When diving long and deep it is important to track your CNS and OTU exposure. Here is a brief overview of what they are and how to calculate them.

### What is CNS Oxygen Exposure?

Although it is possible to reach CNS Oxygen Toxicity by going below the maximum allowable depth of your breathing gas, you should also track your CNS Oxygen Exposure from your dive. It is calculated using Oxygen Exposure Tables. Taking the total time of the dive and dividing it by the allowed exposure at that partial pressure then multiplying your answer by 100 will give you your CNS Oxygen Exposure as a percentage. There is a 50% reduction in your %CNS for every 90 minutes of surface interval that you have.

### What is the Oxygen Tolerance Unit (OTU)?

One OTU or one UPTD (Unit Pulmonary Toxic Dose), the names can be used interchangeably, is the equivalent to breathing 100% oxygen for one minute at 1 ATA. As a safety margin we should not exceed a total of 300 units in a 24 hour period. Exceeding this limit can cause lung damage and reduce your vital capacity.

### Example

You are diving 32% Nitrox at a depth of 30m (99ft). What would your %CNS and OTU exposure at that depth for 25 minutes. To do this we need to know what the partial pressure of 32% Nitrox is at a depth of 30m.

First of all we have to calculate the ATA pressure at 30m. We can do this by using the appropriate formula below.

This gives the ATA pressure of 4.

Now we need to work out the partial pressure of the gas at that depth. This is really easy we use the following formula.

Notice that this formula is the same regardless if you use metric or imperial. So using it with our 32% Nitrox gives the following 0.32 x 4 = PO2 1.28

If we look at out above table we see that 1.28 doesn’t appear so we use the next highest which is PO2 1.3. Also there isn’t 25 minutes on the table so we need to make up 25 minutes by using the value for 20 minutes and 5 minutes and then adding them together.

Time | %CNS | OTU |

20 minutes | 11.11 | 29.54 |

5 minutes | 2.78 | 7.39 |

25 minutes |
13.89 |
36.93 |

So there you have it. We have calculated both our %CNS and OTU for a 25 minute dive to 30m using Nitrox 32%.

*First and foremost let me state that you must receive formal training from one of the many dive organisations before attempting a dive. Without formal training you risk injuring yourself. Please get training before attempting anything you have read or seen in this post. I will not be held responsible for you attempting to use the methods contained in this post. The following post is for illustration purposes only.*

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