Backflow preventers ensure fluids move in one direction and stop contaminants from entering the system due to back-siphonage or -pressure. Back-siphonage occurs during sudden pressure reduction. Back-pressure occurs when inlet pressure is lower than outlet pressure. Reduced-pressure zone backflow preventers rely on the low-pressure area between its two check valves to prevent back-siphonage and -pressure in high-hazard applications such as medical facilities. Double-check backflow preventers rely on the mechanical force of backward-flowing media to stop back-siphonage in medium-hazard applications such as irrigation and sprinkler systems. Dual check backflow preventers rely on the mechanical force of backward-flowing media to stop back-siphonage in low-hazard applications such as commercial or residential water supply systems. Combination fill backflow preventers ensure liquids enter the valve chamber at the desired level and stop backflow from contaminating the source.