Estimating Oxygen Exposure

As part of any technical diving class, the topic of oxygen exposure/tracking must be discussed. For the depths required for decompression diving, technical divers are exposing themselves to elevated partial pressures of oxygen (PPO2). Not only in using oxygen rich mixes to accelerate decompression, but bottom gas mixtures can also have a high PPO2. So tracking our exposure to oxygen is a must to reduce the risk of CNS oxygen toxicity. The goal of this article isn’t to teach you how to track your oxygen exposure. A thorough technical diving class should do that. Also, Steve Lewis has posted some great articles on his blog about managing oxygen exposure, Daily Limits for Oxygen Toxicity and About Oxygen Part One. Instead, this article will discuss how to estimate the CNS % (i.e. CNS Clock) when the software or tables are not readily available.

The Assumptions

First of all, this method of tracking the CNS clock requires the use of standard gases (i.e. 21/35, 18/45, etc.). The reason for this is that the depth range for these standard gases gives us an average PPO2 of 1.2. For technical diving, this level is desired to reduce the risk of oxygen toxicity during the working phases of the dive. We raise the PPO2 to 1.6 during rest at deco.

How it is Calculated

Now the risk of oxygen toxicity isn’t an exact science. To calculate the CSN %, you simply divide the dive time by the maximum allowable PPO2 exposure time. The result is expressed as a percentage. The theory is that the closer a diver comes to 100% or more exposure, the risk of oxygen toxicity increases significantly. To look up the maximum allowable PPO2 exposure time, you will need the NOAA tables. However, if you do not have those handy, there is a quick way to calculate the CNS clock.

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.

OK, but how do deco gases and 100% Oxygen work?

Again, we must refer to the standard deco gas of 50% Nitrox. If we use that as our deco gas, it gives us an average PPO2 of 1.2 (i.e. 70 feet to the surface). We simply add our bottom time with our deco time and divide that by 2. This gives us the total CNS % for the dive. If we do a dive to 150 feet on 21/35 with 50% Nitrox for our deco gas for 30 minutes with 30 minutes of deco, calculate the CNS % for the dive. We add our bottom time and our deco time together, giving us 60 total minutes. We can do this as the average PPO2 is 1.2 for both bottom gas and deco gas. We divide that by 2 for a total 30% CNS exposure. Running tables, V-Planner, or Deco Planner will give us 22% CNS. So again, we pad a little conservatism into our O2 exposure by using this estimation procedure.

When we deco on 100% O2, we are at an elevated PPO2 of 1.6. So how does decompressing with 100% O2 work? If we look at the NOAA tables, our single allowable exposure of a 1.6 PPO2 is 45 minutes. If we divide 100% by 45 minutes, this gives us 2.2 or 220%. Since the percentage is higher than 100%, we can multiply our deco time by 2 rather than divide by 2. Multiplying by 2 does put us on the aggressive side. For greater conservatism, we can multiply by 2.5. So if we have 15 minutes at 20 feet on 100% O2, this gives us 30% CNS. So if we were to do a dive to 150 feet on 21/35 with 100% deco gas only for 30 minutes of bottom time with 30 minutes of deco of which 15 minutes on 100% O2, calculate the CNS %. We have our bottom CNS % of 15 (30/2=15) added to our deco CNS % of 30 (15*2=30) we get 45 % Total CNS. Deco Planner and V-Planner give us a total CNS of 57%, so you can see the lack of precision in some scenarios. But if we multiplied our deco exposure by 2.5 instead of 2, we now get 67.5% which is now more conservative than Deco Planner or V-Planner.

Summary

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.

I welcome your questions and comments.

Dive Safe,
Duane Johnson
Precision Diving