ORP probe claims questioned.(Letters)(Letter to the Editor)

In the April 2004 issue is a column by Lin and Rouse called "All Fouled Up." It is a very appropriate title because it contains some fundamental errors.

Some time ago, Drs. Lachocki and Lin published an article that aimed to prove that ORP measurements were unreliable because at 180 ppm of isocyanuric acid (CYA) the "ORP probes foul quite rapidly." This piece was intended to show that ORP was itself invalid as a technique for gauging water quality. But controller manufacturers recommend an upper limit of 20 ppm for CYA in pool water. All Lachocki and Lin showed was that rapid "fouling" apparently occurs at excessive CYA levels.

The April article states: "Automatic probe-cleaning systems clean probes at a preset time interval, such as weekly or daily." While a few operators set an acid flush on a midnight timer, the manuals supplied by most controller manufacturers would not specify such short time intervals because they know it to be unnecessary and detrimental to the probe's performance.

Finally, we have the following: "After physical or chemical cleaning, the ORP probe takes awhile to stabilize and forms an oxidative film on the metal surface, which enables it to read the electrical potential of the sanitizer and oxidizer in the water."

I don't know what the authors mean by an "oxidative film." Platinum does not oxidize. The ORP probe measures the electron activity in the water. To perform the measurement, one needs a surface that can supply or absorb electrons without being changed itself. That is why platinum or gold (or any other inert metallic element) is used for those probes; these noble metals are electrical conductors, but not subject to oxidation.

The stabilizing of the probe should take about 10 to 15 minutes; longer times indicate that the probe is being cleaned too often.

Wolfram Hartwig, Ph.D.