HI-720-11 is a high-quality set of calibration standards that allows users to validate the Calcium Hardness Checker® HC – HI-720. The HI-720-11 includes one sealed cuvette for zero ppm CaCO3 and one cuvette that is tinted to an equivalent 1.50 ± 0.25 ppm CaCO3 sample @ 25°C. The calibration check set is produced using reagent grade chemicals in a temperature controlled environment.