Definition
A standardized measurement of the pressure exerted by the vapors that evaporate from a fuel sample held at 100°F (37.8°C) in a sealed test container. It indicates how readily a fuel will vaporize, which affects engine starting, vapor lock tendency, and fuel handling characteristics.
Plain English
A standard test number that tells you how easily a fuel turns into vapor. A higher number means the fuel evaporates more readily; a lower number means it stays liquid more easily.
Context Anchor
Seen in aircraft fuel and powerplant discussions, especially when comparing fuel specifications and understanding vapor formation in fuel systems.
Derivation
Named after Dr. Joseph Reid, who developed the test method in the 1920s. The name simply identifies whose procedure is used — the value itself only has meaning when measured under his standardized conditions.
Why Pilots Care
Correct Reid vapor pressure prevents vapor lock or inconsistent fuel delivery in hot conditions.
Analogy
It is like the pressure that builds in a closed soda bottle when gas comes out of the liquid. Warmer conditions make the gas come out more readily and increase the pressure inside the container.
Grounding Statement
Warm fuel in a closed fuel system can give off vapor, and that vapor can create pressure or bubbles that affect fuel flow.
Intuition Check
Reid vapor pressure is not fuel pump pressure. It is a lab-measured value showing how easily the fuel itself gives off vapor at a set temperature.
Example Sentence 1
The mechanic checked the fuel specification sheet to confirm the Reid vapor pressure was within the approved range for summer operations.
Example Sentence 2
High Reid vapor pressure readings can indicate a greater risk of vapor lock during summer operations.