Series 757DCDA Double Check Detector Assemblies are used to prevent backflow of non-health hazard pollutants that are objectionable but not toxic, from entering the potable water supply system. The 757DCDA may be installed under continuous pressure service and may be subjected to backpressure and backsiphonage. Series 757DCDA is used primarily on fire line sprinkler systems when it is necessary to monitor unauthorized use of water.