Distance Amplitude Curve 

DACDistance Amplitude Curve – is a method of compensating for the fact that the pulse-echo response of a reflector will decrease as the distance of the reflector from the ultrasonic probe increases. This occurs because the transmitted ultrasonic beam spreads out as it travels from the probe to the reflector and so the further the reflector is from the probe the lower the energy of the sound that actually hits the reflector. Similarly, the further the reflected pulse has to travel the lower the energy that is received back at the probe.

The DAC is generated by plotting the amplitude of a known calibration reflector at different distances from the probe. Generally, the gain of the flaw detector is set so that the amplitude of the nearest reference reflector (the one at the shallowest depth) is at 80% FSH (see last month for description of FSH). The amplitudes of the same calibration reflector at further depths are then plotted on the flaw detector screen whilst the gain setting is maintained constant. The DAC is generated by plotting a curve through these amplitudes – see Figure.

During the inspection, the amplitude of any echo which appears on the screen can easily be compared to the signal amplitude of the reference reflector at the exactly the same range.

For more information on how to generate a DAC see:
BS EN ISO 16811:2014 Non-destructive testing – Ultrasonic testing – Sensitivity and range setting.

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org