Perhaps it will be interesting to someone.
Under normal conditions, the reduction in O2 is never critical before the increase in CO2 becomes critical.
Atmospheric air contains 21% O2 and 0.03% CO2 by volume.
At normal atmospheric pressure in the compartments:
- a noticeable decrease in the efficiency of the crew due to CO2 appears when its volumetric concentration rises to a level of 1 - 2% and above;
- signs of oxygen starvation appear when O2 drops to 16% and below;
- loss of consciousness and death due to high CO2 content occurs when its volumetric content is at the level of 9 - 10%;
- Loss of consciousness and death due to low content of O2 occurs at its concentration at the level of 10 - 11%.
Oxygen consumption by one person:
25 ... 67 liters of O2 per hour
(depending on the state: rest ... hard physical work, respectively)
Excretion of carbon dioxide by one person:
22 ... 60 liters of CO2 per hour
(depending on the state: rest ... hard physical work, respectively)
Let us assume that the average respiration values for the crew serving at battle stations are 28 liters of O2 per hour and 25 liters of CO2 per hour per person.
For these values, I will give a calculation of how the composition of the air in the compartments changes over time after the submarine has gone submerged and isolated from the atmosphere under the condition of normal air pressure inside the pressure hull and provided that the air purification and air renewal systems are inactive.
Let's say we have a U-Boat VIIC with its free space inside a pressure hull of about 400 cubic meters and a crew of 44 people. After complete ventilation with atmospheric air and sealing, the compartments contain 84,000 liters of O2 (21% of 400 cubic meters of air) and 120 liters of CO2 (0.03% of 400 cubic meters of air).
In order for the O2 and CO2 content to require intervention to normalize the situation (16% O2 and 2% CO2), the amount of O2 must decrease to 64,000 liters, and the amount of CO2 must increase to 8,000 liters.
44 people consume oxygen at a rate of 28 * 44 = 1232 liters per hour and emit carbon dioxide in the amount of 25 * 44 = 1100 liters per hour.
Before the situation deteriorates in oxygen from 21 to 16%, it will pass (84,000 - 64,000) / 1232 = 16.2 hours.
Before the situation deteriorates in carbon dioxide from 0.03 to 2%, it will take (8,000 - 120) / 1100 = 7.2 hours.
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