Altimeter

The internal arrangement of a sealed diaphragm pressure altimeter.

At sea level and standard atmospheric conditions, the linkage attached to the expandable diaphragm produces an indication of zero.

When altitude increases, static pressure on the outside of the diaphragm decreases and the aneroid expands, producing a positive indication of altitude.

When altitude decreases,atmospheric
pressure increases. The static air pressure on the outside of the diaphragm increases and the pointer moves in the opposite direction, indicating a decrease in altitude.

Altimeters that measure the aircraft’s altitude by measuring the pressure of the atmospheric air are known as pressure altimeters.

A pressure altimeter is made to measure the ambient air pressure at any given location and altitude.

In aircraft, it is connected to the static vent(s) via tubing in the pitot-static system. The relationship between the measured pressure and the altitude is indicated on the instrument face,which is calibrated in feet.

These devices are direct-reading instruments that measure absolute pressure. An aneroid or aneroid bellows is at the core of the pressure altimeter’s inner workings. Attached to this sealed diaphragm are the linkages and gears that connect it to the indicating pointer.

Static air pressure enters the airtight instrument case and surrounds the aneroid. At sea level, the altimeter indicates zero when this pressure is exerted by the ambient air on the
aneroid.

As air pressure is reduced by moving the altimeter higher in the atmosphere, the aneroid expands and displays altitude on the instrument by rotating the pointer.

As the altimeter is lowered in the atmosphere, the air pressure around the aneroid increases and the pointer moves in the opposite
direction