Dirty Pool

By Drs. Shawn Lin and Thomas M. Lachocki Special to Aquatics International

June 2002

lectronic controllers fitted with chemical-sensing probes are becoming more prevalent in the pool and spa marketplace. Public health departments and facility managers are driving this growth in an attempt to better ensure that sanitary conditions are being continuously maintained.

Figure 1

Figure 2

Figure 3

Manufacturers offer many effective controllers, but we will focus on the probes used to measure sanitizer levels. These probes send signals to the controllers that operate feeders, which introduce more sanitizing chemicals into the water.

The most common probe used to measure sanitizer levels is the Oxidation-Reduction Potential probe, aka ORP probe. Unfortunately, these probes “foul” or get dirty, causing the ORP reading to change even though the true ORP of the water does not. Manufacturers recognize that probes get dirty and include instructions in their manuals to clean them.

Signs that the probes are dirty include a slow response to sanitizer level changes in the water and low or inaccurate readings. Probes get dirty when materials accumulate in a thin film on their surfaces.

The time it takes for ORP probes to foul varies from pool to pool, depending on what is present in the water and on the probe itself. Components in water that contribute to probe contamination include minerals, organics, perspiration, suntan lotions, body oils, urine and cyanuric acid.

In Figure 1, we see how important it is to keep an ORP probe clean as well as how cyanuric acid affects the readings.

In this experiment, three commercial ORP probes were exposed to balanced pool water containing cyanuric acid for five days. Secondly, with different balanced pool water, a cyanuric acid solution was slowly pumped in at the rate of 10 ppm/hr. There was a dramatic difference in the ORP readings. Two probes (X and Z) differed by more than 100 mV, showing that dirty probes are not precise.

The experiment also shows that probe fouling can occur in less than a week. After the probes were cleaned and the experiment was repeated, higher and more consistent readings (+/- 15 mV) were observed. This means that dirty probes also are inaccurate.

In certain cases, ORP probe fouling is so rapid that an automatic ORP cleaner is more desirable than manually cleaning them. Manufacturers make automatic ORP cleaners designed to resolve the probe-fouling problem and permit long-term use of a probe. However, most of these cleaners are complicated and expensive and require an ultrasonic device, a high-pressure water/air jet spray or concentrated acids.

A simple new, patent-pending method to clean ORP probe surfaces continuously and mechanically was tested (auto-clean). Figure 2 illustrates how this new method performed at keeping ORP probes clean.

Pool water with a cyanuric acid level of 180 ppm was used to make the test more challenging. A commercially available ORP probe was compared to an auto-clean probe. The chlorine level in the balanced pool water was alternated quickly between 2.0 and 4.0 ppm. The response of the commercial ORP probe (Brand X) was slow and did not reflect changes in the true ORP. The auto-clean ORP probe responded more quickly and gave a more accurate true ORP, almost 50 mV higher than the traditional probe. The improved accuracy, precision and responsiveness would have resulted in better control of the sanitizer level.

Three other general types of sanitizer controllers also are commercially available: amperometric probes, amperometric titration systems and a colorimetric system. For the operators who prefer not to handle chemical reagents, the first type is preferred.

Amperometric probes detect sanitizer residual level changes by measuring the amount of current flow between the electrodes in the probe. They have a faster and more linear response to changes in the chlorine level in the water than ORP (see Figure 3, CYA = 200 ppm). Also, the membrane protects the sensing surface of an amperometric probe, so probe fouling often is greatly reduced.

An amperometric probe is more expensive than the ORP probe and it’s relatively new to the U.S. pool and spa market. However, it provides some features and benefits that ORP probes do not.

Editor’s Note: The authors performed their research over the last three years to understand how all system components (controllers, probes, feeders, circulation systems and chemicals) work together to properly maintain pool water quality. They have presented their findings at conferences, including last year’s International Pool & Spa Expo in Phoenix.