How to Calculate Partial Pressure of Oxygen

When divers go through Nitrox classes, one of the things they should learn is that the human body doesn’t respond to the percentage of oxygen in the gas mixture, but rather our bodies are affected by the pressure of oxygen. For example, if we travel from sea level to an elevation of 14,000 feet, we will start to breath harder and feel like we aren’t getting enough air. The percentage of oxygen at sea level is 21% and the percentage of oxygen at 14,000 feet is also 21%. So the difference is that there is less pressure at a higher elevation. This is what our bodies are responding to. Our bodies have evolved to breath oxygen at sea level.

Since our bodies are affected by the pressure exerted on the oxygen we breath, it can have a dangerous affect on scuba divers breathing high oxygen mixtures. Breathing high pressures of oxygen can be toxic to our central nervous system, thus resulting in oxygen toxicity. Therefore it is extremely important that Nitrox divers know how to calculate the partial pressure of oxygen in order to stay within safe limits for recreational diving.

In order to understand how to calculate the partial pressure of oxygen, we must have an understanding of Dalton’s Law. Dalton’s Law states that the total pressure of the gas is equal to the sum of pressures of each gas in the mixture. This number is represented in decimal form with 1.0 equal to 100%. At the surface (i.e. 1 ATA of pressure), the gas mixture we breath is made up of 78% nitrogen, 21% oxygen, and 1% trace gases. For the sake of our calculations, we can simply assume that those trace elements are not much of a concern and use 79% nitrogen and 21% oxygen. So .79 + .21 = 1.0.

To calculate the partial pressure of oxygen we must first know two things; 1) percentage of oxygen in the breathing gas and 2) the depth we are diving. The depth must be in atmospheres absolute (i.e. ATA). To convert depth in feet to ATA, we simply divide the depth by 33 for salt water or 34 for fresh water and add 1 ATA (to account for gravity at the surface). So a depth of 99 feet would be 99/33=3 and add 1 gives us 4 ATA.

Now that we have the depth in ATA, we can take advantage of Dalton’s law and simply multiply the percentage of oxygen in the breathing mix by the depth in ATA. If we had a Nitrox mixture of 32% and a dive to 100 fsw it would look like this: .32 * 4 = 1.28 partial pressure of oxygen (ppo2). This number doesn’t really mean much unless you know what the partial pressure of oxygen limits are. The generally accepted ppo2 maximum exposure limit for recreational scuba diving is 1.4, with 1.6 being an emergency limit. By staying within these limits, scuba divers can significantly reduce their risk of oxygen toxicity. Oxygen toxicity can cause convulsions, which can result in a diver losing the regulator out of the mouth and drowning.

In summary, it is very important to know what the percentage of oxygen in your breathing mixture and it’s partial pressure in order to stay within the 1.4 maximum limit for recreational diving. Of course, this article should not replace a sound Nitrox course. Do not dive Nitrox mixtures if you are not properly trained.

Dive Safe,
Duane Johnson
Precision Diving

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