Up to about a week ago I've only had OTO and FC test strips for chlorine testing. Worked fine all last year, but my dad's pool has to be run at much higher FC levels due to stabilizer overload, so his OTO kit wasn't cutting it. Ordered FAS/DPD for him from Dave and thought I'd get some too while I was at it, just to see what I've been missing
Got home the afternoon the stuff came in, kids had been swimming all afternoon with the pump off (I've been running it at night only). Water was crystal clear. Took a sample from my usual spot, about a 1' under water at the skimmer. Found out I had 5 ppm TC, with 4.5 FC and .5 CC. Not bad I thought, first time I've needed to shock all year. Shocked to 12 ppm that evening, circulated, tested the next morning and got 14.5 ppm FC, no CC. I figured either my 6% bleach was stronger or I was off on calculating my pool volume (I was, I'll start a new thread to explain). I planned to let the FC fall to match my 25 ppm CYA, so I added no more bleach that night.
The next day I tested again in the early evening to see how much my FC had dropped over night and through the next day of full sun. I still had 12 ppm TC, but again I had .5 CC. I was confused. I added twice my normal bleach that night, tested the next morning, and was back at a higher all FC count the next morning (can't recall exact numbers, my sheets at home). I let it drop again overnight, and went to test the next early evening. Again, the pool had been sitting all day in full sun with the pump off, but no one had been swimming in it. Ran the test. 4 ppm FC and 2 ppm CC! No way! I figured it had to be from sitting in full sun without circulation, and the top 1' or so of water was taking the full brunt of the sun and organics falling into the pool (dust and pollen, bugs that float till they die and sink, leaves, etc). I turned the pump on and let it run overnight, without adding any bleach. The next day I had 5 ppm FC, and no CC.
So, I learned I guess it matters when you test and after what specific conditions the pools previously encountered (swimmers vs no swimmers, pump running vs pump not running, etc) to get a true picture of the status of your TC. If I hadn't stumbled onto this, I would have been thinking I was fighting something and shocking every day. Now I know I just need to ensure full circulation and well mixed water prior to testing