Manometers measure pressure by comparing it to the gravitational force exerted on a column of liquid. There are several types of manometers: 1) A U-tube manometer is the simplest, using two legs of equal area connected by a tube filled with a liquid like mercury to measure gas or liquid pressure. 2) An enlarged manometer has one leg with a larger area than the other and a float to indicate the pressure reading. 3) A well manometer has one simple tube leg and one enlarged well leg, providing greater sensitivity with small liquid level displacements in the well.