How a surface deflects radio waves is influenced by the so-called dielectric constant of the material. William M. Healy of the National Institute of Standards and Technology and Eric van Doorn of Intelligent Automation, Inc., exploited the fact that water content affects the magnitude of the dielectric constant to develop their approach to searching for water within walls. They built simplified wall sections out of gypsum board, drywall, insulation and a material similar to plywood, subjected the makeshift walls to varying levels of humidity and then exposed them to radio waves. The amount of time it took for a signal to return to the antenna helped identify the presence of water, and computer software allowed the scientists to create three-dimensional maps that identified wet sections (see image). In a paper to be published in an upcoming issue of the journal ASHRAE Transactions, they report that, so far, they have been able to locate pockets of moisture to within one centimeter.
The next step for the researchers is to determine how the system performs when it examines real structures, which include studs, wires, pipes and other components that may interfere with the readings. They note that their preliminary results "indicate that the development of a handheld, UWB based, non-contact moisture sensor is feasible." That could make the job of a plumber searching for a leaky pipe a lot easier.