CYA Problems

[chem geek]

Chlorine lock doesn't ever really occur in the sense that at some CYA level disinfection stops completely regardless of FC level. At higher CYA levels you simply need to have higher FC levels to maintain the same level of disinfecting, oxidizing and prevention of algae capability. With a lot of CYA in the water, it is possible to slow down the dissolving of slow-dissolving Trichlor (that happened to me when my CYA got over 100).

See this chart which really could continue on at even higher CYA levels having even higher Min FC levels.

The issue is that with higher FC levels at higher CYA levels you are wasting more chlorine because about half of the FC is going to get consumed by sunlight each day and half of a larger number is a larger number. However, as waterbear said, with too low an FC, you simply won't be able to stay on top of your chlorine fast enough. It's a tradeoff between chlorine waste (at higher CYA and FC) vs. how quickly and frequently you are able to replenish the chlorine level. Most people don't want to spend every hour by their pool adding more chlorine.

For SWG pools, there is a completely separate issue as to why higher CYA levels are required and this has to do with SWG cell efficiency, but that's discussed on this thread.

Richard

[CarlD]

Here's the piece that everyone's leaving out:

At higher CYA levels, yes, it's true, you need a higher FC level to maintain sanitation.

BUT what happens is that you get a residual of chlorine that generally remains, so you are only chlorinating for the extra you need.

Say, for example, your CYA is 110. The suggested FC for that CYA is 8 to 15 ppm (and shock at 25). You should find it very easy to keep your FC in this range and not use any more chlorine on a weekly basis to maintain it than you use at lower CYA and FC levels.

Actually, your chlorine usage may go DOWN because the CYA gives you such a high residual of chlorine.

One of the moderators, Aylad, leaves in the Deep South. She gets lots of factors that drive FC down (consuming her chlorine). So Aylad runs her pool consistently at higher CYA levels than we Northerners do--80ppm. Since Aylad has her FAS-DPD test kit, she can test higher levels of chlorine easily--and keeps her FC level in the 5-10ppm range. This allows her to avoid constant monitoring and chlorine additions several times a day.

But since I live in Northern NJ, I tend to keep my CYA in the 30ppm range for the early part of the summer, running it up to 40-50 when we approach the Dog Days.

CYA is a two-edged sword. It slows the metabolizing of chlorine, so it doesn't burn up instantly in the ultra-violet. But it slows down the metabolizing, so you need more chlorine. Still, if you follow the guidelines AND have the FAS-DPD test, there's no reason not to run higher levels of CYA. It CAN make your life easier if you have VERY intense UV (at lower latitudes), and/or lots of organic material you are metabolizing.

CYA should NEVER be used in an indoor pool.