A manometer measures the difference between two different points of pressure. A manometer works by balancing the weight of a column of fluid between the two points of interest.
Different fluids are used in the fluid column depending on the nature of the pressure difference. If the different is large, then a heavy fluid, such as mercury may be used. If the difference is relatively small, such as that measured in a wind tunnel, then a lighter fluid, such as water is used.
There are many different designs of manometers, including digital models. The simplest design uses a sealed length of glass tubing bent into a U-shape. The tube is then filled with the liquid to the sealed end so that no air remains trapped in it. The open end of the tube is then attached to the system whose pressure is to be measured. Gas will exert pressure on the open end with the result that the liquid will be at a higher level in the sealed end than in the open end. The difference between the two levels is an indication of the pressure of the gas in the system.
Manometers are used in a variety of laboratory applications to measure the pressure of gases and liquids and also in the field of medicine.