Question 1

What is CNS oxygen toxicity?

Accepted Answer

Central Nervous System (CNS) oxygen toxicity is a potentially dangerous condition caused by breathing high partial pressures of oxygen (PPO₂). It can lead to convulsions, loss of consciousness, and drowning. Recreational divers manage CNS risk by limiting PPO₂ to 1.4-1.6 bar and tracking cumulative exposure.

Question 2

How does the CNS clock work?

Accepted Answer

The CNS clock tracks cumulative oxygen exposure using NOAA limits. Each PPO₂ level has a maximum exposure time (e.g., 150 minutes at 1.4 bar). Time at a given depth contributes proportionally to the allowed limit. The total CNS% represents the fraction of maximum allowable exposure. After 100%, oxygen toxicity risk increases significantly.

Question 3

What is MOD and how does it relate to CNS?

Accepted Answer

Maximum Operating Depth (MOD) is the deepest depth you can dive with a given nitrox mixture without exceeding a safe PPO₂. For the standard 1.4 bar limit, EAN32 has an MOD of 34m, EAN36 at 29m. Diving deeper than MOD dramatically increases CNS risk and should be avoided for recreational diving.

Question 4

How to calculate CNS for nitrox dives?

Accepted Answer

To calculate CNS, you need PPO₂ (at depth), oxygen percentage, and time. PPO₂ = (depth/10 + 1) × O₂%. Then use NOAA exposure tables to find maximum allowed time at that PPO₂. Divide your actual dive time by the max time to get CNS%. This tool calculates it automatically.

Question 5

What happens during surface intervals?

Accepted Answer

CNS oxygen toxicity decreases with a 90-minute half-life during surface intervals. After 90 minutes on the surface, your CNS exposure is reduced by 50%. After 180 minutes, it's reduced by 75%. This allows divers to conduct multiple dives while managing cumulative oxygen exposure.

Question 6

Why 1.4 bar PPO₂ is recommended for recreational diving?

Accepted Answer

The NOAA-recommended limit of 1.4 bar PPO₂ provides a conservative safety margin for most divers. This allows 150 minutes of exposure before reaching 100% CNS. Many technical divers use 1.2-1.3 bar for longer exposures. The 1.6 bar limit (45 min) is considered aggressive and reserved for short, planned exposures.

Question 7

Can I calculate CNS for a single dive?

Accepted Answer

Yes! Click 'Add Dive' to create a dive entry, then adjust depth, oxygen percentage (O₂%), and duration. Results update instantly. For multiple dives, add more entries and specify surface intervals between them to see cumulative CNS with half-life reduction.

CNS Oxygen Clock Calculator

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NOAA Oxygen Exposure Limits

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