Chlorine Kill Factor, pH and the Relationship with CYA in the Water


The traditional thinking is that the lower the pH in your pool or spa the more effective the chlorine will be in the pool – the kill factor or killing form of chlorine (HOCI). This is perfectly true and scientifically proven. A pH of 8.5 means that there is only 10% of HOCI working in the water. A pH of 7.0 means that there is almost 80% of HOCI working in the water. This simply means that the chlorine will “kill” any viruses, bacteria and disease-causing pathogens faster at a lower pH. But what if there was another factor in the pool water that could short circuit this relationship? Recent research leads us to believe that there is.

One of my hobbies is flying drones (Quadcopters). If you haven’t flown any drones before I suggest you try it as it is tons of fun and challenging. In the drone industry range is everything. I have one drone that has a 1.2-mile range if the signal is not obstructed. But in my area, there are a lot of things that will interfere with the signal between the controller and the drone. Wi-Fi signals from neighboring houses. Power lines. And any object like a building that gets between the controller and the drone (line of sight). This can cut the range of the drone down by half or even more. The 1.2-mile range is achievable only in ideal conditions with no interference. We now believe the same is true with the relationship between the pH level, HOCI and Cyanuric Acid (CYA level).

Cyanuric Acid plays an important role in helping protect the chlorine from being used up rapidly by the Sun’s UV Rays. Cyanuric Acid works by bonding and un-bonding with the chlorine molecule which allows the chlorine to last longer in a pool during the peak of the season. The ideal CYA level is 30-50 ppm and anything higher than that can have a significant effect on the relationship with chlorine and the kill factor. New research points to the fact that the CYA in a pool has more of an effect on the HOCI effectiveness than the pH level.

If you pull up this chart you will see how the pool’s CYA level makes a pool with a pH of 7.4 just as effective as a pool with a pH of 8.0 with 30 ppm of CYA in the water. As opposed to a pool with no CYA in the water the pH at 7.4 allows for 50% of HOCI to be present and effective in the water. With a CYA level of 30 ppm, that same 7.4 pH reading allows for just 3% of HOCI to be present which is about the same as a pool with a pH of 8.0 and CYA at 30 ppm.

View Chart here: https://drive.google.com/open?id=1Cr0Z_pBu_s5k5jUaukrY8TCTH3L6VdTY

What does this mean? Basically, that if you are constantly chasing your pH level to keep it at 7.4 to make the chlorine in your pool more effective, with a CYA level of 30 ppm or above you are not making it more effective. A pH of 8.0 is just as effective in a pool with a CYA level of 30 ppm or higher. Just like the example of my drone above, the box says it has a 1.2-mile range, but in my area with the Wi-Fi interference and the line of sight obstructions the true range is maybe ½ mile. So ideally if I was in Idaho in an open field I could take that drone out to the 1.2-mile range. And in a pool with no CYA, a pH of 7.4 will yield 50% HOCI. But with the interference of the CYA in the water that same 7.4 pH will yield just 3% HOCI. My $1,000 drone has the same range as a $250 drone because of all of the interference in my area. A pH of 8.0 will have the same 3% HOCI as a pH of 7.4 in a pool with a CYA level of 30 ppm or more.

I think in an outdoor pool treated with CYA you can stop chasing your pH and focus more on the LSI of a pool. The LSI (Langelier Saturation Index) is a better indicator of where your pH should be vs the potential kill factor rate. Keeping your pH at 7.4 with your LSI out of balance can make the pool water aggressive/corrosive. So instead you should be adjusting your pH based on what the LSI Index says about your pool water. Is your pool water scale forming or corrosive in relation to your pH level? Or is your water scale forming based on your pH level? That is the better application of the pH versus chasing the lower pH level to make your chlorine more effective in light of this new evidence of the CYA, pH and HOCI relationship.

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