The main difference between Hydrometer and Aerometer is that the Hydrometer is a laboratory instrument for measuring of density of liquids and Aerometer is a instrument to measure properties of air or gasses
A hydrometer is an instrument used for measuring the relative density of liquids based on the concept of buoyancy. They are typically calibrated and graduated with one or more scales such as specific gravity.
A hydrometer usually consists of a sealed hollow glass tube with a wider bottom portion for buoyancy, a ballast such as lead or mercury for stability, and a narrow stem with graduations for measuring. The liquid to test is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer correlates to relative density. Hydrometers can contain any number of scales along the stem corresponding to properties correlating to the density.
Hydrometers are calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits.
The hydrometer makes use of Archimedes’ principle: a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. The lower the density of the fluid, the deeper a hydrometer of a given weight sinks; the stem is calibrated to give a numerical reading.
An aerometer is an instrument designed to measure the density (among other parameters) of the air and some gases.The word aerometer (or Ärometer, from Ancient Greek ἀήρ -aer “air” and μέτρον -métron “measure, scale”) refers to various types of devices for measuring or handling of gases. The instruments designated with this name can be used to find: the density, the flow, the amount or some other parameter of the air or a determined gas.Several different designs of aerometer have been developed for the study of speech. These make use of a mask fitting closely to the speaker’s face, usually with a partition separating airflow from the nose from airflow from the mouth. Various designs of transducer convert the flow of air into electrical signals.
Another instrument called areometer (from Ancient Greek ἀραιός -araiós “lightness” and μέτρον -métron “measure, scale”), also known as hydrometer, used for measuring liquids density, is often confused with the term aerometer here defined.
An instrument that floats in a liquid and measures its specific gravity on a scale.
An instrument used to measure the mass and density of gases.