Scuba CNS & OTU Oxygen Exposure Calculator
Enter up to four constant-depth gas segments to estimate PPO2, CNS oxygen clock loading, OTUs, REPEX allowance, and remaining CNS time at the last segment PPO2.
Oxygen exposure statusWithin entered CNS and OTU limits
- Peak CNS exposure
- 16.13%
- Ending CNS exposure
- 16.13%
- Total OTUs
- 43.46
- REPEX daily allowance
- 850
- Remaining REPEX OTUs
- 806.54
- Remaining CNS time at last PPO2
- 156
Segment exposure table
| Segment | Ambient | PPO2 | NOAA limit | CNS added | Running CNS | OTUs |
|---|---|---|---|---|---|---|
| 1 | 4 ata | 1.28 ata | 186 min | 16.13% | 16.13% | 43.46 |
Assumes constant depth, constant oxygen fraction, and one atmosphere of surface pressure for each segment.
How to use this calculator
- Choose meters or feet and salt or fresh water for the whole profile.
- Enter the current consecutive exposure day and any starting CNS percentage from earlier exposure.
- Add each active depth, gas oxygen percentage, time, and any surface interval before the next segment.
- Read the segment table first, then check the CNS, OTU, REPEX, and warning rows.
How the oxygen exposure is calculated
This calculator treats each row as a constant-depth, constant-gas oxygen exposure segment. Ambient pressure is calculated as P = 1 + depth / k, where k is 10.0 meters or 33.0 feet in salt water and 10.3 meters or 33.9 feet in fresh water. PPO2 is then oxygen fraction x P.
For CNS exposure, the calculator uses the NOAA single-exposure oxygen table from 0.5 to 1.6 ata PPO2. Between table points it uses straight-line interpolation. A segment CNS percentage is 100 x minutes / NOAA limit. PPO2 below 0.5 ata contributes zero CNS in this model; PPO2 above 1.6 ata is reported as outside the supported NOAA range rather than extrapolated.
Surface intervals reduce the modeled CNS clock with a 90-minute half-time: recovered CNS = CNS x 2^(-surface minutes / 90). An entered interval of 1,440 minutes resets residual CNS to zero for this model. OTUs are different: for PPO2 above 0.5 ata, OTU = minutes x ((PPO2 - 0.5) / 0.5)^(5/6). OTUs do not decay during the selected exposure day.
What moves the result most
Depth and oxygen percentage drive PPO2, so small changes in either can move the CNS and OTU totals quickly. Time matters directly: doubling minutes at the same PPO2 doubles the segment CNS contribution and OTUs. Surface time affects CNS but not OTUs, which is why a repetitive profile can show a lower ending CNS while still using the same daily OTU allowance.
How to read the warnings
The status line flags PPO2 above 1.4 ata, PPO2 above the 1.6 ata calculation range, CNS at or above 100%, OTUs above the selected REPEX daily guideline, and hypoxic gas entered at or near the surface. The REPEX allowance is 850 OTUs on day 1, then 700, 620, 525, 460, 420, 380, 350, 330, and 310 for days 2 through 10. Days 11 through 30 use 300 OTUs per day.
What this calculator leaves out
This is not a decompression model, a gas-switch validator, or a replacement for a dive computer. It assumes sea-level surface pressure, constant depth within each segment, and open-circuit gas with a known oxygen fraction. It does not account for workload, carbon dioxide retention, immersion effects, thermal stress, medications, illness, individual susceptibility, altitude diving, decompression obligations, bailout gas, or team procedures.
Worked example
A default segment of 30 meters in salt water on 32% nitrox for 30 minutes gives an ambient pressure of 4.00 ata and PPO2 of 1.28 ata. Interpolating the NOAA table between 1.2 ata at 210 minutes and 1.3 ata at 180 minutes gives a limit of 186 minutes.
The CNS contribution is 100 x 30 / 186, or 16.13%. The OTU calculation is 30 x ((1.28 - 0.5) / 0.5)^(5/6), or 43.46 OTUs. On exposure day 1 the REPEX allowance is 850 OTUs, so 806.54 OTUs remain. The remaining CNS time at 1.28 ata is 156 minutes.
Common questions
What are CNS percentage and OTUs?
CNS percentage is a planning clock for central nervous system oxygen exposure, based on PPO2 and time. OTUs estimate pulmonary oxygen exposure using a different dose formula. They measure different risks, so a dive can look acceptable on one scale and limiting on the other.
How is PPO2 calculated from depth and nitrox percentage?
The calculator first converts depth into ambient pressure, then multiplies by the oxygen fraction of the gas. In salt water, 30 meters is about 4 ata, so 32% oxygen gives 0.32 x 4, or 1.28 ata PPO2.
How does a surface interval reduce the CNS oxygen clock?
Entered surface intervals apply a 90-minute CNS half-time. A 90-minute interval halves the modeled residual CNS, a 180-minute interval quarters it, and a 24-hour interval resets it to zero in this calculator. OTUs remain accumulated for the selected exposure day.
What do the 1.4 and 1.6 ata PPO2 warnings mean?
Many dive plans use 1.4 ata as a working PPO2 limit, so the calculator flags any segment above it. The NOAA CNS table used here ends at 1.6 ata; above that, the calculator does not extrapolate a CNS time limit and marks the profile outside the supported range.
How should gas switches and multilevel dives be entered?
Split the dive wherever depth, gas, or surface exposure changes. A gas switch at the same depth is a new segment with a zero surface interval. A repetitive dive after surfacing is another segment with the entered surface interval before it.
Can this be used for trimix or rebreather profiles?
It can estimate oxygen exposure for any segment where you know the inspired oxygen fraction or setpoint equivalent, but it does not model inert gas decompression or rebreather control behavior. For closed-circuit diving, compare the result with your controller, backup computer, and team procedures.