1) Instructor and student are swimming at depth, student is on an eCCR at high setpoint (say 1.3).
2) Instructor shuts off O2
3) Student's PO2 slowly drops over several minuts
4) If student notices the drop, they address it (try MAV, check tank valve, bailout if unfixable)
5) If they don't notice it for several minutes and the PO2 drops to, say 0.7, the instructor calmly turns on the O2, and the only way the student realizes what happened is they get a "you fail" flash card
I think you guys are also forgetting: as soon as ppo2 hits 1.2, the solenoid continues to fire over and over and over again trying to resolve the deficit. This is an apparent, loud and empty click. On the Optima and Liberty, it'll probably fire 30 times in a minute. If you can't catch that before you drop to a 1.1ppo2, I show you a card that reads, Your Oxygen Is Off".
Seriously, 30 clicks in a minute, instead of a few shots of oxygen every minute, and you don't have the where with all to notice. You're not getting a card this day.