Catching water penetration early so it can be stopped before it causes damage always has been difficult. Now, with a new portable electronic device it's much easier. With this device, you can instantly measure a material's moisture content. Unlike electrical resistance moisture meters, which measure the moisture content between two probes put into or on a wall, the new meter measures the moisture content over an area. HOW DOES IT WORK? The meter measures moisture content by measuring the change in capacitance caused by water in the material. Capacitance exists between any two conductors separated by a dielectric (insulator). As the moisture content of the material changes, its capacitance changes. The meter senses this change in capacitance and converts it into a moisture content reading. WHERE IS THE MOISTURE COMING FROM? To determine where moisture in a wall is coming from all you must do is compare moisture contents of different areas of the wall. Moisture contents from wind-driven rain should be higher on the outside of walls than on the inside. Walls that are drawing groundwater by capillary action must have a moisture content at or above a critical water content, which for brick is about 6%. Water probably is condensing inside a cavity wall if meter readings inside the cavity are higher than readings on the interior or exterior sides of the wall, and if the readings are taken in the upper half of the wall (to rule out groundwater).