How does a digital barometer work?

A: A barometer is a device to measure pressure. From the 17th to the 19th centuries, most designs had the same working principle: air pressure pushed down on an open column of fluid, which was allowed to move into a second column with a closed top. The higher the pressure, the higher the fluid would rise in the closed column.

The aneroid barometer was invented in 1844: air acted on a capsule that was kept from collapsing by a spring. As the capsule contracted under pressure, it moved small levers attached to a needle on a gauge.

Today, devices like smartphones use digital barometers. Here, a micro-electro-mechanical system (MEMS) converts changes in pressure to electronic signals. One way is to use the piezoresistive effect: when pressure is applied on a conductor, its resistivity changes. Another way is to expose one plate of a capacitor to air pressure: as the pressure changes, the inter-plate distance varies.

The rest of the MEMS circuitry amplifies the changed resistance or capacitance signal, converts it into a pressure reading, and displays it on a screen.