Free Chlorine in SW pool rose 2ppm after a weekend of pool use on its own. How?

[just4747]

I looked this up and could not find anyone else asking the same question. I've been testing right at 7ppm FC and .5 combined every few days over the past few weeks in our salt water pool, using the TF-100 chlorine drop test. SWG has been set at the same 40% level the whole time when it runs and even after I've had to add a bit of salt once or twice after rain (had to drain down a bit) the chlorine numbers didn't change. The only time I've had to add liquid chlorine was once about 2 weeks ago when I was first getting the level up from only 2ppm and has stayed at 7 stable ever since.

This past weekend, we had heavy usage of the pool on Saturday and then Sunday it rained on and off. Back to sun today.

Did my tests today and kept getting 9 FC and the same .5 CC. How would FC rise 2 ppm if I haven't added or adjusted anything chlorine related at all since I last tested 6 days ago? All I had done was add 40oz of dry stabilizer one week ago to get my CYA from 50 to 70.

CYA was 75 today and tested at 70 6 days ago. pH was about 7.6 today and about 7.4 6 days ago. I did not add or change anything btw those two readings.

 

[bobloblaw42]

Theory: Heavy usage (different water movement pattern than just pump alone) = more uniform pool water mixture. Possible that your test is reflecting more accurately the average FC and CYA for the entire pool as a result of a good stir.

 

[just4747]

Theory: Heavy usage (different water movement pattern than just pump alone) = more uniform pool water mixture. Possible that your test is reflecting more accurately the average FC and CYA for the entire pool as a result of a good stir.

I guess I could see this. We only have a 12k gallon pool with no drain, 1 skimmer, and 2 return jets, so not the best movement I guess, but there have been other days earlier on that 2 or in one case, 4-5 people have used the pool at the same time and that didn't seem to make the same difference, so just not sure.

Either way, I'm guessing it's not a big enough change to matter much but still just curious.

Thanks.