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Analysis Of The Error Of The Ultrasonic Thickness Gauge
Nov 27, 2018

Analysis of the error of the ultrasonic thickness gauge:

1. Laminated materials, composite (non-homogeneous) materials. It is impossible to measure uncoupled laminates because ultrasonic waves cannot penetrate uncoupled spaces and cannot propagate at a constant rate in composite (non-homogeneous) materials. For equipment made of multi-layer material (like urea high-pressure equipment), special care should be taken when measuring thickness. The thickness gauge indicates only the thickness of the material that is in contact with the probe.

2. The effect of temperature. Generally, the speed of sound in solid materials decreases with increasing temperature. Test data shows that for every 100 °C increase in hot material, the speed of sound drops by 1%. This is often the case with high temperature in-service equipment.

3. The influence of the coupling agent. The coupling agent is used to exclude the air between the probe and the object to be measured, so that the ultrasonic wave can effectively penetrate the workpiece for inspection purposes. If the type is selected or the method of use is improper, it will cause an error or the coupling mark will flash and cannot be measured. In actual use, the instrument indicates the value of the couplant layer thickness due to the excessive use of the couplant, causing the probe to leave the workpiece.

4. The effect of metal surface oxide or paint overlay. The dense oxide or paint anti-corrosion layer produced on the metal surface, although tightly combined with the matrix material, has no obvious interface, but the speed of sound propagation in the two materials is different, resulting in errors, and the thickness of the cover varies with the thickness of the cover. It is also different.

5. When there are defects inside the material (such as inclusions, interlayers, etc.), the displayed value is about 70% of the nominal thickness (in this case, the ultrasonic flaw detector is used for further defect detection).