Detector check valve is designed for installation on portable water connections in fire sprinkler systems to protect against both backsiphonage and backpressure of polluted water into the portable water supply. The main body of the valve is made of ductile iron with fusion epoxy coating. The springs are made of stainless steel material. It detects any leakage or unauthorized use of water from fire or automatic sprinkler systems on an application where no backflow assemblies are required. During time of minimal water flow, the valve clapper remains closed so that the water flows through the by-pass meter. When a major water flow is required, the water pressure will fully open the clapper to maximize water flow.