All fouled up: if you think you can't get accurate ORP readings with trichlor, it's time to check your assumptions, and your probe.(Tech Talk)(oxidation reduction potential)

Many pool operators think you can't get accurate oxidation reduction potential (ORP) readings when using trichloroisocynauric acid (trichlor). What these operators don't realize is that it's not the chemical that is at fault, but rather, the probe technology itself.

To understand this problem, it's necessary to examine the chemical controllers used on most commercial pools today. Many controllers use a common ORP probe to gauge water balance. But these probes have some serious limitations.

That's because ORP probes do not specifically measure chlorine or bromine residual in the water. Instead, they apply a measurement of the electrical potential of the water known as conductivity. These readings determine when the controller sends a signal to feed sanitizer into the water.

An increase in the sanitizer residual results in an increase in ORP. Similarly, a decrease in sanitizer residual yields a decrease in the ORP of the water. However, if the probes become fouled, as is often the case, the ORP readings become inaccurate.

Most ORP probes are susceptible to fouling because the electrode sensing surfaces are made of platinum metal, which can become coated with contaminants from sweat, lotions and other body oils. As these contaminants collect in the water, they begin to form a microscopic coating on the platinum probe, reducing the probe's accuracy--a process known as probe fouling.

Fouling delays the probe's ability to detect changes in sanitizer level and causes the controller to display a lower than actual ORP value. To remain effective, probes must be routinely cleaned.

Then the question becomes, how often is routine? Field reports ...