Where did my free chlorine go?

[CCC712]

Hi all, I am a new homeowner and new pool owner in southern Georgia and up until recently have done pretty well taking care of the pool myself. I plan to get one of the recommended test kits but don't have it yet so I have to rely on test strips and the pool store for the moment.

I have a ~20,000 gal vinyl non-salt pool, and my FC is reading 0 ppm even though everything else (pH, TA, CH, Phosphates) is "perfect" per the pool store and within the acceptable range per test strips (hard to list specific numbers using the strips). The pool store did say I have a CYA of 70 ppm which was slightly high but not enough to eat up all my chlorine and didn't know what to do other than shock my pool.

Any ideas on what is eating my chlorine? Is there too much stabilizer despite what they said? I do not have algae in the pool, the water is clear and blue, I have 3-4 silk tabs in the skimmer basket at the time of testing. With that many chlorine tabs I was surprised that my FC = 0. The test strips are new and the pool store confirmed there was no FC so I don't think faulty strips are to blame.

 

[polyvue]

Any ideas on what is eating my chlorine? Is there too much stabilizer despite what they said?

Welcome to TFP! :-D

Chlorine is consumed by sunlight and swimming (body oils etc.) and from its work oxidizing other things that fall into the pool, so consumption of chlorine is pretty much a given. How and when the chlorine is added impacts consumption. Your CYA level of 70 is high for a non-salt water pool, and its likely that high because of the tabs in your skimmer. If they contain Dichlor or Trichlor, you've found the reason. Switch to liquid chlorine (10-12%) or bleach (6%) to avoid further increase in CYA. So, yes the "stabilizer" (CYA) is too high.

For CYA of 70, the minimum recommended level of Free Chlorine is 5 ppm. You could drain some of the water to reduce CYA, or just adjust the chlorine up to 5 ppm or more.

 

[PaulR]

The most likely explanation is that you have algae at the not-quite-visible stage. The small amount of chlorine introduced by the sticks kills off some of it, and gets used up in the process; the rest of the algae grows a bit, repeat endlessly.

Switching to a form of chlorine that doesn't add extra CYA is definitely in the cards, as is putting in enough chlorine to kill off the algae entirely. Let me point you to Pool School (see the big button in the upper right of every page on the forum), especially the Chlorine/CYA Chart.
--paulr