GB/T 6909-2008 is a standard for the determination of the Hardness of boiler water and chilled down water. In boilers and chilled down systems, the Hardness of water affects the operation of equipment. The standard covers the determination methods of Hardness of different types of water samples, including the introduction of the determination principle. Detail the selection and preparation of required reagents and instruments. Standardize the process of water sample collection and pretreatment, and clarify the determination operation steps, such as the control of the titration process. The calculation, representation and precision requirements of the results are also specified. Provide a scientific process for the accurate determination of water Hardness, which helps to prevent scaling and corrosion of equipment and ensure the safe and efficient operation of boilers and chilled down systems.