I had an interesting opportunity yesterday. The pool builder sent out a tech for routine service under the first year protection plan & I got to watch him test my water. He was packing a Taylor K-2006 & knew how to use it. All the parameters for pH, TA and CYA matched mine perfectly, but the FC was off. We both used the DPD method of testing FC.
At 6:00 a.m. that morning, I tested and added enough bleach to bring it up to 5.5 PPM.
Pool Guy showed up and tested around 3:00 p.m. after the pool baked in the sun, so I expected some loss. He got 3.0 PPM.
I did not do anything after he left until 6:00 a.m. today. When I tested this morning, I got 4.0 PPM
So where did the extra chlorine come from?
Some notable differences between how we tested: I test early in the morning at the deep end, pulling water from probably 6-8" below the surface. Pool Guy tested in the blistering afternoon sun in the shallow end. I did not notice how deep he sampled.
Could there really be a 1 PPM difference within my pool, based on sample location?