Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from 0 (completely dry) to the value of the materials' porosity at saturation. It can be given on a volumetric or mass (gravimetric) basis.
#Measurement#
Direct methods
Water content can be directly measured using a drying oven.
Gravimetric water content, u, is calculatedvia the mass of water {\displaystyle m_{w}}:
{\displaystyle m_{w}=m_{\text{wet}}-m_{\text{dry}}}
where {\displaystyle m_{\text{wet}}}and {\displaystyle m_{\text{dry}}} are the masses of the sample before and after drying in the oven. This gives the numerator of u; the denominator is either {\displaystyle m_{\text{wet}}} or {\displaystyle m_{\text{dry}}}(resulting in u' or u", respectively), depending on the discipline.
On the other hand, volumetric water content, θ, is calculated via the volume of water {\displaystyle V_{w}}.
{\displaystyle V_{w}={\frac {m_{w}}{
ho _{w}}}}
where {\displaystyle
ho _{w}} is the density of water. This gives the numerator of θ; the denominator, {\displaystyle V_{\text{wet}}}, is the total volume of the wet material, which is fixed by simply filling up a container of known volume (e.g., a tin can) when taking a sample.
For wood, the convention is to report moisture content on oven-dry basis (i.e. generally drying sample in an oven set at 105 deg Celsius for 24 hours). In wood drying, this is an important concept.
Laboratory methods
Main article: Moisture analysis
Other methods that determine water content of a sample include chemical titrations (for example the Karl Fischer titration), determining mass loss on heating (perhaps in the presence of an inert gas), or after freeze drying. In the food industry the Dean-Stark method is also commonly used.
From the Annual Book of ASTM (American Society for Testing and Materials) Standards, the total evaporable moisture content in Aggregate (C 566) can be calculated with the formula:
{\displaystyle p={\frac {W-D}{W}}}
where {\displaystyle p}is the fraction of total evaporable moisture content of sample, {\displaystyle W} is the mass of the original sample, and {\displaystyle D}is mass of dried sample.