I am having trouble with the FC test lately. I was shocking the pool so I was doing a lot of FC testing. Sometimes after I added the powder, the water did not turn red. It was pinkish and I expected it to be red since chlorine should be around 20. I dumbed it out, rinse the vial with pool water and re-did the process, then the water turned red. Wonder what I did wrong?
Another problem or related problem is the test result. I would do the FC test in the morning and FC was 2 or 3ppm less than it was the night before. So I thought I had to continue shocking. Before I added more chlorine, I re-did the test from the same batch of pool water and the test results would be different. One morning I actually went to the pool and got a different batch of water from the other side of the pool and the FC test results were different too. Now I am beginning to wonder if there is a water circulation problem in the pool. What do you think?