Calibration

    Three variables need to be calibrated in analyzing guided wave testing data: velocity, percent reflection, and attenuation. The velocity calibration is used for accurately pinpointing the defect location. It can be easily performed by

1. measuring the distance between the probe and known geometric feature or measuring the distance between two known geometric features
2. matching the measured distance with its signal position. If a known geometric feature is not available (for example, buried pipeline), the nominal velocity of torsional mode can be used in a shear wave calibration of the steel pipe.

    The calibration of percent reflection is performed for properly estimating the defect cross-sectional area with the detected signal. The percent reflection is usually calibrated by using the signal reflected from a weld. The impedance of guided wave testing is proportional to cross-sectional area of pipe wall because the guided wave fills the whole cross-sectional area of pipe when it propagates. We can calculate the percent reflection with these three variables:

    We can calculate the percent reflection with these three variables:

1. cross-sectional area of the pipe wall (calculated with pipe OD and wall thickness)
2. height and width of weld bead
3. wavelength of the operating center frequency.

The following figure shows the equivalent percentage reflection calculated with these variables.

    The attenuation calibration can be done by setting the amplitudes of two or more weld signals to be a similar amplitude. If there are big geometric features such as a pipe support, elbow, or clamp, the local attenuation should be applied to compensate for the local loss due to these geometric features together with the attenuation calibration. The attenuation calibration is displayed with distance amplitude correction (DAC) or Time-Corrected Gain (TCG) plots.