an instrument for measuring the moisture content in gases, liquids, and solids (including free-flowing solids).
The moisture in the air is usually measured with hygrometers and psychrometers. In hygroscopic electrochemical moisture meters the moisture of a gas is determined from the changes in properties of an electrolyte in the meter’s bulb.
The moisture in liquids (that is, the content in a water-liquid mixture where water is not the chief component, such as in petroleum or alcohol) is measured by means of capacity moisture meters that determine the dielectric constant or dielectric loss in the liquid, by conductance moisture meters that measure the liquid’s electrical conductivity, and by hygroscopic electrochemical moisture meters for gases that have built-in evaporators.
The moisture in solids is determined by means of capacity and conductance moisture meters. Use is also made of the resonance absorption of radio waves by the hydrogen nuclei in water molecules. With this type of meter the material being tested is placed within the coil of the oscillatory circuit of a radio-frequency transmitter, whose frequency can be changed continuously. At the frequency corresponding to the nuclear magnetic resonance the energy absorption increases sharply in the oscillatory circuit; the magnitude of the absorbed energy provides a measure of the material’s moisture. Radio isotope moisture meters compare the quantitative characteristics of interactions between the nuclear radiations and atoms of hydrogen and atoms of other elements. Those most commonly used depend on the attenuation of-γ-rays and the slowing down of fast neutrons.