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Factors Affecting The Indication Value Of Ultrasonic Thickness Gauge
Sep 26, 2018

Factors affecting the indication value of ultrasonic thickness gauge

Let's talk about the factors that affect the indication value of the ultrasonic thickness gauge.

(1) The surface roughness of the workpiece is too large, resulting in poor coupling between the probe and the contact surface, low reflection echo, and even failure to receive echo signals. For surface rust, in-service equipment, pipelines, etc., which have extremely poor coupling effects, can be treated by sand, grinding, frustration, etc. to reduce the roughness, and at the same time, the oxide and paint layers can be removed to expose the metallic luster, so that the probe A good coupling effect can be achieved by the coupling agent with the test object.

(2) The radius of curvature of the workpiece is too small, especially when measuring the thickness of the small-diameter tube. The non-destructive testing resource network is a plane due to the common probe surface, and the contact with the curved surface is point contact or line contact, and the sound intensity transmission rate is low (coupling is not good). A small diameter probe (6mm) can be used to accurately measure curved materials such as pipes.

(3) The detection surface is not parallel to the bottom surface, and the sound wave is scattered by the bottom surface, and the probe cannot receive the bottom wave signal.

(4) Castings and austenitic steels are caused by uneven tissue or coarse grains. When ultrasonic waves pass through them, they cause severe scattering attenuation. The scattered ultrasonic waves propagate along complex paths, which may cause the echoes to annihilate, resulting in no display. . A low-frequency coarse crystal dedicated probe (2.5MHz) is available.

(5) There is some wear on the probe contact surface. The surface of the commonly used thickness measuring probe is made of acrylic resin. The long-term use will increase the surface roughness, resulting in a decrease in sensitivity, resulting in incorrect display. It can be sanded with 500# sandpaper to make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe.

(6) There are a large number of corrosion pits on the back of the object to be tested. Due to rust spots and corrosion pits on the other side of the object, the sound waves are attenuated, resulting in irregular readings and, in extreme cases, no reading.

(7) 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.

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

(9) 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. High-temperature special probes (300-600 ° C) should be used. Do not use ordinary probes.

(10) Laminated materials and composite (heterogeneous) 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.