Definition
An instrument that measures and displays the difference between two separate pressure inputs rather than the absolute value of either one. In aviation pitot-static instruments, one pressure is typically led to a sealed diaphragm or capsule and the other to the surrounding case; the resulting expansion or contraction of the capsule drives the indication.
Plain English
An instrument that works by comparing two pressures and showing how much they differ, instead of measuring just one pressure on its own.
Context Anchor
Seen when learning how the vertical speed indicator works and why it responds to changes in outside air pressure.
Derivation
Differential comes from the Latin differentia, meaning a difference between things. The name simply tells you what the instrument actually measures: the difference between two pressures, not one pressure alone.
Why Pilots Care
It enables the vertical speed indicator to provide real-time climb and descent rate information essential for maintaining desired flight paths.
Analogy
It is like comparing two clocks when one updates instantly and the other updates a little late. The difference between them tells you that something is changing, not just what the current value is.
Intuition Check
Do not read differential as meaning complicated or special by itself. Here it simply means the instrument works from the difference between two pressures.
Example Sentence 1
The vertical speed indicator is a differential pressure instrument because it compares current static pressure with a slightly delayed reference pressure inside its case.
Example Sentence 2
Pilots rely on differential pressure instruments like the VSI to maintain precise altitude control during instrument approaches.