Definition
A sealed, accordion-shaped metal capsule with the air pumped out of it, used as the sensing element in pressure instruments. Because the inside holds near-zero pressure, the bellows expands or contracts in response to the absolute pressure of the air surrounding it, and that movement is mechanically linked to an instrument pointer or other indicator.
Plain English
A small, sealed metal bellows with no air inside. As outside air pressure changes, it squeezes or stretches the bellows, and that tiny movement drives the needle on a gauge.
Context Anchor
Seen in pressure-instrument discussions, especially altimeters, barometers, and manifold pressure gauges.
Derivation
Evacuated comes from the Latin 'evacuare', meaning to empty out. Bellows is an old English word for a flexible, accordion-like air bag, originally used to blow air into a fire. Together the term means an empty, flexible capsule — which is exactly how it works as a sensor.
Why Pilots Care
It provides accurate altitude and pressure readings by eliminating internal air interference.
Analogy
Think of a sealed, flexible metal can with very little air inside. When outside pressure changes, the can slightly squeezes or opens, and that small movement can drive a pointer on an instrument.
Intuition Check
Evacuated does not mean passengers or crew leaving the aircraft here. It means the air inside the bellows has been removed so outside pressure can move it.
Example Sentence 1
The altimeter uses an evacuated bellows that expands as the aircraft climbs and the surrounding air pressure drops.
Example Sentence 2
Mechanics inspect the evacuated bellows during instrument overhaul to ensure it responds correctly to pressure.