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A hydrometer is a sealed floating instrument with a weighted bulb and a narrow graduated stem. The ballast keeps the stem upright. By Archimedes' principle, its gravitational weight equals the buoyancy:It therefore sinks farther into a less dense liquid, displacing more volume, and floats higher in a denser one. The stem makes a small volume change readily visible as a change in immersion. If its cross-sectional area is $A$, then $V_{\mathrm{displaced}}=V_0+Az$ over a uniform part of the stem. A density scale is consequently not generally uniform in $z$.

Float the clean instrument freely in a sufficiently deep sample, without its touching the wall or bottom. Remove adhering bubbles, allow the temperature and motion to settle, and read the liquid surface at eye level using the meniscus convention specified for that instrument. A calibration for a particular temperature or reference density cannot be transferred without correction: both the sample and the instrument expand. An ordinary hydrometer measures density or specific gravity; a special scale for a particular chemical solution does not make it chemically selective. In particular, dissolved substances other than glucose can affect the density of urine.

A hydrometer used to measure the specific gravity of urine; readings depend on calibration conditions and dissolved constituents.