A moisture meter is an instrument that is designed to detect the level of moisture in various building materials. It can measure the moisture content of materials such as wood, tile, fiberglass, roofing and plaster.
There are two different types of moisture meters: pin type and search mode. Some moisture meters contain both options. Pin type meters measure the moisture content of a material's surface or incremental depth using probes. Probes are slender metallic poles that measure the electrical conductivity of the material. The electrical conductivity will indicate the amount of moisture that is on the material.
Moisture meters that operate using search mode emit electromagnetic waves that are usually either radio waves or an electric current. These waves are affected by the presence of moisture in a material. The meter detects changes in the characteristics of the returned emissions and then calculates the moisture level based on the information gathered. Different products make these calculations differently. Some meters assume that a material's resistance will vary inversely from its moisture content. Others detect the characteristics of emitted radio waves.
The International Association of Home Inspectors (InterNACHI) believes that the best moisture meters take measurements using both the pin type and search mode method.