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  • The altimeter is an instrument that measures the height of an aircraft above a given pressure level.
  •  Pressure levels are discussed later in detail. Since the altimeter is the only instrument that is capable of indicating altitude, this is one of the most vital instruments installed in the aircraft.
  •  To use the altimeter effectively, the pilot must understand the operation of the instrument, as well as the errors associated with the altimeter and how each effect the indication.
  • 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.

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