Oxygen exposure

What it's for

Tracks oxygen toxicity. The CNS clock uses the NOAA single-exposure limits, and OTU tracks pulmonary exposure. dailyExposure runs a whole day of dives and surface intervals.

The NOAA table

The single-exposure limits, from the NOAA Diving Manual:

ppO₂ (ata)	Limit (min)
0.6	720
0.7	570
0.8	450
0.9	360
1.0	300
1.1	240
1.2	210
1.3	180
1.4	150
1.5	120
1.6	45

Between rows the limit is linearly interpolated. Below 0.6 ata there is no limit (Infinity, and no CNS accrues).

Above 1.6 ata

NOAA publishes no limit above 1.6 ata. cnsLimitMinutes clamps to 45 minutes there, which understates the exposure. Do not plan above 1.6.

Formulas

$$CNS\mathrm{\%}=100\frac{t}{t_{lim}}$$$$OTU=t{\left(\frac{p{O}_2-0.5}{0.5}\right)}^{0.83}$$

Surface decay, with a 90 minute half-time:

$$CNS(t)=CN{S}_0\cdot {0.5}^{t/90}$$

• $t$: time at the segment's $p{O}_2$, minutes.
• $t_{lim}$: the NOAA limit for that $p{O}_2$, minutes (interpolated).
• $p{O}_2$: oxygen partial pressure, ata. No OTU accrue at or below 0.5 ata.
• $CN{S}_0$: CNS percent at the start of a surface interval.

Assumptions and limits

• CNS percent is summed across segments, and decays only during surface intervals.
• Each dive segment uses one constant $p{O}_2$.
• dailyExposure reports the peak and final CNS percent, total OTU and a per-dive breakdown.
• This is a planning aid, not a guarantee of safety.

Sources

• CNS limits: NOAA Diving Manual, oxygen partial pressure and exposure time limits.
• OTU exponent 0.83 and 0.5 ata threshold, and the 90 minute half-time, are the constants used in the source.

Examples

import { cnsLimitMinutes, segmentOtu, dailyExposure } from 'dive-math/oxygen'
cnsLimitMinutes(1.4) // => 150
segmentOtu({ ppo2: 1.4, minutes: 30 }) // => 48.86
dailyExposure([{ type: 'dive', ppo2: 1.4, minutes: 30 }, { type: 'surface', minutes: 90 }]).endCnsPercent // => 10

API

See the oxygen API reference.