A densimeter or areometer is a measuring instrument that is used to determine the relative density of liquids without first having to calculate their mass, conductivity and temperature. It is usually made of glass and consists of a hollow cylinder with a heavy bulb at one end so that it can float upright. The term used in English is hydrometer; However, in Spanish, a hydrometer is a very different instrument that is used to measure the flow, speed or pressure of a moving liquid.

Hypatia of Alexandria is considered its inventor.[1]​.

Another possibility to determine the densities of liquids and gases is to use a digital instrument based on the principle of the oscillating U-tube, whose resonance frequency is determined by the mass of the materials contained in a known volume, in an analogous way to how the mass of a tuning fork is a determining factor for the pitch of its resonance sound.[2]​.

The densimeter is carefully and vertically introduced into the liquid, and left at rest until it floats freely and vertically. Next, its level of sinking in the liquid is observed on the graduated scale on the stem of the densimeter; that is the reading of the relative density measurement of the liquid. In light liquids (e.g., kerosene, gasoline, alcohol,...) the hydrometer will sink more than in denser liquids (such as salt water, milk,...). In fact, it is common to have two different instruments: one for liquids in general and another for thin liquids, which differ in the arrangement of the measurement marks.

The densimeter is also used in oenology to determine the moment of fermentation in which the wine is. To measure the alcohol content, the Gay-Lussac breathalyzer is used, with which the alcohol content is directly measured in Gay-Lussac degrees.

Contenido

Una de las formas más conocidas de densímetro es el lactómetro, que se usa para medir la densidad de la leche como indicador de su calidad. La densidad específica de la leche de vaca varía entre 1,027 y 1,035. Como la leche contiene otras sustancias aparte de agua (87 %), también se puede saber la densidad de albúmina, azúcar, sal "Sal (condimento)") y otras sustancias más ligeras que el agua.