|A barometer is an instrument used to measure changes in atmospheric pressure. There are two general types of barometer: a mercury barometer and an aneroid barometer. Barometric pressure is most often recorded in inches of mercury, millibars, or kilopascals. Barometric sensors for weather stations and stand-alone sensors usually use an aneroid barometer to measure the air pressure. Remote sensors then relay the information to the base display.
A mercury barometer uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. Although mercury barometers are very accurate, practicality has led to increased reliance on aneroid barometers.
An aneroid barometer uses the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid capsule is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches of mercury and millibars. An aneroid barometer is also called a holosteric barometer.