The altimeter measures height above a given pressure level. When set to 30.10 inches and indicating 5,000 feet, for example, the altimeter is indicating a height of 5,000 feet above the level where we would expect to encounter an ambient pressure of 30.10 inches.
The altimeter functions by utilizing the fact that air density decreases with increasing altitude. As a result, altitude can be inferred simply be measuring ambient pressure. The operation of the altimeter is based on an aneroid barometer, which is a barometer that operates without liquid, aneroid literally meaning "without water". The barometer consists of a small metal diaphragm on which atmospheric pressure acts.
For an altimeter, a much greater range of pressures must be detected. So, it is equipped with a stack of these aneroid diaphragms, often called aneroid wafers, giving it the ability to expand a great deal and detect a large range of atmospheric pressures.
The stack of aneroid wafers is placed inside the altimeter case and evacuated to an internal pressure of 29.92 inches and sealed. Static pressure from the airplane's static line is then routed into the altimeter case. The stack then expands and contracts in response to varying static pressure.
If the static pressure is greater than 29.92 inches, the stack compresses until its internal pressure equals ambient pressure. Conversely, if the pressure is lower than 29.92 inches, the stack expands until its internal pressure is equal to ambient pressure. A mechanical linkage connects translates this expansion and contraction to an altimeter indication.
A sensitive altimeter refers to an altimeter with an adjustable barometric scale allowing the pilot to set a reference pressure from which height is measured. Setting the altimeter correct the altimeter indication for nonstandard pressure.
Types of Altitude
Indicated Altitude: read directly from the altimeter when it is set to the current altimeter setting.
Pressure Altitude: indicated when the altimeter is set to 29.92. This is the altitude above the standard datum plane, which is a theoretical plane where air pressure corrected to 15 degrees Celsius is equal to 29.92 inches. Pressure altitude is used to compute performance data, such as density altitude, true altitude, and true airspeed.
Density Altitude: pressure altitude corrected for nonstandard temperature. In standard conditions, pressure and density altitudes are the same. If temperature is above standard, then density altitude is higher than pressure altitude. At temperatures below standard, density altitude is lower than pressure altitude.
Absolute altitude: the vertical distance of the aircraft above the terrain, or AGL height.
True Altitude: vertical distance of the aircraft above sea level, or MSL height. Airport, terrain, and obstacles elevations on aeronautical charts are true altitudes.