unrounded PPO₂. For example, 1.28 ATA lies 80% of the way from 1.2 ATA (210 minutes) to 1.3 ATA (180 minutes), so its allowance is 186 minutes. PPO₂ and percentage values are rounded only after the calculation for display.
Below 0.6 ATA, this NOAA table model assigns zero CNS because it has no lower row. Above 1.6 ATA, the planner rejects the segment; it never caps PPO₂ at 1.6 or extrapolates beyond the source table.
NOAA's 24-hour limits are a separate check
The NOAA table also gives a maximum total time at each PPO₂ in any 24 hours: 150 minutes at 1.6 ATA, 180 minutes at 1.5 and 1.4 ATA, 210 minutes at 1.3 ATA, 240 minutes at 1.2 ATA, and 270 minutes at 1.1 ATA. From 1.0 ATA down, the daily limit equals the single-exposure limit. The percentage clock in this guide and planner, like most dive computers, uses only the single-exposure rows with a surface recovery decay, which can allow more total daily time than NOAA's 24-hour column. For example, three 75-minute exposures at 1.4 ATA separated by four-hour surface intervals never take the clock above 60%, yet they add up to 225 minutes against NOAA's 180-minute daily limit. On long or repeated exposures at high PPO₂, add up the time at each level and compare it with the 24-hour limits as well.
Worked example: two 30 m EAN32 segments99 ft EAN32 segments
Inputs: each constant-depth segment lasts 25 minutes at 30 m99 ft on 32% oxygen. The segments are separated by 90 minutes on the surface. The sea-level pressure model uses 10 m per additional atmosphere33 ft per additional atmosphere.
The raw PPO₂ is interpolated between the 1.2 and 1.3 ATA rows, giving a 186-minute allowance. The first segment contributes 13.44%. The 90-minute interval retains 6.72%, then the second segment brings the current total to 20.16%. Display values are rounded to two decimals only after the unrounded calculation.
Reaching 100% does not mean a convulsion occurs. It means you have spent the budget the tables allocate, and the underlying data on human CNS toxicity is limited and variable — susceptibility differs between divers and between days, and carbon dioxide retention, exertion, cold, and immersion all appear to matter. Prudent practice plans well below the limit rather than treating it as usable capacity.
What OTU tracks
OTU accumulates as a function of oxygen partial pressure and time, with exposure counting from roughly 0.5 ATA upward. Unlike CNS, the number grows steadily rather than steeply, so it is long dives and repeated days that build it rather than brief deep moments. Because it does not fall during an ordinary surface interval, it is the limit that tends to bind across a series of dives and days rather than within one dive.
Recreational single-day Nitrox diving rarely approaches pulmonary limits, and most recreational computers display CNS rather than OTU. OTU becomes a real constraint on extended exposures, on consecutive days of oxygen-rich diving, and in technical and decompression diving, where daily and multi-day allowances are planned explicitly.
Worked OTU example
For PPO₂ above 0.5 ATA, the NOAA/REPEX calculation is:
The model assigns zero OTU at or below 0.5 ATA. For a 30-minute exposure at 1.4 ATA, substitution gives:
At 1.0 ATA the factor is one, so 60 minutes contributes 60 OTU. The isolated-day reference is 850 OTU; multi-day limits can be lower.
Compare that with the CNS budget: the full 150-minute NOAA single-exposure allowance at 1.4 ATA, which is 100% CNS, adds only about 245 OTU. Within one continuous exposure the CNS budget runs out first. OTU becomes the binding limit when the exposures repeat, because CNS recovers between dives while the day’s OTU total keeps growing, and the daily allowance falls on consecutive days.
The REPEX procedures developed for NOAA habitat diving permit up to 850 OTU for one isolated exposure day, but the allowable daily average falls with repeated exposure toward about 300 OTU/day on long programs. These are reference limits, not default recreational targets; use the version specified by your training and operation.
