The altimeter simply measures the height or altitude of the aircraft above mean sea level (MSL), which can be corrected for atmospheric pressure differences. Inside of the altimeter are stacked and sealed aneroid wafers that a set to an internal pressure of 29.92 inches of mercury.
These wafers are designed to collapse and expand. A high static pressure collapses the wafers showing a decrease in altitude, whereas a low static pressure allows the wafers to expand showing an increase in altitude, allowing the needles on the face of the instrument to rotate.
The short thick needle represents thousands of feet in 1,000 feet increments. The longer needle represents hundreds of feet. The long needle with the triangle on top (left) or short thin needle (right) represents 10,000 feet.
Different face designs
Prior to each flight the pilot needs to set the altimeter to the proper barometric pressure reported at the airport transmitted from either the air traffic control tower, Flight service station (FSS), ATIS, AWOS, or the ASOS.
To set the altimeter to the proper barometric pressure, simple turn the knob at the bottom of the instrument to the proper barometric pressure located in the Kollsman window.
Instrument Errors
When the altimeter is set it should read within 75 feet from the surveyed field elevation, if not it needs to be recalibrated by a certified instrument mechanic.
Remember:
The short needle represents thousands of feet in 1,000 feet increments. The longer needle represents hundreds of feet.
Standard pressure is 29.92.
Instrument Errors - when the altimeter is set it should read within 75 feet from the surveyed field elevation.
References:
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