We monitor and control the atmosphere pretty closely on submarines, as you might imagine. This is a simplification, but we are worried mostly about CO2 (we don’t want it to get too high) and O2 (we don’t want it to get too low).
For fire to burn, the oxygen percentage needs to be above about 16% (sea level is about 21% oxygen). On submarines, we have a huge incentive to make it difficult for fires to burn, so we wanted to keep oxygen percentage as low as possible.
However, what your lungs care about is the “partial pressure” of oxygen. Partial pressure is the pressure attributable to the fraction of the gas in question, so it’s a function both of percentage and pressure. You can actually suffocate breathing pure O2 if the pressure is low enough, like at high altitudes – this is what’s so hard about climbing Everest – climbers can’t get enough O2, even though they are breathing pure O2, because the total pressure and partial pressure of O2 is too low.
At sea level, where the atmospheric pressure is 760 mm of mercury, the “partial pressure” of oxygen is 160mm of Hg (21% of 760). If I recall correctly, the atmosphere monitoring system on my submarine listed 140–160 as the desired range for O2 partial pressure.
In order to reduce the percentage of O2 (to reduce risk of fire) but keep the partial pressure of O2 within the normal range, what we would do as Officer of the Deck (OOD) is to increase the pressure inside the sub by about 10%, while maintaining the partial pressure of O2 right at the lower limit. So, for example, if we were at 860 mmHg of total atmospheric pressure instead of 760mmHg, and at 140mmHg O2, the percentage of O2 would be only 16.3% – and it would be really hard to light cigarettes. Did the O2 partial pressure drop below 140, further reducing the O2 percentage so cigarettes wouldn’t light? Occasionally? Sometimes it would liven up a boring midwatch to have an irate smoker storm up to the control room and complain about not being able to smoke.
Credit : Quora
Picture Credit : Google
