Hydrometer

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 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 can be read directly in grams per cubic centimeter).

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.

Commercial uses Because the commercial value of many liquids, such as sugar solutions, sulphuric acid, alcohol, and wine, depends directly on the specific gravity, hydrometers are used extensively.

Perhaps the best-known form of hydrometer is the kind used in testing milk, called a lactometer. The specific gravity of cow's milk varies from 1.027 to 1.035. Since only the last two digits are important, the scale of a lactometer runs from 20 to 40, which means from 1.020 to 1.040. The specific gravity of milk does not give us a conclusive test as to its worth. In addition to water (which is about 87%), milk contains some substances that are heavier than water, such as albumen[?], sugar, and salt, and others that are lighter than water, such as butter fat. So, in addition to the specific gravity, one needs to determine the amount of fat, and, if possible, of other solids in the milk, to know its richness. Of course, the very important question as to the cleanliness of milk must be left to the bacteriologist.