Definition
An instrument used to measure the specific gravity of a liquid — that is, how dense the liquid is compared to pure water. In aviation maintenance, it is most commonly used to check the state of charge of a lead-acid battery by measuring the density of its electrolyte.
Plain English
A simple tool that tells you how heavy a liquid is compared to water. Mechanics use it to check whether a battery's fluid is strong or weak, which tells them how charged the battery is.
Context Anchor
Seen in aircraft maintenance, especially when checking a lead-acid battery during inspection or servicing.
Derivation
From the Greek hydro- meaning water, and -meter meaning to measure. Literally, a water measurer. The name fits because it measures properties of liquids by comparing them to water.
Why Pilots Care
A weak or undercharged battery can prevent reliable engine starting and cause electrical system failures in flight.
Analogy
Think of dropping a fishing bobber into different liquids. In a dense liquid it floats high; in a thin liquid it sinks low. A hydrometer works on that same principle, with markings on the float to give a precise reading.
Grounding Statement
If the battery liquid is too weak, the hydrometer reading will show that its density is lower than it should be.
Intuition Check
A hydrometer does not measure how much liquid is in a container. It measures how dense the liquid is compared with water.
Example Sentence 1
The mechanic used a hydrometer to check each cell of the battery before signing off the inspection.
Example Sentence 2
During pre-installation checks, any hydrometer reading below 1.240 requires the battery to be charged or replaced.