Definition
An instrument that measures the specific gravity of a liquid — typically the electrolyte in an aircraft battery — and displays the reading at a location remote from the liquid being measured. In aircraft systems, it allows ground crew or maintenance personnel to check battery condition without physically removing or opening the battery.
Plain English
A gauge that checks how strong a battery's fluid is, with the display located somewhere convenient instead of right at the battery itself.
Context Anchor
Seen in aircraft electrical system and battery maintenance references, especially where battery condition is being checked.
Derivation
Hydrometer comes from the Greek hydro (water) and meter (measure) — literally 'water measurer.' It measures the density of a liquid relative to water. 'Remote reading' simply means the display is located away from the sensor, so you can read it from a distance.
Why Pilots Care
Battery specific gravity indicates state of charge. A weak battery can mean a failed start, lost avionics on backup power, or trouble during an in-flight electrical failure. Being able to check battery health quickly during preflight or maintenance supports dispatch reliability.
Intuition Check
Don't confuse hydrometer with hygrometer. A hydrometer measures liquid density (like battery acid). A hygrometer measures humidity in air. One letter, completely different instrument.
Example Sentence 1
The maintenance technician checked the RRHS reading on the panel before signing off the battery as serviceable.
Example Sentence 2
During the inspection the mechanic used the remote reading hydrometer to verify battery condition without removing the access cover.