Test results at different times

[Rangeball]

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

 

[KurtV]

Interesting. I've never noticed the phenomenon you describe because I read once that you should only test after the water's been circulating for a while, that you should collect the sample as far away from returns as you can, and that you should get the sample from as deep in the pool as you can reach (18 inches or so) and I've always followed that advice.

Out of curiosity I'll try to replicate your results.

 

[JasonLion]

A reading of CC 0.5 can normally be ignored. Any time someone is swimming in the pool or other organics are falling in the pool there will be a little CC and even a very little will test as 0.5 using the standard test where one drop is 0.5. Sunlight and chlorine will break down the CC over time, usually faster than it accumulates. If CC tests higher than 0.5 then shock the pool.

 

[chem geek]

The 0.5 ppm CC is especially meaningless if using the 10 ml sample where 1 drop = 0.5 ppm. Odds are that the CCs are actually less than 0.5 ppm if it takes one drop to clear them. With a 25 ml sample where 1 drop = 0.2 ppm, I often find that I have a very faint pink color when testing for CCs and that one drop of titrant clears it, but it's likely to be less than 0.2 ppm so I never worry about it. Even if I measured 0.4 ppm CC (two drops in a 25 ml sample) I still wouldn't worry about it.

 

[Rangeball]

I agree. However, since I hadn't shocked since opening my pool, I figured it was time. Probably just wasted $5 worth of bleach

More interestingly was the 2 ppm combined chlorine result when uncirculated in full sun, then no CC after a full night circulating with no bleach added. This was the best learning experience that could save others headaches if they test in a similar situation.

I think it lends credence to CYA and depth protecting FC better at deeper levels?