Another Perspective on the Debate About PPM vs. ORP in Swimming Pools

Over the last couple years, there has been much talk about the need for parts-per-million (ppm) chlorine monitoring, with several chemical controller companies developing ppm technology. Some have even suggested the new California Title 22 mandates ppm monitoring and recording.

But there is good reason why the industry has and probably will continue to use Oxidation Reduction Potential (ORP) as the standard for chlorine control, even though ppm sensors, using both amperometric and DpD measurement devices, have been around for decades.

There also is good reason to measure ppm. Most county health agencies require, measure and evaluate water disinfection based on the chlorine’s ppm. This is also what most industry test kits provide for us with colorimetric, titrimetric and electronic testing.

Quantity vs. quality

The real question is this: Why didn’t ppm monitoring and control catch on before? It’s simple: The ppm sensors and water testing provide a quantitative measurement of chlorine. That is, how much chlorine is present in the water.

But the largest issue to understand is that quantity may have little to do with quality. The ppm value only tells us how much chlorine is present, not whether it is really working. ORP, on the other hand, is a qualitative measurement of the oxidizer’s or sanitizer’s “work value.” Although most state health departments call for the use of test kits to measure ppm, many states continue to address ORP as the standard for chlorine measurement and control with automatic chemical control devices.

Sure, ORP is affected by both pH and cyanuric acid (CYA). But that is exactly why ORP is a better choice.

We need to understand that ORP measures the “oxidation potential,” or the ability to break down organic compounds in water (a long and boring subject about the removal of electrons from a compound resulting in a change to the compound). This is necessary because the chlorine compound, hypochlorous acid (HOCl), is unstable and very active by nature.

Many studies have proven that disinfection occurs at an ORP level above 650, the minimum recommended by the World Health Organization. However, most industry experts target 700 ORP or more.

The pH factor

Unfortunately, rising pH values result in the ionization of HOCl. The hydrogen splits off, and the remaining hypochlorite (OCl-) does little to sanitize or oxidize. Yet both HOCl and OCl- show up as free chlorine on the DpD test.

The chart above (click here to view original article with chart) illustrates what happens to the HOCl compound as the pH rises. Note that at a pH of 8, which is legal in California, the amount of free chlorine that remains as HOCl is less than 20%. The rest is OCl-. Yet at a pH value of 7.2, the free chlorine is almost 70% HOCl, making it more than three times as active with about three times the work value. So 1.0 ppm can have the same disinfection and oxidation value of almost 5.0 ppm, depending on the water’s pH.

As pH rises, ORP drops, because more OCl- is formed. With less HOCl present, the controller will compensate by adding more chlorine to maintain ample disinfection and oxidation. Of course, it will also turn on a chemical feeder to adjust the pH back to the desired set point. A controller measuring ppm would recognize a pH change only, not a change in work value.

Effects of CYA

With regards to CYA, much has been written by water chemistry experts about its effect on the work value of chlorine, with studies showing a significant increase in the time required for chlorine to inactivate various pathogens.

The chart above (click here to view original article with chart) shows this relationship between CYA and kill times.

As CYA increases, oxidation — or the ability to break down organic compounds — is significantly reduced, and ORP drops significantly. However, the ppm readout on either an amperometric or Dpd reader will remain unaffected, even though the chlorine’s ability to do its work is dramatically affected by the CYA increase. Therefore, a ppm controller will remain unaffected, even though the work value of the measured ppm is significantly and adversely affected.

Because of CYA’s effect, virtually no ORP controllers use isocyanurate feeders: The dichlor or trichlor used in these feeders contains CYA as a stabilizer.

The chart above (click here to view original article with chart) shows data from a non-empirical study completed some 20 years ago by a San Diego County health inspector of hotel spas. Examine the various ppm and pH values that meet code. Then look at the plate count and pseudomonas levels. Here, the culprit is CYA in affecting (or not) the destruction of pathogens. Now, examine the same plate count and pseudomonas levels by examining the ORP. In every instance, ORP values above 650 millivolts result in far more favorable pathogen destruction.

Although not an empirical study, I believe there is enough information to establish that ORP is by far a better predictor of pathogen destruction than ppm, even with acceptable pH values.

There are actually some control systems that have the ability to measure and record both ppm and ORP. The best of both worlds!

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