is there any non obvious reason that Chlorine would go up?


Well-known member
Jun 4, 2016
Portland OR metro
First year pool owner, I had a pretty good feel/system going. i was checking my water with the TC and pH test every other day, occasionally letting it go 3 days, and adding about 1 qt of 10% LC every 1-2 days depending on how sunny it was and how much we were swimming. I was consistently losing 1.5-2 ppm per day, and with this regimen consistently staying at 4-5 ppm TC.

3 days ago the TC was 3 and I added 2 quarts of LC because the weather was turning sunnier and warmer. Today I checked and the FC is 10.5!!! It’s never been above 6 before. We did get about 1-2 inches of rain this week.

I don’t knowingly add any other source of chlorine to the pool. No tabs or additives.

CYA has been 30, now it’s halfway between 20 and 30. I’m thinking of adding some more stabilizer but the drop from 30 shouldn’t have caused chlorine to go up, right?


Bronze Supporter
Jun 1, 2018
Hernando, Ms
I have noticed that the oto is not that accurate- maybe you’re fc was actually higher than 3 when u added lc . At least it wasn’t wrong in the other direction 🤣
Its more of a “oh good i have chlorine!” Type test.


Bronze Supporter
Mar 5, 2020
Melbourne, Australia
Did you use a different batch of chlorine? Maybe you got used to old chlorine and now got a fresh batch. Did you check the date code? CYA shouldn't interfere with the FC test. Did you use FAS-DPD?