Moisture meters are measuring instruments used to measure the percentage of water in a given substance, as physical properties are strongly affected by moisture content and high moisture content for a period of time may progressively degrade a material. Meters exist for various substances, including wood, building materials, concrete, and soil. Direct gravimetric measurement of free moisture requires removing, drying, and weighing of a sample, so moisture sensors measure the volumetric water content indirectly by using some other property of the substrate, such as electrical resistance or dielectric constant. Moisture content can be used to determine if the material is ready for use, unexpectedly wet or dry, or otherwise in need of further inspection. Wood and paper products are very sensitive to their moisture content. Physical properties are strongly affected by moisture content and high moisture content for a period of time may progressively degrade a material.