An altimeter, also referred to as the aneroid barometer, is an important instrument for proper operation of the pitot-static system and navigation of an aircraft. Without proper altimeter functionality, an aircraft would not be able to measure altitude, which is necessary for safe flight. Because altitude varies in different settings, it is extremely important to ensure that the altimeter is properly measuring the correct amount of atmospheric pressure. In order to ensure this, altimeters not only need to be set before every fight, but also calibrated frequently.
Essentially, an altimeter is a type of pressure gauge. The inside of the altimeter consists of a sealed casing which contains a stack of sealed aneroid diaphragms, also referred to as wafers. These wafers are calibrated to a standard atmospheric pressure and are responsible for reading the intake of pressure. Calibration of an altimeter ensures that pressure measurements from the wafers are accurate and reliable. During calibration, an accredited calibration laboratory will apply known pressure to the altimeter to check that the instrument is indicating pressure levels that correspond with the International Standard Atmosphere (ISA). Similarly to other pressure gauges, if calibration of an altimeter is ignored, high chances of inaccurate results, unreliability and extreme safety risks are likely.
It’s important to note that the calibration of an altimeter before each flight is much different than actual instrument calibration. The instrument itself should be calibrated regularly by an accredited calibration lab to ensure that the absolute altitude measurements are calculating accurately. Calibration of an altimeter is essential to assure proper functionality of the pitot-static system as a whole.