I had my FC level up to 30 ppm for a shock. At the end of a full day of not adding any chlorine -- which was a full day of sun where the air temperature got up to around 90 -- the FC was down to 15 ppm. I read that the chlorine level drops much more quickly when it's at high levels, but I was wondering if a 15 ppm drop is still normal in this situation. I don't have a SWG, there was no tablet feeder in the pool, my CYA level is around 60, and the pool water temp was in the high 70s to perhaps the very low 80s.
The 15 ppm level was around dusk, so I did an OCLT by measuring the level again the next morning about 30 minutes before sunrise (which was a little under 9 hours later.) To measure the higher Cl levels, I diluted the water with half bottled water to do the chlorine tests, and then doubled the results. It still showed 15 ppm, and no measurable CC (using a Taylor test kit.) As far as I've read, this should indicate that there was no algae in the pool, but I just want to confirm that the 15 ppm drop is normal in this situation.
To summarize, my evening measurements at about 8:45 pm where the same as my measurements at 5:30 am: