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, 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, 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 = 3.12Po2 = 4 * 0.21 = 0.84
This is how we can calculate the MOD (Max Operating Depth) when using Nitrox 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.