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Advanced Techniques

In addition to the common modes of measurements listed on the previous pages, several specialized modes are used in advanced thickness gages.

Echo-to-Echo measurement

This is essentially a Mode 2 measurement implemented with dual element transducers in a corrosion gage. It is typically used to measure the thickness of metal under a relatively thin layer of paint, resin, or similar nonmetallic coating, without including the coating thickness. Because multiple backwall echoes occur in the metal only, the measured time interval between two backwall echoes represents the metal thickness only.

ThruCoat measurement

ThruCoat is a patented Olympus NDT measurement technique utilizing selected dual element transducers that permits separate measurement of thin nonmetallic coatings like paint over metal as well as the metal thickness. It has two advantages over echo-to-echo mode: it provides a reading of coating thickness rather than simply ignoring it, and it permits metal measurement down to approximately 1 mm (0.039"), whereas echo-to-echo with duals is typically limited to a minimum of approximately 2.5 mm (0.1").

Frequency domain measurement

Frequency domain measurement is a specialized technique available on some advanced gages that extends the minimum measurable thickness range lower than what would be possible with conventional time domain measurement only. It is most commonly used for measurement of thin barrier layers in multilayer plastics. Traditional ultrasonic gaging requires clear separation of the echoes from the front and back surfaces of each layer. However, in relatively thick multilayer products such as fuel tanks and performs, it may not be possible to penetrate the outer structural plastic at a frequency high enough to cleanly resolve the thin barrier. Frequency domain measurement involves digitizing the waveform to capture the interference between the signals being reflected from the front and back surfaces of the barrier layer, and then performing a mathematical analysis of the resulting pattern to calculate the thickness of the layer that is producing it.

B-scan display

A B-scan is a way of plotting thickness data as a cross-sectional profile of a test piece. A series of measurements are made along a line and then plotted on a position-versus-thickness scale, creating a side view of the part.

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