What device is used to prevent backflow in plumbing systems?

The backflow preventer is the correct device used to prevent backflow in plumbing systems. Its primary function is to allow water to flow in only one direction and to block any backflow that could contaminate the water supply. This is crucial in maintaining the safety and quality of drinking water and preventing cross-contamination between potable water and potentially harmful substances.

Check valves are similar devices that allow flow in one direction but are not specifically designed to handle the unique pressures and conditions associated with backflow prevention. A flow switch is used to monitor the flow rate of water in a system and does not play a role in preventing backflow. The pressure regulator is used to control and maintain water pressure within a system but does not specifically address backflow issues. Therefore, while other devices have their respective functions, the backflow preventer is specially constructed to address the key concerns of backflow in plumbing systems.