A hydrometer measures the specific gravity, or relative density, of a liquid. Resembling a thermometer with a weighted bulb at one end, a hydrometer is gently lowered into the liquid to be tested, which is usually contained in a graduated cylinder, and allowed to float freely. When it comes to rest, the point at which the surface of the liquid aligns with the calibrations on the instrument's side is noted.
Hydrometers are used for a variety of purposes, such as determining the alcohol content of wine or spirits, measuring the salinity of aquarium or sea water, and checking the purity of milk or cream. Depending on its intended use, hydrometers can be marked with a variety of calibration units, such as a combination of alcohol content, brix and specific gravity units for use by winemakers. This type of instrument is called a triple-scale hydrometer.
The operation of a hydrometer is based on Archimedes' principle of buoyancy. A solid object, such as a hydrometer, will be buoyed up when placed in a fluid to a degree equal to the weight of the portion of the object suspended below the surface of the liquid. The degree to which the hydrometer is submerged below the surface is an indication of the liquid's density. Lower density liquids will cause the hydrometer to sink further. The degree of density, which reflects the liquid's specific gravity, can be read by noting the calibration mark on the stem of the instrument at the fluid's surface.