Pressure altimeters are devices primarily used in planes to measure altitudes. A pressure altimeter is essentially just a barometer that has been calibrated to measure the current altitude of a plane rather than barometric pressure. Altimeters usually must be set by the pilot before taking off and sometimes adjusted in flight.
Barometers measure air pressure using mercury, air, or sometimes water. Air pressure, also called barometric pressure, is simply the pressure of the air at a given level. Air particles press on the air and the ground underneath them, creating pressure. This is why the barometric pressure at sea level is higher than it is in the mountains.
A pressure altimeter works by measuring the air pressure of its present location and subtracting that from the pressure at sea level. Barometric pressure is not a constant, so the pilot will have to set the sea level pressure of the altimeter before take off. There are two ways to set the pressure: the first is in the Kollsman window, which sets the inches of mercury, or the second is to manually adjust the dial on the altimeter to set the current altitude above sea level at the ground level. Air traffic controllers at the tower will generally give the pilot the correct setting before take off.
Many times a change in elevation will not create a change in the sea level pressure, but occasionally there are times when the sea level pressure changes from take off to landing. For example, drastic weather changes will cause a change in the barometric pressure. When this occurs, the pilot must adjust the sea level air pressure reading while in flight to ensure the altitude reading is accurate.
A pressure altimeter is mechanical. Its main case is attached to the exterior of a plane, while the readout display is inside the cockpit. The readout is similar to an analog speedometer: a graduated circular dial, normally marked off in the hundreds or thousands, along with small plastic pointers indicating the actual altitude.
A small air intake valve on the back of the casing allows air to enter the altimeter. Mechanical instruments, called aneroid barometers, expand or contract depending on the air pressure. This movement causes a series of gears, springs, and pinions to move, finally turning a crankshafts connected to the pointers on the readout. The crankshaft moves these pointers around the dial to show the correct altitude.
There are several types of errors that are a concern when using a pressure altimeter. Mechanical errors, scale errors, and hysteresis errors are usually due to an issue with the internal gears or the material's elasticity. Reversal errors and installation or position errors cause a false pressure reading because of airflow issues.