CYA/FC ratio?

[Daveacksh]

Hi everyone. Can someone provide a CHEMICAL explanation for why increased CYA levels require MORE free chlorine that the recommended 1-3ppm? It doesn't make sense to me why chlorine/hypochlorite would stop working in the presence of elevated CYA. I've watched the videos, read all the articles.. I haven't found an actual CHEMICAL explanation. My pool is ~11000 gallons (~41000L). I clean it regularly, keep visible debris to a minimum, vacuum at least once a week, do my best to keep pH, alkalinity, etc in the recommended ranges. However, my CYA is currently ~110 ppm (due to puck usage all summer). I use a metal free algicide once a week (~100 mL) and usually add about 100g of Cal-Hypo every week. I keep the alkalinity in check with sodium bicarbonate and the pH in check with sodium bisulfate (it only ever needs to be lowered, a phenomenon which I understand). I have had ZERO problems with algae or anything. My water is crystal clear, no smells, etc. I understand that Calcium (currently ~280ppm) and CYA will continually increase over the season with this routine and partial drain/refills will be required if they get to high before the winter closing, but again... WHY do elevated CYA levels supposedly require more free chlorine? Thanks for any insight!

 

[NeuronBasher]

There's discussion on this topic as well as a link to academic source material in the further reading page linked below:

CYA Chlorine Relationship - Further Reading

www.troublefreepool.com

 

[jark87]

I realize that you state that you’ve read all of the articles, but here’s one that goes into the chemistry. It includes links to the references if you want to dive deeper. I’m no chemist, so other than recognizing that there’s a CYA/FC relationship, I’ve not dug any deeper.

CYA Chlorine Relationship - Further Reading

www.troublefreepool.com

 

[IceShadow]

See this article:

CYA Chlorine Relationship - Further Reading

www.troublefreepool.com

It has a lot of the chemistry in it and citations to other scientific articles and forum posts with more in-depth explanations. Mostly it has to do with how much chlorine is free to sanitize, in HOCl form.

EDIT: Wow, double ninja-ed

 

[IceShadow]

Also the article in the wiki page is gone, but there's a new one from the same source here with the same info:

CYA and Chlorine in Plain Language

The CMAHC now recommends the ratio of cyanuric acid to free chlorine be reduced from 45-1 to 20-1. Here’s their explanation, in layman’s terms.

www.aquamagazine.com

 

[Daveacksh]

Ok thank you! All of these explain things in a way that is finally satisfying.

Basically, high levels of CYA/trichlor can decrease HOCl concentration (which is not specifically tested for), so with elevated CYA/trichlor in your pool, you may not have adequate HOCl.

Thanks again! Feel free to chime in if you have more insight!

 

[PoolStored]

Maybe a deeper rabbit hole than you want to crawl into, maybe not:

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

 

[Daveacksh]

That's the type of material I'm looking for! Thanks guys!

 

[randytsuch]

Some TFP history. I think a lot of the hard core chemical posts were made from chem geek. Don't think he posts here anymore, but he provided a lot of knowledge back when he did.

Randy