Definition
The difference in pressure between two points in a system, measured by comparing one pressure against another rather than against a vacuum or against the surrounding atmosphere.
Plain English
It is simply how much higher the pressure is on one side compared to the other side. You are not measuring either pressure on its own — you are measuring the gap between them.
Context Anchor
Seen in cabin pressure systems, air-pressure instruments, fuel and hydraulic systems, and maintenance checks where two pressure readings are compared.
Derivation
‘Differential’ comes from the Latin differentia, meaning ‘a difference.’ In this context it signals that the value being measured is the difference between two pressures, not either pressure on its own.
Why Pilots Care
Accurate differential pressure readings are essential for correct airspeed indication and for detecting clogged filters before they cause system failure.
Grounding Statement
Picture a sealed cabin at altitude: the air inside can push outward harder than the thinner air outside, and that difference is differential pressure.
Intuition Check
Differential pressure is not a special kind of pressure by itself. It is the amount of difference between two pressures.
Example Sentence 1
The cabin pressurization system maintains a differential pressure of about 8 psi between the inside of the cabin and the outside air at cruise altitude.
Example Sentence 2
During preflight the mechanic checked the differential pressure across the fuel filter to see if it was restricted.