3.3.1 Wall Thickness
Several methods have been proposed to nonintrusively measure the thicknesses of walls, corrosion profiles, and macrodefects [i. e., 49]. Two methods at room temperature that require point contact with the cold face of the furnace are known. The first is impact-echo method, used in construction concretes and pavements (Sect. 1.4). The second method is the frequency-modulated continuous-wave (FM-CW) radar technique , which can produce wall thickness data in real time.
Use of ultrasonic testing techniques has been attempted in harsh environments including high temperatures and radiation [i. e., 51]. Testing is complicated because wave-guides with special high temperature couplants and cooling systems are necessary to protect ultrasonic transducers from reaching their Curie point. Newer transducers, based on AlN films, capable of emitting and receiving ultrasonic energy at temperatures exceeding 900°C and pressures above 150 MPa have also been reported .