CNS & OTU calculator — track your oxygen exposure
Oxygen keeps you alive; oxygen at pressure injures you. Technical diving manages that trade with two separate budgets — the CNS clock for acute central-nervous-system toxicity and OTUs for cumulative pulmonary toxicity. DecoLog tracks both across your whole plan, stop by stop and gas by gas.
Two different poisons
CNS toxicity is the acute one: at high ppO₂ the risk is a seizure with little or no warning, and a convulsion underwater usually means drowning. It's driven by how high your ppO₂ is and for how long. Pulmonary toxicity is the slow one: long hours of elevated oxygen inflame the lungs — burning, coughing, reduced vital capacity. It matters on long deco dives, chamber treatments, and multi-day trips. One dive can be fine on one budget and over the other, which is why planners track them separately.
The CNS clock — NOAA limits
The standard bookkeeping comes from the NOAA Diving Manual's oxygen exposure limits: each ppO₂ has a maximum single-exposure time, and your dive spends a percentage of it. Key single-exposure values: 45 min at 1.6 bar, 120 min at 1.5, 150 min at 1.4, 180 min at 1.3, 210 min at 1.2, and 300 min at 1.0. Ten minutes at 1.6 bar spends 10/45 ≈ 22% of the clock. Segments at different ppO₂ add up; DecoLog interpolates between table rows and — a common planning convention — decays the balance with a ~90-minute half-time on the surface, so repetitive dives start with the leftover.
Below about 0.5 bar ppO₂ no CNS loading is counted at all — which is why bottom mix at modest depth barely moves the clock, and the deco gases dominate it.
OTUs — the pulmonary budget
OTUs (oxygen toxicity units, also called UPTDs) use the Bardin–Lambertsen formula: one minute at 1.0 bar ppO₂ is one OTU, and other pressures scale as ((ppO₂ − 0.5) / 0.5)0.83 per minute. Unlike the CNS clock, OTUs are managed per day and across multi-day series — the REPEX method (R.W. Hamilton) assigns a daily allowance that shrinks as consecutive diving days stack up. A single aggressive deco dive rarely threatens the pulmonary budget; a week of them can.
Worked example — a trimix deco dive
Take a 45 m dive for 25 minutes on trimix 21/35, deco on EAN50 from 21 m and oxygen at 6 m:
- Bottom, 25 min at 45 m — ppO₂ = 0.21 × ~5.6 ata ≈ 1.2 bar → limit 210 min → ≈ 12% CNS.
- EAN50 stops, ~15 min between 21 m and 9 m — ppO₂ ≈ 1.55 at the switch, falling as you ascend; interpolating the table over the stops adds roughly 10–15% CNS.
- Oxygen at 6 m, 20 min — ppO₂ = 1.6 bar → limit 45 min → 20/45 ≈ 44% CNS.
Total ≈ 65–70% of the CNS clock — and notice the shape: the twenty minutes on oxygen cost more than everything else combined. That's the general lesson of CNS planning: the shallow, oxygen-rich end of the dive is where the clock actually runs. The same dive accrues on the order of 100–150 OTUs — comfortably inside a single day's allowance, but worth watching on a repetitive-day trip.
Why 1.4 working / 1.6 deco
Risk climbs steeply with ppO₂ — the jump from 150 minutes allowed at 1.4 bar to 45 at 1.6 is the table telling you so. Exertion makes it worse: working muscles produce CO₂, and retained CO₂ is a recognised amplifier of CNS oxygen-toxicity risk. So mainstream technical training draws the line twice: 1.4 bar while you're swimming and working, and 1.6 bar only at rest on decompression stops. This is the same logic behind the standard MOD calculation — the two limits define how deep each gas may go. Breathing-gas density piles onto the same CO₂ story, which is why it gets its own budget.
Track it across the whole plan
Doing this by hand for a multi-gas deco profile is tedious and error-prone. DecoLog's planner computes ppO₂ for every runtime segment, accumulates CNS% and OTUs across the dive, flags limit violations, and shows the totals alongside your decompression schedule. Free, offline, no signup.
⚠ DecoLog is a planning aid, not a substitute for a dive computer or proper training. Oxygen-exposure limits are planning conventions, not guarantees — stay well inside them and dive within your certification.