Double-check backflow detectors monitor leaks and system usage and have two check valves to stop the back-siphonage in medium-risk applications, such as fire protection and commercial water supply systems. Sudden drops in system pressure or downstream stoppages (back-siphonage) can send media flowing backward through the line, generating enough mechanical force to actuate the check valve's disc, block the flow path, and prevent contaminants from entering the system. Along with the two check valves, these preventers also consist of shut-off valves for repair and maintenance if one or both of the check valves fail and test ports to check system media. Detector units can be paired with alarms, sensors, and meters to send alerts, identify backflow, and measure flow rates.