The OTO test that comes with the TF100 gets darker and darker as FC levels increase.
Last week, my FC was 10 as tested with the FAD-DPD test in my TF-100, but the OTO test looked like it was around 5. Any idea why that would be? I'm thinking of getting a K-2000 to replace the K-1000 as my CYA level of 80 for my SWG pool has a target FC of 6, but maybe I'm not understanding my OTO test.