The Altimeters used in Service Operations have Either Mechanical or Electrical Systems
An Altimeter is a device that measures the height of the ground or any object, such as an aeroplane. The pressure altimeter, also known as an aneroid barometer, measures atmospheric pressure to approximate altitude above sea level, while the radio altimeter measures absolute altitude (distance above land or water) based on the time it takes for a radio wave signal to travel from an aeroplane, a weather balloon, or a spacecraft to the ground and back.
The average air pressure drops linearly with height, according to the pressure Altimeter. The illustration depicts a common pressure altimeter. The instrument is housed in a casing with an air pressure inlet at the back that connects it to the outside of the plane. Near the intake are two or more aneroid capsules, which are thin corrugated iron bellows from which air has been drained. When the outside air pressure drops (as in rising), these capsules expand, and when the outside air pressure rises, they contract (as in descending). The expansion or contraction of the aneroid capsules is transformed to the movement of pointers on a dial by a mechanical arrangement of sector gears, pinion, backlash spring, and crankshaft.
The graduated scale dial is labelled in metres or feet, and a series of gear-driven pointers that look like clock hands can be used to indicate altitude in hundreds, thousands, or tens of thousands of feet. The air pressure is measured in millibars on the barometer scale dial (mb). A pressure altimeter must be adjusted with a barosetting knob to compensate for modest variations in barometric pressure induced by changes in local weather since atmospheric pressure is measured relative to sea level.
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