hydrometer hy·drom·e·ter (hī-drŏm'ĭ-tər)
An instrument used to determine specific gravity, especially a sealed graduated tube, weighted at one end, that sinks in a fluid to a depth indicating the fluid's specific gravity. Also called gravimeter.
An instrument used to measure the density of a liquid as compared to that of water. Hydrometers consist of a calibrated glass tube ending in a weighted glass sphere that makes the tube stand upright when placed in a liquid. The lower the density of the liquid, the deeper the tube sinks.