is NOT the same as measured hypochlorous acid.

So how much hypochlorous acid do we need?

The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have determined that a minimum of 650 mV (ORP) is required for instantaneous inactivation of most pathogens. That corresponds to about 0.01 ppm hypochlorous acid. Meanwhile, some studies have shown that 0.05 ppm hypochlorous acid kills most algae. So basically, we know we want a hypochlorous acid concentration somewhere between 0.01 and 0.05 ppm.

Next, we need to figure out how that range corresponds to our measured chlorine and cyanuric acid. Unfortunately, the exact math to get there is tricky and involves solving a bunch of equilibrium equations.

Fortunately, someone else has done the work for us.

In the early 2000s, water chemistry expert Richard Falk solved those equations to determine the amount of cyanuric acid and chlorine that taken together result in a given hypochlorous acid concentration. See accompanying graphic.

The graph shows free chlorine increasing at the top from left to right and cyanuric acid increasing on the left from top to bottom. Colored numbers on the inside are calculated hypochlorous acid concentrations. Those marked red are hypochlorous acid concentrations lower the WHO standard for pathogen inactivation, while those marked in blue are higher than 0.05 ppm hypochlorous acid needed to kill common algae. Green numbers indicate chlorine and cyanuric acid combinations that should be effective for most pools.

The graph also shows that a desired hypochlorous acid range of between 0.01 and 0.05 ppm can result from a variety of chlorine and cyanuric acid combinations, from 0.1 to 10 ppm chlorine, and from 0 to 100 ppm cyanuric acid.

All that is left is the constraints of industry guidelines and regulations.

The 2023 Model Aquat ic Health Code (MAHC) states that a cyanuric-acid-to-free-chlorine ratio that exceeds 45:1 is grounds for closure. That means that chlorine should be greater than 2.2 percent of the cyanuric acid concentration. It stipulates a maximum cyanuric acid concentration of 90 ppm and chlorine levels between 2 and 10 ppm. It’s a very broad allowance, and pool operators adhering to the standard may find that some combinations within this range may actually succumb to algae.

According to Falk and others, the sweet spot for protection against algae has a higher chlorine-to-cyanuricacid ratio.

Their experience is that maintaining the chlorine at 7.5 percent of the cyanuric acid is generally adequate to protect typical pools from green algae, and that a ratio of about 15 percent is good for yellow/mustard algae. Lowering your phosphates (algae food) may allow operators to follow the MAHC’s low estimate of 2 percent.