What device is used to prevent water from flowing back into a main?

A backflow preventer is specifically designed to prevent the reverse flow of water in a plumbing system, ensuring that contaminated water does not flow back into the main water supply. It works by using one or more check valves that are set to open only in one direction, allowing water to flow through the system while blocking any reverse flow. This is critical for maintaining clean and safe drinking water, especially in systems where backflow could occur due to pressure changes or other disruptions.

In contrast, other devices listed serve different purposes. For instance, a pressure valve regulates the pressure within a system, ensuring it remains within an acceptable range, but does not address backflow concerns directly. A check valve, while it may seem similar, is a general term for a device that prevents backflow in various contexts but does not specifically cater to water supply systems with contaminated sources involved. A flow meter measures the quantity of water flowing through a system, rather than controlling the flow direction or preventing backflow. Thus, the backflow preventer stands out as the correct device for the question at hand.