Moisture meters are an important and effective tool for inspection and restoration professionals, though ironically they do not directly measure moisture content. Instead, they measure electrical resistance/conductivity, radio wave reflectance or other factors that correlate closely to moisture content in many materials. However, they are not foolproof. It is important to understand the operational differences of the various types of meters.
Many factors may cause false-positive readings with moisture meters that measure electrical resistance. Structural components or finishing materials made of metal will read positive, including such common materials as metal framing, drywall corner metal, and foil wallpaper. Accumulations of certain salts due to efflorescence can also read positive, despite their being dry.
Moisture meters can also give false-negative readings. An obvious example would involve simply taking the measurement in the wrong location; there are non-penetrating meters that cannot read across even a small air gap, potentially leading you to believe a wall interior is dry when it is in fact not.
Moisture meters may drift off accurate calibration over time. This is a major reason technicians should always use the same meter throughout a project.
Using meters in inappropriate ways can also cause problems. Each type has its own peculiarities of operation. It requires comprehensive training by experts who fully understand both effective restoration practices and proper use of the instruments to become truly proficient in their operation, and even then a familiarity with one type of meter may only partially transfer to another type, even though they may appear quite similar.