A differential pressure switch for water is a device used to detect changes in the pressure of a liquid or gas. It is typically used in water systems to detect changes in pressure between two points, such as between the inlet and outlet of a pump. The switch is designed to detect changes in pressure that are outside of a predetermined range, and when this happens, it triggers an alarm or shuts off the pump. This helps to protect the system from damage caused by excessive pressure. Differential pressure switches are also used in other applications, such as in HVAC systems to detect changes in air pressure, and in oil and gas systems to detect changes in pressure between two points. They are an important part of any system that requires pressure regulation, and can help to ensure the safety and efficiency of the system.