The main issue here is that the operating conditions for cells get worse during the dive, thus the likelihood of a new failure (or an existing small error becoming dangerous) increases during the dive. A cell may be current limited at 1.2 for example, which you won't pick up at the 6m flush but will be noticeable by mV readings on deco for example. Linearity deviations may not be linear at depth either, so knowing what mV to expect for a given PO2 and given sensor on that day, gives an early diagnostic tool to hopefully notice the deviations before they become life-threatening.
Okay, now I feel REALLY lost.
I get to 6m and do an O2 flush and you're telling me that I won't be able to detect a sensor that is current limited to the equivalent of 1.2 ATA ppO2?
I thought that was the whole point of the O2 flush at 6m. If a sensor doesn't read around 1.55 ATA ppO2 or better, then it is current limited.
@stuartv
The computers are programmed for y=x+b, only the Meg and Liberty have the ability to put M into the formula with multi point calibration to adjust the pitch.
Noting the expected mV value for 1.6 is important because when you do the 1.6 check for current limiting, you can also validate your linear deviation.
[snip]
Like
@RainPilot said, you also want to validate the cells at various points during the dive. You can do this with a dil flush if you have a hot dil since the computers will tell you your dil ppO2 and you can validate that during the flush instead of the mV, but not all divers run hot dil mixes. A dil flush to check for cells at the dil ppO2 is pretty useless if the dil ppO2 is below 1.0. If you have a rich dil mix though, you do want to check that if you can. On ocean dives this may not be that significant due to shorter run times, but the cells can get pretty strange when you're on really long dives.
I'm not getting this and I apologize if I'm just having a huge brain fart.
You said computers are programmed for y=x+b.
I'm taking Y as ppO2 and X as the mV reading. So, if my sensor is good and it reads 11mV in air, then I would expect it to read 52mV when I calibrate it in O2.
Y=X+B means: 1.0 = 52 + B. So, B = -51? That is obviously not right. Otherwise, when it's back in air, it would be saying my ppO2 is 11 + (-51).
It seems to me that computers would be programmed with Y=MX+B, and only the Meg and Liberty could determine B. All other computers would assume 0 = 0 as one point and be unable to calculate B. Those single-point computers would use 0,0 and the calibration point to determine M.
So, if calibrated at 1.0 ATA in O2, with the sensor giving 52mV, it would assume that 11mV means 0.21 ATA ppO2.
However, I could be wrong. But, if so, can you please give me a couple of real numbers as an example (for a single-point computer) of the Y=X+B calibration?
Anyway, moving on to the REAL problem I'm struggling with:
You calibrate your computer however you calibrate it. 1 point, 2 point. I don't care. When you're done calibrating, the computer reads 68mV (we'll just say for sensor #1) when your sensor is good and you're at 1.3.
So now, during your dive, sensor #1 is showing 1.2 ATA ppO2. I'm thinking that that means the mV reading for that sensor is going to show as 63mV. Am I correct, so far?
Assuming so, how does looking at the mV help me? I have one sensor reading 1.2 and two sensors reading 1.3. I can see that it's reading low. If I could do the math in my head, I would know without looking that it is showing 63mV on that sensor - because the two numbers are mathematically tied together. So, I know sensor 1 is limited just by seeing the ppO2 readings. What does looking at the mV # tell me that I don't already know?
The way you're talking about it makes it sound like the computer might still be showing me 1.3, but if I checked the mV I would see 63, instead of 68. I don't think that is possible. That would imply that the computer's calibration has changed during the dive.