CNS interval: when to dive again
Your computer shows a CNS % when you surface? See how long to wait before diving again, or what you will have left after your break.
End of dive
The CNS % read on your computer (or worked out) when you surfaced.
Target before the next dive
The CNS % you want to start with. Start + next dive must stay under 80 %.
Decay speed
Sources and computers do not all agree. 90 min is the most common rule.
Wait at least
1h 30
to go from 60 % to 30 % · the next dive can add up to 50 %
How your CNS comes down
It halves every 1 h 30 spent at the surface.
After 6 half-lives (9 h), only about 2 % of the surfacing value is left: the load is then considered cleared.
How it is calculated
The CNS % measures the oxygen load on the central nervous system: the time spent at each ppO2, divided by the NOAA exposure limit for that ppO2. It never goes down during the dive. At the surface, on air, it is made to decay exponentially with a half-life T:
CNS left = end CNS × 2^(−t / T)
Interval t = T × log₂(end CNS ÷ target CNS)
- T = 90 min by default: the convention of nitrox and trimix courses, Erik Baker, Shearwater and Subsurface. NOAA tables give none: they only set per-dive and 24-hour limits.
- T = 120 min as an option, a value some authors quote as more cautious.
- Safe-side rounding: interval up to the next minute, CNS left up to the next %.
- Examples (T = 90): 80 % → 40 % = 90 min; 100 % → 40 % = 90 × log₂(2.5) = 119 min.
Guidelines: stay under 80 % per dive and never go over 100 %. The next dive must also finish under 80 %, residual included. If you fill in “The next dive will add”, the tool checks that start + next dive stays under 80 %.
A planning aid. Your computer and your training remain the reference. Some computers do not lower CNS at the surface and reset it after 24 h: always follow what yours shows.
Understanding the CNS interval
How long to wait before diving again?
Your CNS % halves every 90 minutes spent at the surface, on air. Going from 80 % to 40 % therefore takes 1 h 30; to 20 %, 3 h. The tool reverses the formula and gives you the minimum interval, rounded up to the minute, and the time you can dive again if you enter when you surfaced.
What CNS should you start with?
The usual guideline is to finish each dive under 80 %, residual included. If you know what the next dive will cost (plan or computer), enter it: the tool checks start + next dive and suggests the starting target that keeps you under 80 %. To work out a dive’s CNS from its profile, use the CNS % calculator. For the reverse, from an interval length, use the CNS reduction tool.
And lung toxicity?
CNS is about the nervous system. Over several dives a day or several days in a row, oxygen also loads the lungs: that is the OTU dose, tracked with the OTU calculator. In recreational diving, CNS is usually the first limit.
Frequently asked questions
How long to go from 80 % to 40 % CNS?
1 h 30 with the standard 90 minute half-life, 2 h with the cautious 120 minute half-life.
How do you work out the surface interval for a target CNS?
Interval = 90 × log₂(end CNS ÷ target CNS), in minutes, rounded up to the minute. Example: from 100 % to 40 %, 90 × log₂(2.5) = 119 min.
Should I follow the tool or my computer?
Your computer. Some do not lower CNS at the surface and reset it after 24 h. The tool is for understanding and planning.
Why target 30 % at the start?
With 30 % at the start, the next dive can cost up to 50 % and still finish under 80 %. You can pick 20 % or 10 % for more margin.