Thickness and Coating Measurement Echo to Echo Technique on the Alphagage+

When inspecting coated components using pulse echo inspection, the first echo depth will include the coating thickness, if this is not taken into account the inspection can show inaccurate results, with areas of internal loss this could skew the results from a rejectable state to an acceptable result. The solution to this is to remove the paint, causing additional expense, often the shutdown of the line and the requirement to reinstate the coating after the inspection.

Using the E-E technique the results discount the paint thickness giving an accurate result for the material under the coating.

Since coatings have roughly 2.5 times slower sound speeds than steel, a .010” coating would automatically introduce an error of .025’ (.010” x 2.5). ECHO To ECHO measures the sound bouncing in the steel only thus ignoring the coating. We now also have the ability to measure the coating and the substrate at their independent velocities with the Alphagage+ CW or Alphagage+ CDLW. The use of the live waveform is recommended to look for mode converted echoes (Longitudinal waves becoming shear waves) as well as the ability to adjust gain and blanks…both main bang blank and blank after first detected echo.

To activate E-E mode on the Alphagage, turn the setting on in the measurement menu, this will assign a coating velocity if unknown.

Fig. 1 - Measurements menu showing coating velocity assigned when E-E mode is active

Fig. 2 - A-scan display showing the coating thickness in yellow (top right), and the material thickness in white (top right)

Using E-E measurements the technician can acquire accurate thickness measurement of materials under coatings without the need for expensive paint removal and shutdown times.

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