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A hydrometer or areometer is an instrument that measures the specific gravity (relative density) of liquids—the ratio of the density of the liquid to the density of water.
A hydrometer is usually made of glass, and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. 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 specific gravity. Hydrometers usually contain a scale inside the stem, so that the person using it can read specific gravity. A variety of scales exist for different contexts.
- Lactometer: used to check the general purity of cow's milk.
- Alcoholometer: indicates the alcoholic strength of liquids (concentration) which are essentially a mixture of alcohol and water. Alcoholometers have scales marked with volume percents of "potential alcohol", based on a pre-calculated specific gravity.
- Saccharometer: used for determining the amount of sugar in a solution or a fine suspension. It is used primarily by winemakers and brewers, as well as confectioners when making sorbets and ice-creams.
- Thermohydrometer: a hydrometer that has a thermometer enclosed in the float section.
- Urinometer: medical hydrometer designed for urinalysis.
- Barkometer: for testing the strength of tanning liquors used in tanning leather.
- Battery hydrometer: used to determine the density of the sulfuric acid solution used as electrolyte in lead-acid batteries. They are calibrated to read specific gravity relative to water at 16 °C.
- Antifreeze tester: used for testing the quality of the antifreeze solution used for engine cooling.
- Acidometer: used to measure the specific gravity of an acid.
- Salinometer: used to measure the salt content of saline water (ocean, sea, salt lake).
Hydrometers can be bought from lab suppliers. They can also be found in hardware stores.