hy·drom·e·ter 
(hī-drŏmĭ-tər)
(hī-drŏmĭ-tər)n.
An instrument used to determine specific gravity, especially a sealed, graduated tube, weighted at one end, that sinks in a fluid to a depth used as a measure of the fluid's specific gravity.
hy′dro·metric (hī′drə-mĕtrĭk), hy′dro·metri·cal adj.
hy′dro·metri·cal·ly adv.
hy·drome·try n.