Scale error is like when a thermomether reads 32F at 32F but only 93F at 100F. The amount of change is not correct even though it's correct at 32F
I've yet to come accoss a new sensor that had enough scale error to make a difference. However, just like any battery, as it ages and gets used the voltage will drop and your readings will change. A sensor's offset error doesn't change much but it's scale error continually increases until the sensor is dead.
Unfortunately, this is not true. If you looked at this post
you would find HUGE discrepancies in the mV ORP per doubling of HOCl (or in FC at constant pH). The differences between Chemtrol, Oakton, Aquarius, Sensorex, etc. are HUGE even for brand new sensors using the manufacturer's own calibrated tables! As described in this post
, 23% of pools that had built-in ORP controllers were off by more than 100 mV from the Oakton portable ORP sensor measuring THE SAME WATER (and yes, both sensors were properly equilibrated).
So while you might be able to get some consistency with a particular manufacturer's sensor, they do not work reliably enough to report the same values over a wide range, even when one fixes offset error at one point. Also, they have absolutely no direct correlation to the thermodynamics (like they are supposed to) since theoretically a 2-electron transfer should have 9 mV per doubling while 1-electron transfer should have 18 mV per doubling. Yet sensors vary 22 mV for Chemtrol
to 28 mV for Oakton to 46 mV for Aquarius
to the absolutely ridiculous 84 mV for Sensorex
. These higher mV per doubling imply thermodynamics involving less than 1 electron transfer per molecule. No one has explained such discrepancies and since ORP measurements vary so much there is probably chemistry related to the specific composition of the electrode material itself and to contamination of that electrode.
As James noted, ORP is NOT measuring anything to do with oxygen, but rather is a thermodynamic measurement of the oxidation-reduction (redox) potential difference between the water being measured relative to a silver/silver chloride reference electrode. If you don't have any strong oxidizers such as chlorine in the water, then ORP will roughly measure the dissolved oxygen since that is the next strongest oxidizer in most water.
As for pH, ORP varies with pH not only because of the hypochlorous acid vs. hypochlorite ion, but also DIRECTLY due to the pH itself. Though this would be explained if the standard reference electrode were hydrogen, it's not explained with the silver/silver chloride standard electrode so is yet another mystery and the "best fit" formulas for different manufacturers vary in their pH dependence.
For your 762 mV example, assuming 80ÂºF (since there is also temperature dependence), at a pH of 7.5 this is 0.8 ppm FC and 0.41 ppm HOCl with Chemtrol, 1.5 ppm FC and 0.73 ppm HOCl with Oakton, about the same with Aquarius as with Oakton, and 1.6 ppm FC and 0.78 ppm HOCl with Sensorex. What manufacturer of sensor are you using for your analysis? Whose "PPM vs PH vs ORP chart" are you using? The links I gave above show completely different charts for different manufacturers -- wildly different, not just a little different.
In all cases above the controller has the FAC exactly where it needs to be in order to maintain the same pathogen destroying power. This is exactly what then ORP controller is supposed to do. The "problem" with the sanitizer levels above is the pH, not the FAC.
This is also not true. ORP is affected by things that are not related to killing power. For example, the use of non-chlorine shock (MPS) will register as high ORP yet MPS does not kill pathogens nearly as quickly as hypochlorous acid. Similarly, hydrogen gas from saltwater chlorine generators can affect ORP readings usually lowering them yet has nothing to do with the killing of pathogens. ORP is a thermodynamic measurement, not one of reaction rates for specific compounds which would be much more relevant to determining kill times. Our bodies are thermodynamically favored to oxidize with the oxygen in the air, yet this process is fortunately very slow otherwise we would burn up almost immediately. Also, some compounds can kill pathogens slowly, such as metal ions, linear and Polyquat algaecides, and boric acid yet these do not show up in ORP because their mechanisms do not involve oxidation (electron transfer).
The net of all this is that ORP can be useful for process control with a setpoint for separately measured FC levels as well as knowing CYA, pH and temperature. It is only a very rough indicator for absolute water quality at the extremes and even then only when one understands the specific characteristics of their particular manufacturer's sensor.