Coating thickness measurements using pulsed eddy currents
"The nondestructive measurement of the thickness of one metal coated or clad upon another metal as a base has been studied for the past twenty years or more. If one of the metals is magnetic, the problem is relatively simple. If the clad and base metals are both nonmagnetic, the measurement problem is more difficult, particularly if both metals have nearly equal electrical conductivities. The phase measurement of sinusoidal eddy currents has been tried with some success although there is difficulty with low sensitivity and the presence of harmonic frequencies. The use of pulsed eddy currents in the manner of echo sounding appeared to have promise and was tried. A brief theoretical treatment indicated that the necessary pulse length depended upon the thickness of the coating. For thickness of the order of ten mils of coating, pulse lengths of approximately one microsecond were found satisfactory. A thyratron was used to pulse the magnetic probe, while a balancing circuit was employed almost to eliminate the echo from the air-to-metal surface so the echo from the metal-to-metal boundary could be detected. Several different forms of the balancing circuit were tested. Comparison of thicknesses measured using the pulsed probe is made with those measured optically."--Page 1.