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U tube manometer
A device for measuring gauge pressure or differential pressure by means of a U shaped transparent tube partly filled with a liquid, commonly water; a small pressure above or below atmospheric is measured by connecting one leg of the U to the pressurized space and observing the height of liquid while the other leg is open to the atmosphere; a small differential pressure may be measured by connecting both legs to pressurized space for example, high and low pressure regions across an orifice or venturi.