There are different types of pressure measuring devices that use various techniques. Absolute pressure is measured relative to a vacuum while gauge pressure is measured relative to atmospheric pressure. Common pressure measuring devices include manometers, barometers, piezometers, and differential manometers. Manometers use a column of liquid, such as mercury, to measure pressure based on the height of the liquid. Barometers measure atmospheric pressure using a mercury-filled glass tube. Piezometers and u-tube manometers also use columns of liquid but have advantages over simple piezometers. Differential and inclined manometers are used to measure small pressure differences more accurately. The specific type of liquid used is important, with mercury being heavier than water, allowing for smaller,