Yokugawa pH/ORP Free Online Handbook - Very Detailed

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That's an excellent document. While the information on the pH electrode is excellent and indeed pH electrodes follow the Nernst equation such that pH (negative log of hydrogen ion activity) and voltage are linearly related, that is NOT true for real ORP sensors. The RT/nF that they call the Nernst number tells you the mV per doubling of hypochlorous acid concentration in chlorinated pool water. The half-reaction of hypochlorous acid getting reduced to chloride ion is a 2 electron reaction and using n=2 implies 9 mV per doubling. However, actual ORP sensors all have much higher mV per doubling (usually 20 to 28) and they vary depending on the sensor manufacturer. This implies an "n" electron transfer of LESS THAN 1 electron. For example, this post describes ORP going from 700 to 640 mV when the FC goes from 3-4 to below 2. That's at least 60 mV per doubling (or halving) of chlorine concentration and implies an "n" of only 0.15 electrons!

You've had reasonable results from using your ORP sensor because you regularly calibrate and don't have as many interfering factors. You also replace your sensors annually.
 
Interesting- I wonder if, in a real pool, other reactions (chloramines, ...) could also account for such a difference by producing a 'gain' effect as FC varies.

I noticed that as the pH electrode ages (w/o calibration), it tends to read a higher-than-actual pH, causing the controller to add excess acid. The ORP sensor ages by becoming less responsive (under-reading), so that more chlorine gets added (after reading the handbook, maybe also by seeing a lower pH). This shows up in the test kit readings, and helpfully fails towards a more sanitary pool.

I was also intrigued by the semiconductor pH sensor, but couldn't find much info on it.

Parched
 
It happens in standard chlorinated water, not just in a pool. If you look even at George Clifford White's "Handbook of Chlorination and Alternative Disinfectants", you will find all kinds of inconsistencies even in pure water with only chlorine added. There is no ammonia or organics added so there are no chloramines. See this post for some of the craziness associated with ORP. The Figures in White's book varied in mV/doubling from 10 in distilled water to 12-20 in pools to 35, 30-40, 15-20 in three different figures of non-pool chlorinated water. The absolute ORP level (i.e. the offset) varied as well as the slope.
 
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