Definition
A maintenance tool used to check the sealing condition of a reciprocating engine's cylinders by introducing regulated compressed air into a cylinder through the spark plug hole and comparing the pressure held inside the cylinder against a fixed reference pressure supplied to the tester. The pressure difference between the input air and the air retained by the cylinder indicates how well the piston rings, valves, and head gasket are sealing.
Plain English
A piece of test equipment that pushes air into an engine cylinder and measures how much of that air leaks out, so a mechanic can tell whether the cylinder is still sealing properly.
Context Anchor
Seen during aircraft engine inspections, especially when checking the condition of cylinders on a piston engine.
Derivation
Differential' comes from the Latin 'differre', meaning to carry apart or differ. In this tool the key reading is the difference between two pressures — one going in and one held inside the cylinder — which is what tells the mechanic how much air is escaping.
Why Pilots Care
Identifies internal engine sealing problems that could cause power loss or in-flight failure.
Intuition Check
Do not confuse this with simply seeing whether the engine “has compression” while it turns. A differential compression tester measures pressure loss in one cylinder while air is held against it.
Example Sentence 1
During the annual inspection, the mechanic used a differential compression tester on each cylinder and recorded the readings in the engine logbook.
Example Sentence 2
A reading below 60/80 on the differential compression tester showed excessive leakage past the exhaust valve.