An instrument used to determine the pressure difference between two points, usually by measuring the height difference of liquid in two vertical legs. There are many variations of instrument, and all consist of a vertical leg up which a liquid moves in proportion to the pressure applied.
A gas-filled manometer is used to measure the pressure of process gases and consists of a sealed container at some pressure and the other end is attached to a process via a U-tube containing a liquid. The pressure of the process is determined from the levels of manometer liquid in the U-tube and the pressure in the sealed vessel.
A differential manometer is used to measure the pressure of a process fluid and measures the difference in pressure between two points in a process, such as the pressure drop across a heat exchanger. An inverted manometer is essentially an inverted differential manometer in which the process fluid is used to measure its own pressure. A head of trapped gas (usually air) is used in the device.
Single leg manometers replace the U-tube containing a manometer liquid, with a large sump containing the liquid that extends up one leg when a differential pressure is applied. An inclined leg manometer is a variation of the single leg manometer but with the protruding leg from the sump inclined at some angle. This provides a magnification of the movement of the liquid along the leg to an applied pressure and is particularly useful for measuring small pressures.