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Wall thickness measurements are based on the measurements of the time-of-flight of longitudinal waves in the examined object. At each point where the acoustic impedance changes the ultrasonic waves are reflected more or less strongly (see above). The wall thickness measurement takes advantage of this. At the surfaces of the examined object waves arriving at right angles are partly reflected. When these reflected waves hit the probe an electric impulse is generated (echo), which is processed. By measuring the time difference of the time-of-flight between two echos of known origin, conclusions can be drawn to the distance passed with a known velocity of sound. The material thickness may be concluded out of this.

For pipe testing with UT rotaries the outer diameter can additionally be determined with a known distance of the probe to the center of the rotation and the measurement of the so-called water path (sound path in the coupling water surrounding the pipe). The inner diameter, eccentricity, ovality (dimensions for the deviation from the cylindric form) can be determined by the combination of the measurement results of the water path and wall thickness.

When signal amplitudes are generated before the back wall echo arrives (reflected sound wave from the back plane /interior of the pipe) or without any further generation of a back wall echo, the ultrasound is reflected from an area within the examined object. These kinds of laminar reflectors are called laminations. Rules regulate whether such laminations are allowable for the product.