I'm missing the logic behind doing an O2 flush between 20 and 30 ft. 20 ft is essential because it's the known pressure and expected PP02 of 1.6 that you're testing the cell against with the O2 flush.
The problem with accepting a reading of 1.6 at greater than 20.5' as proof the cell isn't current limited is that in the early stages of the cells demise it may well produce readings of 1.6 and higher, when hit with a high enough PPO2. Consequently, all that tells you is that it can produce enough millivolts to read 1.6 PPO2, if the PP02 is high enough - it won't tell you what it reads when the PPO2 is actually 1.6
For example, a pure O2 flush at 28' should produce a PPO2 of 1.85. A current limited cell may well read 1.6 at that actual PP02 of 1.85, but read significantly less (1.45 or 1.5) at an actual 1.6 PPO2. Worse, it may be reading 1.2 when the actual PPO2 is 1.3 or 1.35. That will give you a very conservative deco profile, since you are in fact at 1.3 or 1.35, and basing the deco on an assumption of 1.2. On the other hand, the CNS calculations will be garbage and you'll in fact have a lot more on the CNS clock than your computer is telling you.
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Before each trip I check my cells in a pressure pot and I check the percentage of error at .7 (our low set point), 1.0 (our shallow cave high set point), 1.2 (our deeper cave high set point), and at a PPO2 of 1.6 to tell me what I should expect on an O2 flush at 20'. If I'm only getting something like 1.54 at an actual PPO2 of 1.6, I won't pitch the cell just yet, but I will be aware that if all my cells are reading a bit low, I can stop O2 flushing when I reach 1.54, and if I can't reach it, then I'm getting even poorer performance at the end of the dive with wet cell membranes, and I need to take that into account.
Since I am calibrating at 1.0, I'm mostly concerned with how the cell performs at 1.2. I am ok with the cells reading a bit low (1.16 to 1.19) at 1.2, since maintaining "1.2" on the dive won't result in an actual PPO2 of more than 1.21 to 1.25 or so. I can live with that. However, if the cells start falling off more than that at 1.2, they are replaced.
That tendency for an early onset current limited cell to read low is also one of the (many) reasons why I'm not inclined to dive at a PPO2 of 1.3 during the deep/working portion of the dive, since it doesn't take much current limiting for that to result in your 1.3 set point maintaining an actual PP02 of 1.4 (or higher).
