OTO is not super accurate, but it covers the range from 0 - 50 ppm, and it's pretty much goof proof. The FAS-DPD test covers the same range, but it *can* be screwed up. The DPD color match (K2005) covers the range from 0 - 5 ppm, (0 - 10 ppm claimed, but few people can read the difference between a 5 ppm red and a 10 ppm red!) and there are multiple ways it can go wrong.