Commonly used for locating rebar, this technique involves passing an alternating current through a coil to generate a magnetic field. When a magnetic material is encountered, the field is disturbed. The magnitude is related to the size of the object and proximity to the probe. This NDE technique is recommended for use in conjunction with other techniques to determine the presence of hidden metallic objects. The operating principle is that an object having a temperature above absolute zero will radiate electromagnetic waves.
Wavelengths fall within certain bands, depending on temperature.
Wavelengths at room temperature are outside the visible spectrum, while those at very high temperatures are shorter and fall within the visible spectrum. Cameras or video equipment are used to photograph the surface temperature of the subject. The resulting photograph or video images indicate surface temperature variations, which are also affected by ambient humidity, time of day, and other micro-environmental factors. In masonry construction, the different wavelengths often indicate the presence of moisture. The technique can distinguish between dense and porous or deteriorated masonry, as well as identifying voids in walls, such as flues.