How did they know that oxygen was getting low?

Even without a gauge of some sort, there were signs that oxygen was getting low. Probably the first would have been labored breathing. Men would have to breathe deep in order to get enough oxygen into their lungs. This gets to be the feeling that there is an elephant sitting on your chest. Added to that would be headaches among the crew. They can be pretty intense and painful when oxygen is very low.

Another indicator was that the air no longer supports combustion. It becomes impossible to light a cigarette because the match or lighter can’t burn. This puts the “smoking lamp” out and slows the rate of oxygen consumption ever so slightly. There was still free oxygen in the air, but the level has definitely decreased. It would certainly have been time to do something about the poor air in the boat.