CYA Levels for ORP Controllers

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CYA narrows the range of the signal that an ORP sensor picks up. Without any CYA the ORP level can vary from about 550 to almost 900. As you add CYA the range narrows. At typical CYA levels it is often down to perhaps 550 to 700. Eventually the range gets so small that the natural noise in the ORP sensor is larger than the signal you are trying to read.

The exact CYA cutoff at which the sensor becomes unreliable varies from brand to brand, but all of them are somewhere a little above 50. Keeping the CYA level below 50 insures that the ORP sensor will be able to reliably detect variations in the FC level.

It is important to keep in mind that ORP sensors are affected by a number of factors other than just the FC level. For reliable automation, you want FC to have the largest effect on the reading and for all of the other variations to be treated as noise. Keeping the CYA level low keeps the sensors sensitivity to FC high and the system behaves as expected.
 
JasonLion said:
It is important to keep in mind that ORP sensors are affected by a number of factors other than just the FC level. For reliable automation, you want FC to have the largest effect on the reading and for all of the other variations to be treated as noise. Keeping the CYA level low keeps the sensors sensitivity to FC high and the system behaves as expected.
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From the charts I've seen, pH is another of those factors. I'm all for eliminating as many of these factors as possible. My controller does a great job of keeping the pH at 7.5 and I've kept the CYA under 30ppm. I'm curious though, what else should I be keeping an eye on? Also, if CYA is such a problem with ORP controllers, why use CYA at all? At first glance at least, it would seem a reasonable alternative would be to use an oversized cell and just generate the FC as needed rather than rely on CYA to "release" it. I did that last season and had better luck with ORP control than this season.

Now, my ORP readings are all over the chart. This happens even when I have the set point set so low as to never generate any chlorine. Within a period of 24-48 hours after cleaning the sensor (both a new one and an "old" one) I get readings that *might* allow for a set point to be chosen. By getting this controller have I chosen to replace daily chlorine additions with daily sensor cleanings?

According to "The Pool Calculator" (Thanks Jason) my CSI is about -0.1 should I be looking for a different target to keep the sensor cleaning down? The goal of my automation was to be able to leave the pool for four or five days and come back to it needing only a vacuum and rinse of the filter cartridge. Was I expecting too much?

I apologize if my frustration is starting to show... I really do appreciate all of the great response I've been getting and the lively discussions on the forum.
 
A great number of things affect ORP sensors, but most of them are uncommon in swimming pools. The most common factors in swimming pools are FC, PH, temperature, other oxidizers (like MPS), sunlight, metals in the water, stray currents, and dissolved hydrogen gas.
 
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