This 6" double check detector backflow preventer assembly is designed to prevent the reverse flow of fire protection substances and water of non-potable quality, but not toxic, from being pumped or siphoned into the potable water supply due to backsiphonage and or backpressure in accordance with national plumbing codes and water authority requirements. It is ideal for protection at cross-connections identified as non-health hazards such as on fire protection systems and from other contaminants found at the service line entrance. This compact modular design with a single access cover for easy maintenance is approved for horizontal and vertical flow up installation. It consists of a light weight stainless steel body and sleeve, domestic outside stem and yoke resilient seated gate valves, captured spring check valves, a gallons per minute metered bypass to detect underground leaks and unauthorized water use, and four test cocks.