We see many compressed gas cylinders of Asphyxiants in labs within large industrial facilities. We even see these labs increasing their quantities and moving to using Dewars (cryogenic liquid storage) to store their asphyxiants. These dewars are a massive game-changer, as both Liquid N2 and Liquid Ar have expansion ratios of 1:700 and 1:860, respectively. This means 1 gallon of liquid will make 700/860-gallon jugs of pure gas. A large Dewar is 2000 L (528 gallons).
If the room has a 2000 L DEWAR of LIQUID asphyxiant, such as Liquid N2 or Liquid AR, you could be in a room that is approximately 10,594 cubic feet, and that Dewar is enough to drop the O2 level to less than 19.5%.
Now, are we starting to see the need for basic Oxygen detection when the room contains known asphyxiants?