LF5D series differential pressure switch / pressure control, normally use in water, oil series control difference of the outlet tube and inlet tube. One typical application is , put valve install  in the next to water (oil) system pumps by-pass valve line ,when the pressure difference rise (or reduce) of two side of the systems tube exceed the setting data of the pressure control, so the valve open big (or close small), so  the system tubes two sides pressure difference is reduce, to reach the system normal circulate.