So part-way through a SLAM and got a higher FC reading than I would have expected according to PoolMath and the amount of bleach I put in. I am using TF-100 for testing. One possibility I was thinking is that maybe my pool size is off, although 35,000 should be close. Reason I know is we got a new liner installed last season and used the garden hose to fill -- the time it took to fill supported 35k. And pool store who originally installed it for the previous owners (and replaced the liner) also say 35k.
Here is the sequence.
Tested last night around 9pm
- FC 0
- pH 7.5
- TA 170
- CYA 55
Based on this, I calc'd a shock value of FC 22 which would require 1161 oz (around 9.5 jugs) of 8.25% bleach. Poured that in and went to bed..
This morning at 7:15am, FC tested to a reading of 1 using the FAS/DPD with a CC of 0.5 (green had become much lower - lot of progress overnight visually).
Put in another 9.5 jugs to shock again (lots of scrubbing the walls here) and went out for a bit.
Tested again at 12:30pm, FC now tested to a reading of 28 with CC still at 0.5. Color had changed to blue, still cloudy although much less so!
So what worried me was the FC at 28 .. when I had only intended to shock to 22. What else could cause this, beyond my pool size actually being much less than I had previously calculated? I will say it has been quite cloudy today -- it is bright but direct sunlight has only peeked out a few times.