An instrument used to determine the differential pressure between two points such as across an orifice plate in a pipeline. It consists of a U-tube of glass or clear plastic with the top of each vertical leg being attached to the tapping points across the flow (p. 395) meter. The U-tube contains a manometric fluid that is opaque, unreactive, has a low volatility, and has a higher density than the process fluid. The difference in levels between the two vertical legs of the manometer provides a measure of the differential pressure. It is also known as a differential manometer.