Is 3 PPM of Free Chlorine the Magic Number?

 First off credit for all of this goes to the Legendary Bob Lowry who before his passing last year cemented this theory in the industry through his books, seminars, Certification course, and here on my podcast show as well as others. So all credit of course goes to Bob Lowry and I am just expounding on his teaching here.

The formula is thus:

Your pool’s FC level should be 7.5% of your pool’s Cyanuric Acid (CYA) level to effectively prevent algae and kill viruses and bacteria (summarized). Bob Lowry devised this 7.5% number and under the surface, there was a lot of complicated math involved. With Borates in the pool that magic number drops to 5% of FC to the pool’s CYA level. 

Here are two examples. 

A pool with a CYA level of 50 ppm will require 3.75 ppm of FC (4ppm to make it easy).

50 x 7.5% = 3.75

A pool with a CYA level of 100 ppm will need an FC level of 7.5 ppm

100 x 7.5% = 7.5

If you have a pool with constant algae or other water quality issues try this formula out. I have tested it in the field and have found it to be spot on in all of my test pools. My wife jokes all the time that the answer to almost all water chemistry issues is to “add more chlorine!” Which as it turns out is not too far from the truth.  Note that this FC to CYA level is something that needs to be constant all week and month. If your pool with a CYA of 50 ppm drops down to 2 ppm mid-week, there is a chance that algae can gain a foothold. Or that the chlorine will not effectively protect bathers from viruses and bacteria in the water. 

Why hasn’t this FC to CYA formula been adopted by more organizations out there? In part because the 3 ppm of FC has been entrenched in the industry for decades. It would be like taking the Strawberry ice cream out of Neapolitan ice cream. It is entrenched in the mindset of the entire pool industry. Indeed why Bob Lowry called himself an outsider or Maverick of the industry. 

So is there a magic FC level? Yes and no. According to Bob Lowry, it all depends on the pool’s CYA level and if you adhere to the 3 ppm as being the ideal level then the pool’s CYA level needs to be at 40 ppm constantly all season long. Which of course is hard to do in a realistic pool setting. 

If you want to adopt this method I suggest adding borates to your pools to bring that level to 50 ppm as an extra insurance policy. Remember the magic number drops to 5% of the CYA level. You may also want to keep a higher FC in your pool, something seasoned pool pros have been doing for years during the Summer, sometimes effectively duplicating Bob Lowry’s formula unknowingly. And in all reality a bather cannot tell if the pool has 3 ppm or 7 ppm of FC in the water since most of it will be bound to the CYA in the water and reality, a ppm is a very tiny amount of anything. Adding more chlorine then is better than trying to keep it at the unmagical 3 ppm mark. 

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