CNS % of a dive
Your oxygen dose for the nervous system, worked out segment by segment on the NOAA table, just like a dive computer.
Before the dive
What is left from a previous dive. 0 if it is the first of the day.
On the bottom
Your mix, your max depth and your bottom time (from descent to the start of the ascent). Multi-level dive: add the next levels.
Bottom mix
Stops
Length of each stop. With a deco gas, tap a stop’s gas to switch it.
Deco gas
Total CNS
23%
Comfortable margin
Dive only: 22.2 % · exact value 22.2 %
What will be left at the surface
Breakdown
What each part of the dive adds.
| Segment | ppO2 | Time | Limit | CNS |
|---|---|---|---|---|
| BottomNx32 · limit 186 min | 1.28 | 40 min | 186 min | 21.5 |
| Ascent 30 → 3 mNx32 · 10 m/min | ≤ 1.28 | 2.7 min | – | 0.7 |
| Stop 3 mNx32 · limit ∞ | 0.42 | 3 min | ∞ | 0.0 |
| Ascent 3 → 0 mNx32 · 10 m/min | ≤ 0.42 | 0.3 min | – | 0.0 |
| Total | 22.2 → 23 % |
NOAA single exposure limits (rows used are highlighted)
| ppO2 (bar) | Limit (min) | % CNS / min |
|---|---|---|
| 0.6 | 720 | 0.14 |
| 0.7 | 570 | 0.18 |
| 0.8 | 450 | 0.22 |
| 0.9 | 360 | 0.28 |
| 1.0 | 300 | 0.33 |
| 1.1 | 240 | 0.42 |
| 1.2 | 210 | 0.48 |
| 1.3 | 180 | 0.56 |
| 1.4 | 150 | 0.67 |
| 1.5 | 120 | 0.83 |
| 1.6 | 45 | 2.22 |
Settings
ascent 10 m/min · precise
Reading the NOAA table
Surface half-life
Sea water: 10 m = 1 bar. The whole bottom time is counted at max depth (descent included), which errs on the safe side.
How it is calculated
CNS (central nervous system) measures how much of your oxygen tolerance you have used. At 100 %, you have reached the maximum exposure time NOAA sets for a given ppO2.
ppO2 = FO2 × (depth ÷ 10 + 1)
Segment CNS = time × 100 ÷ NOAA limit(ppO2)
Total CNS = residual + Σ segments, rounded up to the next %
Between two rows: in “Precise” mode, the limit is linearly interpolated. In “Row above” mode, the row of the next higher ppO2 is used, which is stricter.
Below 0.5 bar: no CNS load. Between 0.5 and 0.6 bar, the 0.6 bar limit applies.
Beyond 1.6 bar: the table stops and the exposure must be avoided. Like Shearwater computers, the tool raises the rate from 2.2 to 15 % per minute between 1.60 and 1.65 bar, then keeps it at 15 % per minute. The figure shows the order of magnitude; it does not make the dive acceptable.
Guidelines: stay under 80 % per dive and never go over 100 %. The next dive must also finish under 80 %, residual included. At the surface, CNS halves every 90 minutes (120 min as a cautious option); it is practically back to zero after 6 half-lives.
Several levels: each level is a separate segment, on the same mix as the bottom. Stops deeper than the last level are ignored.
Ascent: if the option is on, ppO2 is integrated continuously between depths, with the gas breathed at that moment (the gas switch happens on arrival at the stop).
Sources: NOAA Diving Manual (oxygen exposure limits); Shearwater, “The CNS oxygen clock”. A learning tool: it does not replace your computer, your training or your planning.
Understanding CNS
CNS %, the oxygen clock
The CNS % measures how much of your oxygen tolerance for the central nervous system you have already used. Each minute at a given ppO2 uses a fraction of the maximum time set by NOAA: 150 minutes at 1.4 bar, 45 minutes at 1.6 bar. The tool adds up every segment of your dive (bottom, levels, stops, ascent), just like your computer. For your ppO2 at each depth, use the PpO2 tool.
80 % per dive, never over 100 %
The guideline used by computers and training courses is to finish each dive under 80 % and never go over 100 %. The tool also gives you the maximum bottom time to stay at 80 % with the same stops, and what a next dive could cost after 3 h at the surface. To pick a mix that stays within your ppO2 limit, see the MOD tool.
Repetitive dives
The CNS from a previous dive halves every 90 minutes at the surface. Enter the residual read on your computer, or let the tool work it out from the surfacing CNS and the interval. To know how long to wait, use the CNS interval tool; to see what is left after your break, the CNS reduction tool. Lung toxicity is tracked in OTU with the OTU calculator.
Frequently asked questions
How do you work out the CNS % of a dive?
For each segment: time × 100 ÷ NOAA limit at its ppO2. Add up the segments and the residual, then round up to the next %. Example: 30 min at 1.0 bar (300 min limit) + 15 min at 1.6 bar (45 min limit) = 10 % + 33.3 % ≈ 44 %.
What CNS for EAN32 at 30 m for 40 min?
About 23 % (22.2 % exact) with a 3 min stop at 3 m and the ascent counted at 10 m/min: the bottom ppO2 is 1.28 bar, the NOAA limit about 186 minutes.
What CNS limit should you not exceed?
100 % is the NOAA maximum time. Computers and training courses advise finishing each dive under 80 %, residual included.
What happens beyond 1.6 bar?
The NOAA table stops. Like Shearwater, the tool counts up to 15 % per minute beyond 1.65 bar to show the order of magnitude: such an exposure must be avoided.