A high echo does not mean a large flaw. Two identical reflectors at different depths produce different amplitudes: without a term of comparison, the height of the peak says nothing.
- ✓ DAC: curve built on known reflectors at different depths
- ✓ DGS: evaluation based on theoretical distance, gain and size curves
- ✓ Direct comparison between actual indication and reference
- ✓ Sizing toolkit with DAC, DGS, TCG and AWS on the FX family
The amplitude of the echo returning from a discontinuity depends on several factors at once: the size of the reflector, its orientation with respect to the beam, and the distance the pulse has had to travel. The farther the flaw, the more the signal is attenuated, regardless of its severity.
The DAC curve solves the problem experimentally. Identical artificial reflectors are machined into a reference block of the same material, at increasing depths; their echoes are measured and the points obtained are joined. The resulting curve describes how, in that material, the echo of a reflector of known size is attenuated as distance increases.
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From that moment the operator has a yardstick. An indication found in the part is compared with the curve at the same depth: one assesses by how much it exceeds or falls below it, and the applicable code establishes how to classify it. The DGS method follows a similar logic but relies on theoretical curves linking distance, gain and equivalent reflector size.
What is needed to build a DAC
| Element | Why it is necessary |
|---|---|
| Reference block | It must be of the same material as the part to be inspected |
| Known artificial reflectors | They provide echoes of comparable size at various depths |
| Same probe and same wedge | The curve is valid for the configuration with which it was built |
| Reference gain | The curve is valid at the gain with which it was plotted |
| Applicable code | It sets the criteria by which the indication is to be classified |
When the configuration changes
A DAC curve is not transferable. It is valid for the combination of material, probe, wedge and gain with which it was built: if one of these elements changes, the curve must be redone, because the way the signal is attenuated along the path changes.
This is why the reference block is part of the kit of anyone performing weld inspections, just like the probe. On high-end Dakota NDT instruments the manufacturer indicates the possibility of building 16-point DAC curves.
Instruments with sizing toolkit
| Instrument | Evaluation functions |
|---|---|
| Dakota FX70-DL | Sizing toolkit with DAC, AWS, TCG and DGS |
| Dakota FX81-DL | Same toolkit in benchtop version |
| Dakota CMX10-DL | Diagnostic functions with 16-point DAC |
| Calibration blocks | References for building the curve |
Frequently asked questions
Because the amplitude also depends on the depth at which the reflector lies and on its orientation with respect to the beam. Two identical flaws at different depths produce echoes of different amplitude.
No. It is valid for the combination of material, probe, wedge and gain with which it was built. If one of these elements changes, the curve must be redone.
The DAC is built experimentally by measuring real reflectors on a reference block. The DGS instead relies on curves linking distance, gain and equivalent reflector size.
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Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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