hydrometer (n.)

before vowels hydr-, word-forming element in compounds of Greek origin, meaning "water," from Greek hydro-, combining form of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet"). Also sometimes a combining form of hydrogen.

"device or instrument for measuring," abstracted 1832 from gasometer (in English from 1790), etc., from French -mètre, used in combinations, from Latin metrum "measure" or cognate Greek metron "measure" (from PIE root *me- (2) "to measure").

English already had meter "person who measures, official who checks that measured quantities are correct" (late 14c., c. 1300 as a surname, agent noun from unrelated mete (v.)), which might have influenced this word. As short for parking meter from 1960. Meter maid "woman police official who patrols metered parking sites" is recorded by 1957, meter reader as a job is by 1872 (originally in reference to gas meters).