Calculator
CNS / OTU calculator — Oxygen toxicity
Two clocks track oxygen exposure. CNS is the central-nervous-system clock, plotted against single-exposure NOAA limits. OTU is the cumulative pulmonary-toxicity dose. The calculator takes one flat segment (gas, depth, duration) and reports both — useful for sanity-checking deco plans and estimating multi-day exposures.
How CNS and OTU are calculated
CNS uses a two-segment exponential fit to the NOAA single-exposure table — the standard curve-fit used by most planners. OTU follows the REPEX 5/6 power-law on (ppO₂ − 0.5), with the contribution rate going to zero below 0.5 bar.
CNS% = Σ (t_segment ÷ T_max(ppO₂)) · NOAA single-exposure curve OTU = Σ rate(ppO₂) · t · REPEX (5/6 power-law on ppO₂ − 0.5) ppO₂ oxygen partial pressure during the segment (bar) t segment duration (min) T_max NOAA single-exposure time limit at this ppO₂
Worked example: EAN32 at 30 m for 30 minutes. ppO₂ ≈ 1.28 bar. CNS contribution ≈ 17% — well inside the 80% single-exposure cap. OTU ≈ 30, also modest.
When to worry
80% of the single-exposure CNS limit is a common cap on a single dive; 100% is treated as a hard ceiling. OTU above 300 in a 24-hour period is considered the warning threshold for repetitive deep / decompression dives — sustained exposure above 600 OTU per day produces measurable lung-function decline. The calculator flags both thresholds.
Frequently asked questions
- What CNS does EAN32 produce at 1.4 ppO₂ for 30 minutes?
- Around 17% — about a fifth of the 100% single-exposure limit. The same gas at 40 minutes (1.4 ppO₂) is about 23%.
- How do CNS and OTU differ?
- CNS reflects acute risk — convulsions and loss of consciousness, which can happen in seconds. OTU reflects pulmonary irritation and accumulates over hours / days. Both must be tracked: CNS for the dive, OTU for the day or week.
- Does CNS reset between dives?
- Yes, with a roughly 90-minute half-time. Most planners apply this decay automatically. The single-exposure value here is for one dive in isolation — if you've been on CCR all morning, your starting CNS isn't zero.
- Why does the calculator flag 80% and 300?
- These are the conventional working thresholds: 80% of the NOAA single-exposure limit for CNS on one dive, and 300 OTU per 24-hour day for repetitive technical diving. Operational limits at agencies vary; treat the numbers as guides, not boundaries.