Lost 2x 184oz bottles worth of Chlorine in 6 hours

[sethz]

Opened the pool yesterday, before I put any chemical, the test results are:
FC 0, CC 0, pH 8.0, TA 90, CH 170
I didn't test CYA, CYA is 30 at the end of last season, I think the current number is much lower.

shock with 3 bottles of walmart 184oz (1.42lb) 6% bleach and 1lb of 70% trichlor. pump is running 24.

Today noon, the water looks perfectly clear, after brush/vacuum, I tested FC again:
FC 5.0, CC 0.2

Then I poured 2 bottle of 184oz 6% bleach.

Around 6PM, tested FC again:
FC 6.5, CC 0.2

Today's high temperature is 73 degree, cloudy.

So basically these 2 bottle of 184oz bleach chlorine are dissipated during only 6 hours, raising FC only by 1.5. Based on pool calculator and my own estimation, I was expecting raising FC to ~10.

I am confused. What could be the possible explanation?

 

[frustratedpoolmom]

Sunlight destruction is certainly likely in this situation, given you have very low CYA.

 

[dmanb2b]

absolutely agree w/ FPM...if your CYA is low, you likely burned up any FC added today...eventhough it was cloudy. Time to test CYA and adjust to at least 30ppm

 

[chem geek]

Check the overnight FC loss -- add chlorine at night and see if it's mostly still there in the morning. If it's all gone, especially if it goes away within an hour at night, then you likely have ammonia in the pool from bacterial conversion of CYA and it could take a lot of chlorine. Worst case, 30 ppm CYA would turn into 10 ppm ammonia and need at least 80 ppm FC cumulatively added to clear it.

Even with no CYA in the water, the FC shouldn't drop by more than half in 30 minutes at the most -- usually it's around a 50% drop in FC in about an hour for typical depths of pools at noontime. If you were adding chlorine in the morning and then only checking it at night, then it could effectively be all gone from sunlight breakdown alone when there is no CYA in the water.

 

[sethz]

Thank you all for clear explanations.
I tested overnight FC loss. The pool lost ~1ppm overnight.
As well I tested CYA, unfortunately the minimum CYA mark on Taylor K-2006 test tube is 30. My fill the tube all the way up and the black dot is still clear, so I must have a very low CYA level. Dropped ~4lb cya sock in skimmer.

 

[waskydiver]

Haven't seen mention of your pH since the initial reading of 8.0.

Drop your pH to about 7.2, and your chlorine will work harder for you.