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A hydrometer is an instrument used for determining the specific gravity of liquids. It 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 is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely.

The point where the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity (or density relative to water) can be read directly. Specific gravity is a ratio of one density to that of the density of water. Therefore, specific gravity has no units. See relative density.

In light liquids like kerosene, gasoline, and alcohol, the hydrometer must sink deeper to displace its weight of liquid than in heavy liquids like brine, milk, and acids. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top, and one for light liquids, on which the mark 1.000 is near the bottom of the stem.

Many industries have more than one set of hydrometers, 1.0-0.95, 0.95-0.9 etc, to provide more precise measurements of density. For measuring density of petroleum products, like fuel oils, the specimen is usually heated in a temperature jacket with a thermometer placed behind it since density is dependent on temperature. Light oils are placed in cooling jackets, typically at 15^oC. Very light oils with many volatile components are measured in a variable volume container using a floating piston sampling device to minimize light end losses.

The function of the hydrometer is based on Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid displaced. Thus, the lower the density of the substance, the lower the hydrometer will sink. Some historians credit Hypatia of Alexandria with the invention of the hydrometer although there is little evidence to support this.

A homebrewer using a hydrometer

Because the commercial value of many liquids, including sugar solutions, sulfuric acid, and alcohol beverages such as beer and wine, depends directly on the specific gravity, hydrometers are used extensively.

A lactometer is a hydrometer used to test milk. The specific gravity of milk does not give a conclusive test to its composition since milk contains a variety of substances that are either heavier or lighter than water. Additional tests for fat content are necessary to determine overall composition.

An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids. It is also known as a proof & traille hydrometer.

A saccharometer is a hydrometer used for determining the amount of sugar in a solution. It is primarily used by brewers and winemakers.

A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section.

A modern hydrometer in a sugar solution

A hydrometer analysis is the process by which fine-grained soils, silts and clays, are graded. Hydrometer analysis is performed if the grain sizes are too small for sieve analysis. The basis for this test is Stoke's Law for falling spheres in a viscous fluid in which the terminal velocity of fall depends on the grain diameter and the densities of the grain in suspension and of the fluid. The grain diameter thus can be calculated from a knowledge of the distance and time of fall. The hydrometer also determines the specific gravity (or density) of the suspension, and this enables the percentage of particles of a certain equivalent particle diameter to be calculated.

• Hydrometer Information
• Hydrometers, Ertco Precision.
• Jurjen Draaijer. Milk Testing. Milk Producer Group Resource Book, Food and Agriculture Organization of the United Nations
• Using Your Hydrometer, Winemaking Home Page.

• Elevator paradox (physics)