Accumulating across a diving day
Neither measure resets on surfacing. CNS loading decays — a common planning approximation, introduced for an early dive-computer algorithm rather than measured physiologically, halves it roughly every 90 minutes on the surface — so a surface interval reduces accumulated exposure without clearing it. The planner's single-day OTU total stays unchanged during surface intervals and simply receives the next segment's OTU. It does not estimate pulmonary recovery or carry a REPEX schedule across multiple days.
For this planner, recovery is applied before the next segment:
CNS before is the unrounded accumulated percentage before the interval, Δt is the surface interval in minutes, CNS kept is what remains afterward, and CNS added is the next segment's contribution. The displayed remaining allowance is 100% minus CNS after, clamped at zero. Crossing 100% remains visible as an exceeded planning budget; it is not presented as a prediction that toxicity will occur at a particular percentage.
This runs in parallel with inert gas accumulation, on a completely separate schedule. A day of diving can be comfortable on nitrogen and tight on oxygen, or the reverse, and a rich mix improves one while worsening the other. That trade is the whole reason both get tracked; see surface intervals for the inert gas half.
Two oxygen clocks across one diving day
The CNS & OTU Exposure Planner shows PPO₂, CNS added, CNS retained after each interval, cumulative CNS, OTU added, and the running single-day OTU total without forcing those values into a wide table. Its selected 1.4 or 1.6 ATA PPO₂ check compares each segment with its maximum operating depth; it does not alter either exposure calculation at that segment's actual PPO₂. The planner models ordered, constant-depth segments. It does not model changing-depth profiles, workload, carbon dioxide retention, individual susceptibility, pulmonary recovery across days, or a dive computer's proprietary algorithm.
Planning against the limits
In practice: choose the leanest mix that meets the dive rather than the richest the depth permits, since the rich mix costs oxygen budget for decompression benefit you may not need. Set your computer to the analyzed mix so its tracking is real. Keep workload low, because carbon dioxide retention appears to raise CNS susceptibility. Add conservatism across consecutive diving days, and treat any oxygen-toxicity symptom as a reason to end the exposure rather than something to push through.
A commonly taught planning target is to keep a single dive below 80% CNS and the diving day below 100%, leaving margin for delays and individual variability. On long or repeated high-PPO₂ exposures, also check the total time at each PPO₂ against NOAA's 24-hour limits, and track the OTU total across consecutive days. Your agency, computer, or technical-diving procedure may specify a lower limit.
The mix decision is where most of the leverage sits — the best-mix guide covers why the richest theoretical mix is often not the right operational one. Use the MOD calculator to verify the measured mix's depth ceiling before tracking its exposure.
This is educational background for trained Nitrox divers. Published oxygen exposure limits vary by reference and by the type of diving, individual susceptibility varies, and the underlying human data is limited. Track exposure with a computer set to your analyzed mix and follow the limits and procedures taught by your training organization.
Sources
- NOAA Diving Manual: Diving for Science and Technology (6th ed.) · NOAA — Source for the 6th-edition normal single-exposure CNS rows used by the planner, the separate 24-hour total limits the planner does not enforce, the NOAA/REPEX OTU equation, and the isolated-day and multi-day pulmonary exposure references.
- Tolerating Oxygen Exposure · SPUMS Journal (South Pacific Underwater Medicine Society) — Reproduces the NOAA single-exposure and 24-hour oxygen limits developed with C. J. Lambertsen, gives the OTU equation and its 0.5 bar threshold, and the Repex daily limits from 850 OTU for one day to 300 OTU/day; it describes CNS clock interpolation and the dive-computer recovery decay as empirical, not physiological.
- Developing Oxygen Toxicity Guidelines · Divers Alert Network — Explains the evidence limits behind operational oxygen-exposure guidance.
- CNS function and dysfunction during exposure to hyperbaric oxygen in operational and clinical settings · Redox Biology — Reviews mechanisms, variability, and evidence limitations in CNS hyperbaric-oxygen toxicity.
Always use current training materials and operational procedures for dive planning.