How does an altimeter work?
An aneroid (barometric) altimeter shows a plane’s altitude above the earth by measuring the atmospheric pressure, which lessens as the altitude increases.
An altimeter is an instrument that shows a pilot just how high his airplane is above the earth. Most altimeters are aneroid (without air) barometers, made to read in feet of altitude instead of inches of mercury.
Like all barometers, the aneroid altimeter shows changes in air pressure, which lessens as the altitude increases.
Behind the dial is a small metal can called a diaphragm from which most of the air has been removed. As the airplane goes up, the air pressing against the diaphragm lessens, allowing the diaphragm to expand.
The movement of the sides of the diaphragm move the hands on the face of the altimeter.
Still another kind of altimeter is called a radio or radar altimeter. It measures altitude by sending radio signals to the ground.
It tells the altitude by measuring the time needed for the radio signals to reach the ground and bounce back to the plane. – Dick Rogers