Minimum Free Chlorine

[POOLGOON]

I've noticed on the FC/CYA charts there is a "rule" saying the free chlorine level needs to be at a minimum of 7.5% of the CYA...
Simply out of curiosity, where does this 7.5% come from, just real world experience, or is there some kind of documentation?
I've also noticed that some very large pool stores disregard this when they claim chlorine needs to only be between 2 and 4 ppm, yet they also set their CYA range from 40-100 ppm.
I would think such a fundamental flaw would have been noticed after being in business for decades...
Anyway, If i had hard evidence, I would be interested in writing these stores to point out their somewhat flawed system.

 

[JasonLion]

The specific number, 7.5%, is from experience. The general relationship that when CYA is higher FC needs to be higher has been well documented for a couple of decades.

It is difficult to tell how much of the mis-information about this simply dates back to before the FC/CYA relationship was properly understood without any subsequent updates, and how much of it is intentional mis-information on the part of the large chemical companies that are selling trichlor.

 

[mas985]

There is a little bit of science behind it as well. You might want to read through this:

chlorine-cya-chart-t2346.html

and this

pool-water-chemistry-t628.html

 

[POOLGOON]

Thanks guys, that second chart was helpful. I get what you mean, Jason, because of the many alternatives that exist to counteract the high CYA problem (sodium bromide, algaecides...)
Where I live (Orlando) the predominate market seems to be trichlor tablets, and almost everyone has CYA levels past 100ppm