When measuring pressurized air in other contexts, the choices include gauge pressure (air pressure compared to atmospheric pressure) and differential air pressure (pressure difference between points). Barometric pressure sensors instead measure absolute air pressure. This is the air’s pressure in relation to a perfect vacuum.
For much of our history, barometers depended on the behavior of mercury or some other liquid in response to changing air pressure. The aneroid barometer, whose name refers to the absence of liquid, was invented in 1844. It uses deformation of metal instead.
In the aneroid barometer, a partially evacuated metal cell is subjected to pressure from the atmosphere. As the pressure increases or decreases, the cell contracts or expands. This movement is translated and amplified, via an opposing spring, a system of levers and a pointer, to register a reading on the barometer’s dial.
Modern barometric pressure sensors are, in a sense, aneroid barometers, as their method of operation does not involve liquid. In construction and appearance, though, they are very different from their predecessors-often using the latest microelectromechanical system (MEMS) technology.
In common with the original aneroid barometers, they detect atmospheric pressure via its effect on a flexible structure-in this case a membrane or diaphragm. The degree of deformation in the membrane is proportional to the pressure and is translated into an electrical signal-hence the sensors are sometimes referred to as pressure sensors. These small pressure sensors are built around one of two main measurement approaches: resistive or capacitive.
A resistive barometric pressure sensor is also known as a piezoresistive sensor or a strain gauge. One face of its diaphragm is in contact with the atmosphere. The other face has strain gauges attached to it.
Increasing pressure deforms both the diaphragm and the strain gauges. Deformation of the strain gauge material alters its resistance, due to the piezoresistive effect, and the sensor reflects this change in its electrical signal.