Manometer
Definition
A U-shaped tube of liquid that measures pressure by the height difference between its two columns.
Unit: Pa (via mm Hg or cm H₂O)
Technical definition
A device measuring pressure via hydrostatic balance: ΔP = ρgh, where h is the liquid column height difference. Open manometers reference atmosphere; closed ones measure absolute pressure. Mercury (dense) for high pressures, water/oil for small ones.
Example
Blood pressure cuffs were calibrated against mercury manometers — ‘120/80 mmHg’ literally means millimeters of mercury column height.
Common confusion
A manometer measures pressure difference, not absolute pressure (unless one side is vacuum). An open U-tube reads gauge pressure — you must add atmospheric pressure for absolute.