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A manometer is a device for measuring pressure. This could be atmospheric pressure, or the pressure of a gas or liquid. There are many types of pressure manometer devices, including mercurial manometers and digital manometers.
A simple mercury manometer can be made using a length of glass tubing. The tubing is sealed at one end, and then mercury is placed in the tube such that no air remains at the sealed end of the tube. The tube is then bent into a U shape, with the sealed end at the top of one branch of the U, and the open end at the top of the other branch of the U.
As the mercury in the tube moves to the bottom of the U shape, a vacuum results in the sealed end of the tube. At this end of the tube, there is no air to produce a downward pressure on the mercury. The open end of the tube can then be exposed to the air or gas, the pressure of which is to be measured. The difference in the height of the mercury in the two sides of the manometer provides the measurement that can enable the pressure of the gas to be determined.
Because this way of measuring pressure was used in the early days of physics research, pressure was specified in units of "millimeters of mercury" (mm Hg). A pressure measurement of 1 mm Hg refers to the pressure that will result in a difference of one millimeter in a mercury manometer tube. Modern physicists have a new name for the measurement for pressure, called the torr. One torr of pressure is equal to 1 mm Hg.
In addition to taking pressure measurements, a manometer may also be used to measure the speed at which a stream of air is flowing. Moving air creates a region of lower pressure than normal atmospheric air. The open end of a gas manometer may be placed into an area of airflow in order to determine the speed of the moving air by measuring how the pressure changes.
For modern applications of pressure measurement, a digital manometer is often used. In medicine, manometers may be used to measure blood pressure, and tiny manometers may be used to monitor the pressure in a patient’s airways. Manometers are also used in many industrial settings, in avionics, and in weather forecasting.