# Estimating Oxygen Exposure

*Today marks the first post in a new section on Thailand Tech Diving called “ TECHnique,” devoted to tips, tricks, education and “how to” information for the technical diver. While not Thailand specific, we think all of the knowledge can be of use to tech divers in- and outside The Kingdom.*

Any (good) technical-diving classes will teach students to track their oxygen exposure. Both deco and bottom mixes can subject divers to high partial pressures of oxygen, so measuing PPO^{2} is essential to avoid CNS oxygen toxicity.

In a fine post on its blog, **Precision Diving**, of Michigan in the U.S., discusses how to estimate the CNS % (i.e. CNS Clock) when software or tables are not readily available.

The article assumes standard gases are being used so the average PPO^{2} is 1.2. The CNS percentage is calcuated by diving the dive time by the maximum allowable PPO2 exposure time. The result is expressed as a percentage. The closer to 100 percent the result is, the greater the risk of toxicity. To figure that out, however, you need tables. So what happens if you don’t have them?

According to the NOAA oxygen exposure tables, the maximum single dose limit for a PPO2 of 1.2 is 210 minutes. If we divide 100% by 210 minutes, this gives us the percent exposure for every minute. We use 100% as the maximum as we want to reduce the risk of oxygen toxicity. So for the PPO2 1.2, we get 100%/210 = .48 or just slightly less than 50%. So what does this mean? Well, we can simply divide our bottom time exposure by 2 (i.e. half or 50%) to get our CNS %. Let’s look at an example. We do a dive for 145 minutes at a PPO2 of 1.2, the formula looks like 145/2 = 72.5%. NOAA tables give us a 69%. So we are more conservative (i.e. lowering the risk of oxygen toxicity than NOAA tables) in this calculation.

The article continues with more discussion on how the above works with deco gases and 100 percent oxygen.

So, while calculating the CNS % in this fashion isn’t exact. It will still give the diver a pretty close estimate. This is particularly useful when one does not have access to decompression software or NOAA tables to perform the calculations. This method should also not replace accurate oxygen management and tracking protocols taught in a thorough technical diving class. Instead only act as an estimate when the specific tools are not available.

See the full article here and, for good reference, check out Daily Limits for Oxygen Toxicity and About Oxygen Part One.