Table I: Results of the ranking algorithm showing the top three frequencies ranked by mean error and standard deviation. Error values are normalized.

To determine these coefficients and the frequency to be used, a simple algorithm is performed. First, the measured capacitances are averaged over the four samples for each of the coating levels. For each frequency, the minimum capacitance is subtracted to remove offset errors. Then the values are divided by the maximum capacitance at each frequency to remove the linear errors. The resulting capacitance ratios at each frequency are individually fitted to the quadratic equation above, and the resulting error in estimation is computed. The mean of the estimation errors and the standard deviation is tabulated for each frequency. The frequencies are ranked on the basis of least mean error and the least standard deviation. The product of these rankings is used as a figure of merit for selection of the frequency f₁. Table I shows the results obtained from the algorithm detailed above. It shows that the measurements at 100 kHz are the most suitable for the estimation process. The coefficients K₁, K₂, and K₃ were determined by least square methods. The resulting equation for estimation of coating thickness is

Figure 6 (ALL FIGURES ARE COURTESY OF THE AUTHORS.)