The control sample that came with my TF kit seems to test closer to 30ppm. I can clearly see the dot still at 50ppm. On the plus side, the kit allowed me to determine that my CYA is the cause of my algae issues, as it is off the charts. I will be draining to clean some existing staining anyway, but I was a bit concerned that my very first test (using the control sample) seemed so far off. Is this common?