Factors affecting the indication value of 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 reflected echo, and even the inability to receive echo signals.
For surface corrosion and in-service equipment and pipelines with extremely poor coupling effects, the surface can be treated by sanding, grinding, fringing, etc. to reduce the roughness. At the same time, the oxide and paint layers can also be removed to expose the metallic luster and make the probe A good coupling effect can be achieved with the test object through coupling agent.
(2) The radius of curvature of the workpiece is too small, especially when measuring the thickness of small-diameter pipes. Since the surface of the commonly used probe is flat and the contact with the curved surface is point contact or line contact, the sound intensity transmittance is low (poor coupling). A special probe for small pipe diameters (6mm) can be used to more accurately measure curved surface materials such as pipes.
(3) The detection surface and the bottom surface are not parallel, and the sound waves are scattered when they encounter the bottom surface, and the probe cannot receive the bottom wave signal.
(4) Due to the uneven structure or coarse grains of castings and austenitic steel, severe scattering attenuation occurs when ultrasonic waves pass through them. The scattered ultrasonic waves propagate along complex paths, which may annihilate the echoes and cause no display. . A coarse-grain dedicated probe with a lower frequency (2.5MHz) can be used.
(5) There is some wear on the probe contact surface. The surface of commonly used thickness measurement probes is made of acrylic resin. Long-term use will increase the surface roughness, resulting in a decrease in sensitivity, resulting in incorrect display. You can use 500# sandpaper to polish it 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 side of the object being tested. Because there are rust spots and corrosion pits on the other side of the object to be measured, the sound waves are attenuated, causing the readings to change irregularly, or even no readings in extreme cases.
(7) There is sediment in the object to be measured (such as a pipe). When the acoustic impedance of the sediment and the workpiece are not much different, the value displayed by the thickness gauge is the wall thickness plus the sediment thickness.
(8) When there are defects inside the material (such as inclusions, interlayers, etc.), the displayed value is approximately 70% of the nominal thickness. At this time, an ultrasonic flaw detector can be used for further defect detection.
(9) Effect of temperature. Generally, the sound speed in solid materials decreases as the temperature increases. Experimental data shows that for every 100°C increase in hot materials, the sound speed decreases by 1%. This situation is often encountered for high-temperature in-service equipment. Special probes for high temperatures (300℃ ~ 600℃) should be used. Do not use ordinary probes.
(10) Laminated materials, composite (heterogeneous) materials. Measuring uncoupled laminated materials is not possible because ultrasonic waves cannot penetrate uncoupled space and cannot propagate uniformly in composite (heterogeneous) materials. For equipment made of multi-layer materials (such as urea high-pressure equipment), special attention should be paid when measuring thickness. The indication value of the thickness gauge only indicates the thickness of the layer of material in contact with the probe.
(11) Influence of coupling agent. Coupling agent is used to eliminate the air between the probe and the object being measured, so that ultrasonic waves can effectively penetrate into the workpiece for detection purposes. If the type is selected or used improperly, errors will occur or the coupling mark will flash, making measurement impossible.
The appropriate type should be selected according to the usage conditions. When used on smooth material surfaces, low viscosity coupling agents can be used; when used on rough surfaces, vertical surfaces and top surfaces, high viscosity coupling agents should be used. High temperature coupling agent should be used for high temperature workpieces.
Secondly, the coupling agent should be used in an appropriate amount and applied evenly. Generally, the coupling agent should be applied to the surface of the material being measured, but when the measurement temperature is high, the coupling agent should be applied to the probe.
(12) Wrong sound speed selection. Before measuring the workpiece, preset its sound velocity according to the material type or back-measure the sound velocity based on the standard block. When the instrument is calibrated with one material (the commonly used test block is steel) and then measured with another material, erroneous results will be produced. It is required that the material must be correctly identified and the appropriate sound speed selected before measurement.
(13) Effect of stress. Most of the equipment and pipelines in service have stress. The stress condition of solid materials has a certain impact on the sound speed. When the stress direction is consistent with the propagation direction, if the stress is compressive stress, the stress will increase the elasticity of the workpiece and accelerate the sound speed; vice versa. , if the stress is tensile stress, the speed of sound slows down.
When the stress is inconsistent with the propagation direction of the wave, the vibration trajectory of the particle is disturbed by the stress during the wave process, and the propagation direction of the wave deviates. According to the data, as the general stress increases, the speed of sound increases slowly.
(14) The influence of metal surface oxides or paint coatings. Although the dense oxide or paint anti-corrosion layer produced on the metal surface is closely combined with the base material and has no obvious interface, the propagation speed of sound in the two materials is different, thus causing errors, and the error varies with the thickness of the covering. Also different.
