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Analysis Of Factors Affecting The Distortion Of Ultrasonic Thickness Gauge
Oct 09, 2018

Analysis of Factors Affecting the Distortion of Ultrasonic Thickness Gauge

The following is a small series to explain the factors of the distortion of the ultrasonic thickness gauge

Analysis of the indication is too large or too small. In the actual inspection work, the indication value of the thickness gauge is often significantly larger or smaller than the design value (or expected value). The reasons are as follows:

(1) There is sediment in the measured object (such as pipeline). When the sediment and the acoustic impedance of the workpiece are not much different, the thickness gauge shows the wall thickness plus the thickness of the deposit.

(2) 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).

(3) 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.

(4) 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.

(5) 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.

(6) The sound speed selection is incorrect. Before measuring the workpiece, preset the speed of sound according to the type of material or reverse the sound speed according to the standard block. When the instrument is calibrated with one material (commonly used for steel) and another material is measured, erroneous results will result.