Water has a very intense absorption in both the mid-infrared and near-infrared (NIR). This means that low levels of moisture can be readily measured with infrared spectroscopy, in solids, liquids and gases.
Near infrared is usually preferred because of ease of sampling. For low moisture levels long pathlengths may be used for liquids and fibre-optic probes mean that the measurement can be made in situ and a multiple points. Diffuse reflectance probes can be used for solids and direct coupled NIR systems for measurements of powders in fluidised bed dryers and other flowing powder applications.
Indirect measurements of drying from water or solvents can be made with Mass spectroscopy - in this case the exhaust gas from the dryer is monitored and the amount of solvent in the stream will be proportional to the amount remaining the the solid being dried.
In the lab or at line NIR systems can be easily set up for a rapid measurement by technicians or plant operators.