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hydrometer(hīdrŏm`ətər), device used to determine directly the specific gravityspecific gravity,
ratio of the weight of a given volume of a substance to the weight of an equal volume of some reference substance, or, equivalently, the ratio of the masses of equal volumes of the two substances.
..... Click the link for more information. of a liquid. It usually consists of a thin glass tube closed at both ends, with one end enlarged into a bulb that contains fine lead shot or mercury to cause the instrument to float upright in a liquid. In the glass tube is a scale so calibrated that the reading on it level with the surface of the liquid in which the hydrometer is floating indicates the number of times heavier or lighter the liquid is than water, i.e., the specific gravity of the liquid. The hydrometer is based on Archimedes' principleArchimedes' principle,
principle that states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases.
..... Click the link for more information. . The level at which the hydrometer floats depends only on the densitydensity,
ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the
..... Click the link for more information. of the liquid. Hence this level can be used to measure both the density and the specific gravity, which is proportional to it. Commercial hydrometers are usually calibrated for ordinary room temperature, which is taken to be 20°C; (68°F;), or for 4°C; (39.2°F;). Because of the variation in the depth to which the instrument sinks in heavy and in light liquids, one type is made for use in liquids more dense than water and another for use in those less dense than water. The so-called bulb hydrometer consists of a small commercial hydrometer contained in a larger glass tube into which the solution to be tested is drawn by the action of a rubber bulb. It is used to measure the specific gravity of the sulfuric acid solution in automobile batteries. The two Baumé hydrometers (invented by Antoine Baumé), one for specific-gravity determinations in liquids denser than water and the other for liquids less dense than water, are calibrated with the special Baumé scale (also constructed by him).
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.
REFERENCESTurubiner, I. K., and M. D. Ippits. Tekhnika izmereniia plotnosti. Moscow, 1949. Chapters 2–5.
Kivilis, S. S. Tekhnika izmereniia plotnosti zhidkostei tverdykh tel. Moscow, 1959.
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).