What mechanism does the altimeter use to gauge altitude?

The altimeter uses a sealed stack of aneroid wafers to gauge altitude. These aneroid wafers are sensitive to changes in atmospheric pressure. As an aircraft ascends, the atmospheric pressure decreases; this change causes the wafers to expand, which in turn moves a needle on the altimeter dial, indicating an increase in altitude. Conversely, as the aircraft descends and the atmospheric pressure increases, the wafers contract, resulting in the needle moving in the opposite direction.

This mechanical relationship between pressure change and wafer movement is fundamental to the operation of traditional altimeters, making them effective for measuring altitude based on the principles of barometric pressure. Other mechanisms mentioned, such as static pressure sensors and fluid-based sensors, do play roles in other instruments but are not the primary components of a conventional altimeter. Digital sensors may also be used in modern avionics but are not typical of the classic altimeter design characterized by aneroid wafers.