Definition
Sealed, flexible metal capsules inside a pressure-sensitive instrument from which most of the air has been evacuated. They expand as the surrounding air pressure decreases and contract as it increases, providing the mechanical movement that drives the indication on instruments such as the altimeter and vertical speed indicator.
Plain English
Small sealed metal discs with almost no air inside. When the air around them gets thinner, they puff out a little; when the air gets denser, they squeeze in. That tiny movement is what makes the altimeter needle turn.
Context Anchor
Seen in discussions and diagrams of how a pressure altimeter works.
Derivation
Aneroid comes from the Greek 'a-' meaning 'without' and 'neros' meaning 'liquid' or 'water.' It literally means 'without liquid,' distinguishing this kind of pressure sensor from older ones that used mercury or other fluids. A wafer simply describes the thin, flat, disc-like shape of the capsule.
Why Pilots Care
They are the core sensing element that converts pressure into altitude readings, directly affecting height accuracy and safe flight.
Analogy
Think of a sealed, mostly empty tin can. If you carried it up a mountain, it would bulge slightly because the outside air pressure dropped. An aneroid wafer does the same thing on a tiny, precise scale.
Grounding Statement
As the airplane climbs and outside air pressure drops, the wafers expand slightly; as the airplane descends and pressure increases, they compress slightly.
Intuition Check
Do not picture a food wafer or a solid flat plate. In an altimeter, aneroid wafers are flexible sealed metal capsules that move with pressure changes.
Example Sentence 1
As the aircraft climbs and outside pressure drops, the aneroid wafers inside the altimeter expand, driving the needle to show a higher altitude.
Example Sentence 2
During preflight the mechanic verified that the aneroid wafers moved freely with simulated pressure changes.