An instrument for measuring pressure relative to another pressure. A liquid-column manometer uses hydrostatic pressure; a digital manometer uses a pressure transducer.
The appeal of a liquid-column manometer is that a pressure measurement becomes a visible length measurement: mass density and gravitational acceleration supply the conversion. It can compare two pressure tappings in a flowing fluid, even though the measuring liquid itself must settle into hydrostatic equilibrium. The reference connection determines which pressure difference is being measured.
New to topics? Read the docs here!