Oxygen exposure is tracked as two independent numbers because oxygen hurts you in two unrelated ways. CNS % tracks the risk of a central-nervous-system oxygen toxicity event, a seizure underwater. OTU tracks cumulative pulmonary irritation, which builds over hours and days and does not seize anybody.
The two clocks
Both are driven by the partial pressure of oxygen you are breathing at every moment of the dive, not the label on the cylinder. On a rebreather that is the setpoint the loop is holding.
The CNS clock is a percentage of a single-exposure limit, accumulated minute by minute against the NOAA table, so 100 % means you have used the whole of the exposure that table allows at the pressures you have been breathing. Below 0.50 bar the clock does not run at all. Between 1.50 and 1.60 bar the allowance falls by more than half, which is why 1.60 bar is treated as a decompression-only limit rather than a working one.
Oxygen tolerance units accumulate for any ppO₂ above 0.50 bar, at roughly one unit per minute at 1.0 bar and about twice that at 1.6 bar. OTU is less sensitive to pressure than the CNS clock, because pulmonary irritation is about dose over time rather than peak exposure, so a long shallow nitrox dive can carry more OTU than a short deep one and less CNS.
Where the planner warns
The plan carries warnings at four thresholds, and they do not change with the algorithm; every model reports the same figures against the same limits.
| Condition | Severity | Message |
|---|---|---|
| CNS above 80 % | Warning | CNS 84% exceeds 80% advisory |
| CNS above 100 % | Error | CNS 112% exceeds 100% limit |
| OTU above 200 | Warning | OTU 240 exceeds 200 advisory |
| OTU above 300 | Error | OTU 340 exceeds 300 single-dive limit |
The 300 OTU figure is the recommended single-dive limit, and the 200 advisory sits below it so that a plan flags before it reaches the limit.
In the instrument strip below, O₂ CNS carries a bar as well as a number. The fill is the fraction of the 100 % limit used, and it turns amber at 80 % and red at 100 %, matching the two warning thresholds. OTU is a plain count.
Both figures are for the whole plan, surface to surface, decompression included. On this dive the deco gas is 50 % taken at 21 m, the shallowest depth at which that mix reaches 1.6 bar; take it deeper and the ppO₂ passes 1.6 bar, which the planner raises as an error rather than an advisory. See Switch depths.
What the two numbers do not measure
Neither clock decays. Within a plan they only accumulate: there is no surface-interval recovery, no CNS half-time, and no credit for time spent breathing air. Both are single-dive totals, and a repetitive dive is not modelled by adding them to a previous plan’s.
Neither is a personal threshold. The NOAA limits and the 300 OTU figure are population recommendations with a large individual spread, and CNS toxicity is influenced by CO₂ retention, exertion, cold and immersion, none of which enter the calculation. Divers have seized well below 100 % and dived well past it without incident: a clock at 60 % is a fraction of a tabulated allowance, not a probability of anything.
Next
- Gas density and narcosis covers the other numbers a mix is judged on.
- Plan warnings shows how these four messages appear.