Oxygen Toxicity Calculator (CNS / OTU)

Enter your gas, depth and time to see how much of your safe oxygen dose a dive uses.

Results update as you type.

Quick examples

Load a full example, then change any value.

Oxygen in your gas (% O₂)

From your tank's analysis label. Air is 21%.

Quick picks

Depth (m)

The depth of this part of the dive.

Time at this depth (min)

Minutes spent at that depth.

CNS clock

Within limits

This dive uses 25% of the oxygen limit for a ppO₂ of 1.28 bar (limit: 180 minutes).

ppO₂: 1.28 bar
OTU: 65.1

NOAA single-exposure limit: 180 min

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These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification. High ppO₂ can cause central nervous system oxygen toxicity and underwater convulsions; do not approach these limits without nitrox and technical diving training.

About the CNS clock and OTU

Breathing oxygen at a high partial pressure stresses the central nervous system (CNS). The CNS clock shows how much of the NOAA time limit for your ppO₂ a dive uses. At 100% you have reached the limit and the risk of convulsions rises.

OTU (oxygen tolerance units) measure the slower effect of oxygen on the lungs. They matter on long dives and on trips with many dives in a row.

With your numbers

Enter valid values to see the calculation worked through with your numbers.

Formula

Oxygen partial pressure

ppO2 = FO2 × ((d) ÷ (10) + 1)

CNS clock

CNS = (t) ÷ (tlimit) × 100%

Oxygen tolerance units

OTU = t × ((ppO2 − 0.5) ÷ (0.5))^0.83

Where

FO2: oxygen fraction of the gas (32% = 0.32)
d: depth (m)
t: time at this depth (min)
tlimit: NOAA single-exposure limit, with ppO2 rounded up to the next 0.1 bar row
NOAA rows: 1.2 bar 210 min, 1.3 bar 180 min, 1.4 bar 150 min, 1.5 bar 120 min, 1.6 bar 45 min. Lower rows allow more time, such as 300 min at 1.0 bar.
At 0.5 bar or less, neither the CNS clock nor OTU count.

How it's calculated

ppO₂ (bar) = FO₂ × (depth ÷ 10 + 1).
CNS % = time ÷ NOAA limit for that ppO₂ × 100, with ppO₂ rounded up to the next table bracket (0.6 to 1.6 bar).
OTU = time × ((ppO₂ − 0.5) ÷ 0.5)^0.83, counted only above 0.5 bar.

Good to know

Work out each part of a dive on its own, such as the bottom time and each deco stop, then add the CNS and OTU numbers together.
The CNS clock drops by about half for every 90 minutes at the surface. Adding repeat dives without that credit is the conservative way.
Many divers keep the CNS clock under 80% for a dive. The result turns to caution from 80%.
OTU matter on long trips. The REPEX tables allow 850 OTU on a single day and 300 per day on trips of 11 days or more.

How to use the CNS / OTU calculator

1
Enter the oxygen percentage of your gas and the depth of the dive segment.
2
Enter how many minutes you'll spend at that depth.
3
Read the CNS clock and OTU. Oxygen exposure adds up, so if you dive more than once a day, add the numbers together.

What the results mean

The CNS clock shows how much of the NOAA single-exposure time limit for your ppO₂ the dive uses. 100% means you have reached the limit.

OTU (oxygen tolerance units) track the slower, whole-body (pulmonary) effect of breathing oxygen over long or repeated dives.

Above a ppO₂ of 1.6 bar there is no planning limit at all. The risk of convulsions from oxygen toxicity is too high.

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