I'm in the final phases of clearing our rain-mudded pool. It is mostly clear (slight cloudiness to the water in the deep end) and I wanted to pass the OCLT test and call it done since we are having a pool party Saturday. Our CYA had been at 30 for part of the swamp session but I was working to raise it to closer to 40-50 - we are in a full sun, extreme heat location and 30 did not seem optimal for us last year. So part of my shocking process was done targeting a CYA of 30 and part of it was done at 50 as of a positive test result of CYA at 50 on Saturday.
Two nights ago, I tested just after sundown and found the FC at 9.5, CC was 0. Did not add any chlorine, was going to see if the previous 11 days at shock levels of FC was enough and hadn't attempted the OCLT until now. Got up early and my husband tested that morning and found FC of 11.5, CC 0. We assumed tester error (mine or his, who knows) to explain the rise in FC even though we didn't add any and hadn't floated any pucks in the ducks or anything (now that our CYA is where we want it, ducks are grounded).
So last night, we test together after work so there are no discrepancies . Came up with 7.5 FC, 0 CC. (I thought that jived with a loss of 2ppm during the day if in fact my test the night before was correct but DH's that morning was off, but being a good wife, I didn't rub it in ... too much). Especially since I don't know definitively that I'm right... I try not to let that stop me.
We decided to bring the FC up to the shock level of 20 based on our new CYA of 50. We used the newer pool calc and calculated our 35000 gal pool to need 5.5 jugs of 8.25% 121 oz. We didn't have 5.5 jugs on hand, we had 4 available, so put them in. Messing with the data in the calculator, it seemed that adding four jugs should take our pool to 16.75 FC. We thought if we pass OCLT, it wouldn't matter terribly if we are at 16.75 or 20. I have experienced that the cloudiness left behind can take a long time to completely clear with our sand filter and large pool.
Weeeeelll, when DH tested this morning, he found FC of 21.5! What the what?! Two days where the FC readings are higher than we thought they should be. We are baffled. We have a few theories and would love your input and ideas:
A. Our pool volume is smaller than we thought. This is possible, as various experienced pool guys have looked at it and guessed anywhere from 30-40K gallons, and I've tried to measure and calculate the volume myself but with our steeply pitched deep end it's a little difficult for my remedial geometry skills and have come up with somewhere in the neighborhood of 33K (picture me lying flat on the end of the diving board with a tape measure). Playing with Pool Calc I figured that four jugs of our bleach would take a 23K pool from 12.5 to 21.5 FC. Not sure I would buy that our pool is only 23K.
B. Our jugs of laundry chlorine from Wal-Mart have super powers and/or are higher concentration than listed
C. Testing errors, although I don't think this is the case. We tested together last night and are confident that we are following the instructions well WITH THE EXCEPTION of adding the powder. I'd read here that one scoop was sufficient if the water turns hot pink and so would conserve the powder, and so we've added one heaping spoonful of the powder instead of 2, and it's REALLY pink, and we add the drops from there. Using the 10ml sample method.
I would appreciate any thoughts the people here can provide. I know you are smarties! Thanks!