A direct-reading instrument for indicating the density, specific gravity, or some similar characteristic of liquids. Almost all hydrometers are made of a high-grade glass tubing. The main body is the float section in the bottom of which ballast, such as small shot, is secured. A small-diameter tube, the stem, extends from the upper end of the float section. Inside the stem is the scale, printed on heavy-grade paper, and well-secured within the stem so its position will not change. When the hydrometer is placed in a liquid, the stem extends vertically above the surface for a portion of its length.
Hydrometers may be classified according to the indication provided by graduations of the scale as follows: (1) density hydrometers, to indicate densities at a particular temperature, and usually for a particular liquid; (2) specific gravity hydrometers to indicate specific gravity of a liquid, with reference to water, at a particular temperature; (3) percentage hydrometers to indicate, at a particular temperature, the percentage of a substance such as salt, sugar, or alcohol dissolved in water (alcoholometers are an example); and (4) arbitrary scale hydrometers, indicating the density, specific gravity, or concentration of a liquid in terms of an arbitrarily defined scale, at a defined temperature. The last group includes the saccharimeter (indicates percentage of pure sucrose solutions); the Baumé hydrometer (measures specific gravity of liquids lighter than water); the lactometer (tests milk); and the barkometer (tests tanning extracts).
a device for measuring the density of fluids and solids. The construction of a hydrometer is based on the Law of Archimedes, from which it follows that the weight of a liquid that is displaced by a suspended substance (in this case, a hydrometer) is equal to the weight of the suspended substance. The density of the fluid under investigation can be determined by the depth of the hydrometer’s immersion in the fluid (the volume of liquid displaced by it) and the weight of the hydrometer. In practice hydrometers are of two types: constant-weight hydrometers (most widely used) and constant-volume hydrometers.
Densimeters are constant-weight hydrometers which have scales graduated in units of density (earlier standard hydrometers with scales graduated in conventional units of degrees—Baumé, Brix, Tralles, others—are not permitted in use today); hydrometers for measuring the concentration of solutions use scales which are graduated by percent-volume or percent-mass. Many hydrometers are scaled for measuring the concentration of materials in certain fluids, mixtures, and solutions and have special names: lactometers, which determine the fat content of milk; alcoholometers, which measure the content of alcohol in water; and saccharometers, which measure the amount of sugar in syrup (by mass, in percent), and others. The density in densimeters is determined directly from the scale. The values of scale divisions for standard densimeters are 0.0001, 0.0002, and 0.0005 g/cm3; for fluids (depending on the limits of measurement) from 0.0005 to 0.02 g/cm3; and for hydrometers that measure concentrations, from 0.1 to 2 percent.
Density measurement in a constant-volume hydrometer is based on the invariant volume of the immersed part of the device, which is attained by change of weight of the hydrometer. Density is determined from the mass either removed or added, so that the hydrometer is immersed to the line indicating the volume of the displaced liquid. The density of solids can be determined by a constant-volume hydrometer with an additional pan that is fixed to the bottom of the body of the hydrometer.