Water level meter 100mm" refers to a type of electronic device that measures the depth of water in wells, boreholes, and other water bodies, with the "100mm" likely indicating the diameter of the probe rather than the length. These meters use a stainless steel or corrosion-resistant probe attached to a graduated cable and a reel system; when the probe touches the water, an audible and visual alarm activates, allowing for accurate water level readings. They are essential for monitoring groundwater levels in hydrology, environmental protection, hydropower projects, and water resource management.