The Nitrox gets this name because it's a mix of nitrogen and oxygen. The Nitrox is simply air that has been enriched with oxygen. We usually add a number behind which indicate the percentage of oxygen. The air is a mix of 21% oxygen and 79% nitrogen. The air could also be named: Nitrox 21. The most commonly used Nitrox is the Nitrox 32... Which is a mix of 32% of oxygen and 68% of nitrogen.
In the lesson for 20 meters, we said that our body absorbs pressurized air during the dive. That's correct but it's time to distinguish separately the nitrogen and the oxygen. The oxygen is consumed by your cells. But, your cells don't consume the nitrogen. Therefore, nitrogen becomes the relevant factor in the calculation of the saturation of a scuba diver. So, if you breathe less nitrogen, you saturate less nitrogen. So, your diving computer will allow you to stay longer, underwater. If you don't dive longer time, your body saturates less nitrogen. Therefore, you reduce the risk of decompression sickness.
A smaller percentage of nitrogen means a higher percentage of oxygen. Is this good for the body? It all depends on the partial pressure of oxygen that you breath.
Simply multiply the pressure by the percentage of oxygen. Let's take 2 examples of calculation with the Nitrox 32. At 30 meters depth (4 bars), the partial pressure of oxygen (PpO2) is (4 x 32%): 1,28 bar. At 40 meters (5 bars), PpO2 = 5 x 32% = 1,6 bar.
You expose yourself to the oxygen toxicity.
Yes indeed, the oxygen allows us to live exactly like electricity allows to a bulb to shine. If you give too much electricity, the bulb burns out. This is similar with oxygen in our body. The first signs and symptoms are generally some convulsions: The muscles contract by themselves. If ever you have convulsions, go immediately to shallower waters. The next step of oxygen toxicity is the "burn of the main fuse": Your brain. You may simply lose consciousness. The oxygen toxicity can strike strongly and suddenly. This is why we take a large safety margin and we agree to limit the use of oxygen for recreational diving to a maximum pressure of 1,4 bar.
Multiply 1,4 by 100 and divide by the nitrox number. Here is an example with Nitrox 32: You multiply 1,4 by 100 and divide by 32 (1,4x100/32). The result is 4,375 bars. (4,375 bars minus the 1 bar of the atmospheric pressure) multiplied by 10 m/bar gives you a depth of 33,75 meters. To simplify the calculation, you may use 140 instead of 1,4 and divide 140 by the nitrox number.
Let's suppose that we want to dive till 40 meters depth. There, the pressure is 5 bars. We divide 1,4 by 5 and we obtain: 0,28. This means 28%: Nitrox 28. To simplify the calculation, you may use 140 instead of 1,4 and divide 140 by the pressure of the desired depth.
We use a small instrument: An oxygen analyser. Your instructor will show you how to use it.
This will happen most of the time. For example, if you read 32,45% of oxygen, you may use 32,45% or the next rounded value 33% in order to simplify the calculation and obtain an extra safety margin to determine the maximum depth to avoid oxygen toxicity. Now, on the other side, you should enter Nitrox 32 into your diving computer which calculate your nitrogen saturation. Yes indeed, it is safer to inform your diving computer that you use 68% of nitrogen rather than 67% of nitrogen. Please, read your diving computer manual.
You now understand that it would be unsafe to breathe Nitrox without knowing its limit. We invite you to test your knowledge and do a medical check. Once you'll finish the course, your instructor will give to you your IFDI Nitrox Diver certification. This certification allows you to dive with Nitrox guided or not by an instructor.
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Edited in 2015. Updated in 2023.