## What's partial pressure? Noted `P`, the partial pressure is the notional pressure of each constituent gas in a mixture of gas, as if it occupied alone the entire volume at the same temperature. I.e, on the regular gas we're breathing, we have : - 78% Dinitrogen (n2) - 21% Dioxygen (O2) - 1% of rare gases, like argon (Ar), neon (Ne) and helium (He) At 1 bar of pressure (1 atm), we have `Pn2` = 0.78 and `PO2` = 0.21. In [scuba diving](Scuba%20diving.md), PP (partial pressure) can become a threat because of the pressure. When diving deeper, the partial pressure of gas increase, and some can cause diseases, such as [nitrogen narcosis](Nitrogen%20narcosis.md), due to nitrogen toxicity or [[Hyperoxia]] due to oxygen toxicity. Partial pressure can be calculated using the formula ``` partial pressure = (total absolute pressure) × (volume fraction of gas component) Pi = P x Fi ``` I.e, at -30, the total absolute pressure is 4 bar (1 bar of atmospheric pressure + 3 bar of water pressure, 1 per 10m). So we can say - `Pn2 = 4 * 0.78 = 3.12` - `Po2 = 4 * 0.21 = 0.84` This is how we can calculate the MOD (Max Operating Depth) when using [Nitrox](Nitrox.md) gas. In France, the legal frame allow a maximum of 1.6 PO2. If we're using a Nitrox 32, meaning that we raised the amount of oxygen to 32%, we have at 1 atm, 0.32 PO2. So at 5 atm (-40m), we have `Po2 = 5 * 0.32 = 1.6`. We know here that our MOD is -40m.