1. n. [Drilling Fluids]
A weighted, hollow glass bulb with a long, graduated tube attached for measuring the density of a liquid. A hydrometer is placed in the liquid and the bulb sinks according to the density of the liquid. Graduations on the tube indicate the density. Hydrometers are used in fluids that have no gel strength, such as brine, but are not reliable in drilling fluids because of gelation.