/... You can breath just fine on a CCR with the O2 off, until you go hypoxic and die in a few minutes.
This will start an argument, but "breathe just fine" isn't strictly true.
It'll start a sub argument when I suggest that the training that the high altitude taking that all military and some civilian pilots receive has some relevance to CCR diving.
A very low PPO2 of .10 is equivalent to a barometric altitude of about 20,000 ft. If you experience an O2 failure at altitude you'll have about 10 minutes of useful consciousness, but that's assuming you are at rest. If you suffer rapid decompression at that altitude, that time shrinks to about 5 minutes, and even moderate exercise cuts that in half again to about 2.5 minutes. Using that most conservative figure and applying it to CCR, you've still got some time as the loop PPO2 falls below .16 or so to figure out that something is wrong. Symptoms of rapid onset hypoxia vary a bit from person to person. In my case, I note a feeling of fatigue, and visual impairment including reduced low light sensitivity, and impaired peripheral vision. That's followed fairly quickly by cognitive impairment with an inability to focus and problem solve - which is what leads to inaction that results in a fatality.
They key is to recognize the early stages of hypoxia before you enter the late stage where the cognitive impacts can prevent you from taking appropriate action.
In that regard, I think CCR divers are at a disadvantage by not having exposure to hypoxia in a high altitude chamber where they can experience and recognize their early onset symptoms of hypoxia. It's partially what leads to statements such as "you can breath just fine until you go hypoxic and die in a few minutes", which creates the impression that there are no warning signs.
I prefer Mel Clark's approach of stating "if you feel funny, it means you are probably about to die". Absent training in your responses to hypoxia, a CCR diver is well served to heed that advice and bailout for a couple sanity breaths in the event he or she "feels funny".
You can have multiple O2 sensors go current limited in under 12 months because of a manufacturing defect and they vote out the one that is trying to tell you that you are about to go into convulsions from O2 Toxicity.
That's very true. But that also reflects the need for the CCR diver to fully understand his or her rebreather, both as an overall system and as a collection of sub systems with distinct failure modes. Along with this is the need for a thorough understanding of the cross checks that can be used to detect failures.
Again at the risk of pissing off the non pilots, the training in systems and cross checks to detect failures will be second nature to an instrument rated pilot flying high performance aircraft and like it or not, that approach and the related critical thinking process transfer well to CCR diving.
In the example you describe above, a diluent flush will give you a ball park close to known loop PPO2 based on the O2 percentage of the diluent and the current depth. That derived value can then be compared to the sensor readings to see which of the three, if any, are ball park accurate. I set the dil PPO2 to display on my Petrel so that I can determine the dil PPO2 immediately with no math involved and can compare it directly to the PPO2 reading for each sensor. It tells me which one(s) I need to ignore, and if none of them are accurate, it's telling me I need to bailout.
That should however be CCR 101 level knowledge, so stating it the way you've stated it is perhaps over stating the risk, or it's perhaps a very sad commentary on the state of CCR divers and CCR training
