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.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
39,212
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Keep testing. The data does not lie. It is odd you are losing that low amount of FC but many factors come into play.
 

mguzzy

Gold Supporter
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.
 
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