Puzzled by FC rise

chasut

Silver Supporter
Mar 17, 2020
6
Austin/TX
Just when I thought I had this nailed...

Free chlorine is rising at ~ .5ppm for the last several days to 11ppm this morning even with a progressive drop of the SWG from 65% -> 55% -> 45% -> 35%. I have now turned it off. This last week has been sunny with very high UV, and pretty typical bather load. I have no clue why the chlorine continues to rise given the drop in chlorine generation. With the SWG at 35% Pool Math estimates 1.6ppm generated at my seasonal runtime of 14 hours. Just doesn’t seem reasonable that I would be using only 1ppm in this weather.

I have been testing daily as I get comfortable with the pool and process. Plaster is new as is my use of TFP methods. In the prior two months I was using ~ 2.4 - 2.8 ppm/day depending on weather and bather load. Last week i added CYA to get to my target of 80 (was running at 60). I was almost imediately rewarded with a drop in chlorine usage and was pretty darn pleased with myself. Then it just kept getting better?!?

I suppose it it not the worst of things, but I am getting a bit obsessive with the learning and understanding. The pool water is crystal clear so I am well rewarded for my effort. BTW, if you would have told me last year that my water could be more clear, sparkle more, and even feel better I would have thought you nuts; but it most definitely does!

Thoughts please.
 

mguzzy

Gold Supporter
Jul 8, 2015
2,386
OV, CA
Last week i added CYA to get to my target of 80 (was running at 60). I was almost imediately rewarded with a drop in chlorine usage and was pretty darn pleased with myself. Then it just kept getting better?!?
Last week huh? In my experience it takes almost a week for the CYA to completely stabilize. But then again your pool's average temp is probably much warmer than mine. But I have noticed after CYA additions the chlorine usage would continue to level off for almost a week as I approached the sweet spot above 70ppm.
 

chasut

Silver Supporter
Mar 17, 2020
6
Austin/TX
Thank you for the support. Tested this afternoon after 8 hours of sun and the SWG off. FC = 8! Just as weird to drop 3ppm as rising in the face of all that sun. Now I am thinking that my chosen testing location may be getting some flow from the return and I just happen to be pulling the sample with the SWG on when I test.

I specifically chose that spot away from the return, but who knows. I will run some tests tomorrow from multiple locations and see if I can recreate what i have been observing.

Or maybe I just have a magical pool that doesn’t consume much chlorine in the Texas sun ;)

@mguzzy, its been taking about 48 hours to get the expected test results from CYA add for me. Of course, that test is not terribly precise, so best i can tell is that it is “at least” at the expected amount. Since I cannot pinpoint exactly where I “was” it is just a best guess as to where I should be, but expected around 75. I suppose I could still have some in the DE that will leach out over time. Test was above 70 but not quite 80, but since it is above 70 I am calling it 80. Will test again tomorrow and see.
 

chasut

Silver Supporter
Mar 17, 2020
6
Austin/TX
So, the answer is... “the data does not lie” My daily FC loss is, in fact 1.0 (+/- 0.5ppm).

Of course I was determined to do things the hard way... . So to quote Spock (or Sir Arthur Canon Doyle if you like) “if you eliminate the impossible, whatever remains, however improbable, must be the truth.” I ran tests on everything I could think of that may contribute to the FC rise (and discovered the city has started adding a bit of chlorine to the water for the summer months among other things). Concluding that nothing was having a material impact on testing or chlorine rise. So capped off my testing with a 24 hour chlorine loss test, and sure enough, it was 1.0.

It was all worth it. Thanks @mknauss and @mguzzy for tutoring.