This doesn't appear to be your first
Overnight Chlorine Loss Test, but the testing process may still be a bit new overall, so we want to ensure you are set and any questions are addressed. When you get prepared for the
Overnight Chlorine Loss Test, it would be good for you to see a good/solid FC reading of some sort to start with since previously you had no chlorine showing at all. I suspect this evening you'll probably add that first gallon of chlorine which "should" get your FC up to around 7 ppm. But it may dissipate quickly from organics, so don't be surprised if you check again an hour later and it did drop. Regardless, at some point this evening you will add the last chorine dosage and get prepared for the first (pre-bedtime) FC test for the OCLT. Ideally, I would like for you to start the OCLT with a first FC test somewhere around 5-8 ppm zone.
While it's pointed out on the
Overnight Chlorine Loss Test, I always remind members to ensure the SWG is OFF or set to zero output well in advance before taking the first FC reading at night. In fact, I would ensure that any liquid chlorine added was done at least 30 minutes prior to taking that first sample for testing to ensure it mixed well. IMO, it's best to just leave the pump running all night (low is fine). If you must shut off the pump after the first FC test, be sure to turn it back on and let the water mix at least 30 min in the morning before grabbing the 2nd FC test. Grab that morning water sample before the sun hits the water.
Hope that helps get you started. If you have any questions at all about the
Overnight Chlorine Loss Test let us know. Good luck, and we'll watch to see what your two FC numbers (tonight and tomorrow morning) bring.