5. Eddy Current Techniques
In principle, the same information should be obtainable from. either pulsed or multiple frequency eddy current techniques, provided they utilize comparable frequency ranges. In practice, there are important differences and advantages for each method. Pulse instrumentation is generally cheaper, simpler, and less sophisticated. On the other hand, there has been greater development of theory and instrumentation using sinusoidal eddy currents, so that the equipment is generally more quantitative at present. The basic problem of determining certain paramenters when others may also be varying can be solved by measuring enough quantities to eliminate the unwanted variables, for example, by measuring the pulse response at various time delays or the sinusoidal response at various frequencies. In practice, the number of useful frequencies is strictly limited. Little additional information is obtainable from frequencies for which the skin depth is much greater or much less than the thickness of the sample. Since the frequencies must be spaced to. permit separation by filters, this puts a practical limit of about four on the number of frequencies useful for a given problem. This is not a serious limitation, since one can measure two quantities for each frequency and the total number of pertinent parameters rarely exceeds six. Pulse equipment can more readily handle a wide range of frequencies, but the instrumentation tends to become more elaborate, especially if high frequencies are needed for a particular application, and the repetition rate becomes low if low frequencies are necessary. The reproducibility of pulses is a problem which can be circumvented by the use of bridge techniques, differential coils and other standard techniques. New computer programs and microprocessor equipment have been developed which now make it possible to set up tests and measure parameters directly and precisely without the lengthy optimization calculations once necessary, though the latter will continue to be useful for the design of optimized coils and experiments.
Deeds, W. E., "A Comparison of Multiple Frequency and Pulsed Eddy Current Techniques" (1979). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, July 1977–June 1978. 23.