High FC and CYA -- a buffering effect

[jeffbg]

Weeks ago, I had to shock my pool and then ran my SWG at a pretty high level. I had to finally cut that out to get my FC levels to settle down, and they took DAYS to do so. I had the SWG set to like 15% and still my FC was falling very slowly.

Once I got near my target level, I moved my SWG to 25 then eventually up to 35% to keep those levels. It seems as though there is a buffering effect going on in the pool such that my FC levels were very high for a while till I burned off the excess (bound up in another form along with the CYA), and so it takes my SWG running higher to keep the steady state levels where I want them or to rebuild the buffer.

Can someone who really understands how CYA works to "protect" the chlorine and whether it acts as a buffer shed some light on this situation?

Thanks!

- Jeff

 

[frustratedpoolmom]

It's not exactly simple terms, but perhaps you'll find what you are looking for here:

http://www.troublefreepool.com/pool-water-chemistry-t628.html

 

[chem geek]

The effect you are seeing isn't due to CYA buffering chlorine, per se. That effect has the active chlorine (hypochlorous acid) level not change that much when the pH changes. When the pH goes from 7.5 to 8.0, the active chlorine level in water with no CYA drops by around 50% while with CYA in the water at typical levels the drop is only around 15%.

It took days for the FC to drop because the CYA in the water was protecting the chlorine from getting broken down by sunlight and your SWG still on was putting out at least some chlorine. If you had turned off the SWG completely, the chlorine level would have come down a little bit faster. In your situation, the steady-state chlorine addition from the SWG equaling the chlorine loss from sunlight and oxidation of organics in your pool is with the SWG set at around 35%.

Basically, independent of the CYA, there is a rate of chlorine loss and there is a rate of chlorine production from the SWG. The CYA in the water slows down the rate of chlorine loss. Whether it does this by combining with (buffering) chlorine or whether it does this by shielding chlorine in lower depths of water isn't relevant to the net effect you are seeing. If you were to do the same thing with a pool not exposed to sunlight, such as an indoor pool or a pool with an opaque pool cover, you would find the same effect of a slow drop in FC after shocking even with no CYA. Similarly, some SWG setting would be needed for maintaining the FC at steady-state (probably a lower SWG setting since there would be no breakdown from sunlight at all).

Richard