A stack of sealed aneroid wafers co

A stack of sealed aneroid wafers comprise the main
component of the altimeter. An aneroid wafer is a
sealed wafer that is evacuated to an internal
pressure of 29.92 inches of mercury (29.92 "Hg).
These wafers are free to expand and contract with
changes to the static pressure. A higher static
pressure presses down on the wafers and causes
them to collapse. A lower static pressure (less than
29.92 "Hg) allows the wafers to expand. A
mechanical linkage connects the wafer movement
to the needles on the indicator face, which translates
compression of the wafers into a decrease in altitude
and translates an expansion of the wafers into an
increase in altitude.