Favre-Leuba Bivouac 9000 with altimeter
An altimeter is a mechanical device that uses barometric pressure to display altitude on a calibrated scale. Most use a sealed drum with a geared hand. Because barometric pressure can change over time, an altimeter must be set at a known altitude before being used and must be re-set daily. Altimeters have a maximum altitude specification varying from 4,000 to over 30,000 meters.