Editing Hydrometer (section)

{{Short description|Device used to measure density of liquids}}
{{distinguish|hygrometer}}
[[File:Areometer.svg|thumb|upright=0.65|Schematic drawing of a hydrometer. The lower the density of the fluid, the deeper the weighted float '''B''' sinks. The depth is read off the scale '''A'''.]]

A '''hydrometer''' or '''lactometer''' is an instrument used for measuring density or [[relative density]] of [[liquids]] based on the concept of [[buoyancy]]. They are typically [[Calibration|calibrated]] and [[Graduation (instrument)|graduated]] with one or more scales such as [[specific gravity]].

A hydrometer usually consists of a sealed hollow glass tube with a wider bottom portion for [[buoyancy]], a [[ballast]] such as [[lead]] or [[Mercury (element)|mercury]] for stability, and a narrow stem with graduations for measuring. The liquid to test is poured into a tall container, often a [[graduated cylinder]], and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer correlates to relative density. Hydrometers can contain any number of scales along the stem corresponding to properties correlating to the density.

Hydrometers are calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in [[Distilled beverage|spirits]].

The hydrometer makes use of [[Archimedes' principle]]: a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. The lower the density of the fluid, the deeper a hydrometer of a given weight sinks; the stem is calibrated to give a numerical reading.