The pressure-sensitive component of an aneroid barometer is a row of flexible, evacuated cells, often made of a beryllium-copper alloy, supported by internal springs. Small changes in atmospheric pressure cause the assembly of cells to expand or contract which, in turn, deflects the pointer of a semicircular gauge. The device illustrated in the graphic represents an idealization of an older design, whose mode of operation is easier to understand. Modern aneroid barometers are much more compact, with their components concealed.
Standard atmospheric pressure at sea level is taken as 1013.2 millibars (mb), equivalent to 101.320 kPa in SI units. Barometric readings provide important data for weather prediction, in conjunction with temperature, wind velocity, and so on. The markings on the periphery of the dial give a rough indication of the expected weather—to be regarded, however, "with a grain of salt"!
Atmospheric pressure also depends on elevation above sea level, according to the approximate relation , where is the altitude in meters. It is assumed here that aneroid barometers are available for altitudes in intervals of 500 m.