Notice how the static pressure is introduced into the
rear of the sealed altimeter case. The altimeter’s
outer chamber is sealed, which allows the static
pressure to surround the aneroid wafers. If the static
pressure is higher than the pressure in the aneroid
wafers (29.92 "Hg), then the wafers are compressed
until the pressure inside the wafers is equal to the
surrounding static pressure. Conversely, if the static
pressure is less than the pressure inside of the
wafers, the wafers are able to expand which
increases the volume. The expansion and
contraction of the wafers moves the mechanical
linkage, which drives the needles on the face of the
ASI.