Smell, as such, cannot be measured directly. Therefore, indirect methods have been learned to study odors and their perception, in particular: assessing odor intensity, calculating the threshold of perception of an aromatic substance, and comparing odors by similarity and concentration.

Studies of olfaction show that there is usually a direct correlation between sensitivity to an odor and its threshold of perception. As ephemeral as the sense of smell may seem, there are two sciences that study it: olfactometry and odorometry. Olfactometry examines the quantitative evaluation of odors, while odorometry determines their qualitative characteristics.
And yet, how exactly can you measure olfactory acuity? It turns out that it is determined by the threshold concentration of an aromatic substance, by which we mean the minimum concentration of the substance at which the subject feels its presence.
A device for measuring olfactory acuity is called an olfactometer. There are several varieties of olfactometers, differing in design and principle of operation. The Zwaardemaker olfactometer consists of a cylinder with pores in which an aromatic substance is placed and a glass tube with a scale of divisions: as the tube is immersed in the cylinder, it reduces the spread of the odorous substance through the tube opening to the test person's nose. To quantify olfactory acuity, a special unit of measurement, olfactia, is used; it is expressed in centimeters of immersion of the glass tube of the lfactometer in the cylinder.
Another type of measuring device is the Elsberg and Levy olfactometer. It differs in the fact that the supply of air into the nose of the person examined is carried out impulsively: with the help of a syringe some air is injected into a sealed vessel with an aromatic substance, which displaces the odorous substance vapors through a tube inserted into the nose of the examinee. In this case, the unit of olfactory acuity is the amount of air that is blown into the nose, displacing the volume of the odorant in cubic centimeters.
Both instruments have their disadvantages, so research in the field of olfaction and its acuity measurements are still carried out to this day. There are special systems and classifications of smells, which are used to compare and describe these or those scents.