Stuart,
If I may humbly suggest, start thinking in mV. I had a CCR diver, that I was mentoring, that started getting funky mV numbers and couldn't figure out why. His PPO2 seemed fine however, because I had taught him to think in mV, he knew that something was not right. When we discussed the issue I had him run through his checks with me on the phone. What we found was that he was flushing then closing the DSV and turning off the O2 which resulted in an over pressurization of the loop, as you had mentioned earlier. The problem was that he had already calibrated so it took a minute or so for his PPO2 to display 1.0 however it would not go over that. While on the phone I heard his OPV burp and knew right away that he was calibrated at an over saturated PPO2.
As the presentation, from the OP states, thinking in mV, checking linear deviation with mV, and 1.6 checks for cell limiting with mV will give you a much better understanding of your cells health. It also makes me more comfortable during a dive knowing the health of each of my cells. PPO2 is only a mathematical representation of a divers input, not necessarily an accurate representation.
You should still, IMHO, only calibrate when things are off however always verify by checking the mV output in air, O2, & 1.6 PPO2. If you can't get your mV numbers in air & O2 or your PPO2 is off, then you calibrate before getting to depth to do the 1.6 check.
Hey, Bobby, thanks for taking the time. No need to "humbly suggest"! I know you, respect what you have to say, and I'm always open to hearing advice from people who know what they're talking about.
And I do keep an eye on mV. I'm a numbers guy. I do things like calculate ppO2s, MODs, Best Mixes, etc. in my head, just to keep my brain from getting any duller. So, checking mV readings on sensors is a normal thing for me to note and cross-check.
But, it's not part of my formal, official process. And I still don't understand what value it would give to make it so.
By following my check list, if I made the mistake that your mentoree made, when I got to the step where I do a dil flush (during my Closed check, after assembling the unit at home), I would have seen that the ppO2 of my dil was not reading what it should have been. Calibrating to O2 in a pressurized loop would make my dil read as a lower FO2 than it should (I believe).
And, to me, the whole point of all of this discussion is that you are able to identify when something is wrong. Whether you identify it by seeing a mV reading that isn't right, or whether you identify it by seeing a ppO2 reading that isn't right doesn't seem to be so important. What's important is that your process ensures that you catch it when something is wrong.
In the case of your mentoree, once he saw that something was wrong, it seems like it should have been easy to walk back through his check list and realize where he'd made a mistake, then start over, do it again, correctly, and be good to go. The problem - it seems to me - is if following his check list would not have informed him that something was wrong. If the only way he would have known something was wrong was by looking at his mV output and doing a calculation to confirm the value, then that seems like his check list is failing him. Unless, I suppose, it's part of his check list to do the calculation and compare against the mV reading. But, that seems way more complicated than it needs to be.
Once I'm into a dive, the calibration is set. It's not going to change during the dive. So, looking at a ppO2 reading is the same as looking at a mV reading. It's just scaled to a different actual number. But, the ppO2 number is one that I'm much better equipped to easily recognize as "good" or "problem". If I see ppO2 = 1.5 when I'm on the bottom, I know that that's bad. If I see a mV reading of 75, well, I don't know if that's good or bad except by doing some math or looking at a chart or something. And why would I do that?
If my ppO2 is reading 1.1 and my other sensors say 1.3, then looking at the mV is not going to tell me anything new. That sensor is going to be reading the mV equivalent of 1.1 - whether it's current-limited to that value or not. The only way to know whether that sensor is right and the other 2 are wrong, or vice versa, is to do a dil flush. And with the dil flush, I'm not going to be looking at mV. I'm going to be looking at the Dil PO2 on the computer and the ppO2 readings from the sensors. Those PO2 numbers are what's going to tell me which sensor(s) are right, not mV readings.
When I get to the end of my dive, ascent to 20', and do an O2 flush, my sensors are going to read 1.55 - 1.6 or so - or they're not. Again, I won't need to look at mV readings to know if I have a problem. Again, the ppO2 # and the mV # are the same, just scaled differently.
