(1) The surface roughness of the workpiece is too large, which leads to poor coupling effect between the probe and the contact surface, low reflected echo and even failure to receive echo signals. For the in-service equipment and pipeline with rusty surface and poor coupling effect, the surface can be treated by sand, grinding and other methods to reduce roughness, and at the same time, oxide and paint layer can be removed to expose metallic luster, so that the probe and the measured object can achieve good coupling effect through coupling agent.
(2) The radius of curvature of the workpiece is too large, especially when measuring the thickness of small-diameter pipes. Because the surface of the common probe is flat and the contact with the curved surface is point contact or line contact, the sound intensity transmittance is low (poor coupling). Small diameter special probe (6mm) can be selected, which can accurately measure bending 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, so the probe cannot accept the bottom wave signal.
(4) Due to the uneven structure or coarse grain of castings and austenitic steel, ultrasonic waves will have serious scattering attenuation when passing through them, and the scattered ultrasonic waves will propagate along complex paths, which may annihilate the echo and cause no display. A coarse-grained probe with low frequency (2.5MHz) can be selected.
(5) The contact surface of the probe is worn. The surface of the commonly used thickness measuring probe is acrylic resin, which will increase its surface roughness and reduce its sensitivity for a long time, resulting in incorrect display. It can be polished with 500# sandpaper to make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe.
(6) There are a lot of corrosion pits on the back of the measured object. Because there are rust spots and corrosion pits on the other side of the measured object, the sound wave attenuates, resulting in irregular changes in readings, and even no readings in extreme cases.
(7) There are sediments (such as pipes) in the measured object. When there is little difference between the acoustic impedance of the deposit and the workpiece, the display value of the thickness gauge is the wall thickness plus the deposit thickness.
(8) When there are defects (such as inclusions and interlayers). ) In the material, the displayed value is about 70% of the nominal thickness. At this time, ultrasonic flaw detector can be used for further defect detection.
(9) The influence of temperature. Generally speaking, the speed of sound in solid materials decreases with the increase of its temperature. The experimental data show that the speed of sound decreases by 1% when the temperature in hot matter increases by 100℃. This situation is often encountered in high-temperature in-service equipment. Special high-temperature probes (300-600°C) should be selected, and ordinary probes cannot be used.
(10) Laminated materials and composite (heterogeneous) materials. Uncoupled laminated materials can't be measured, because ultrasonic waves can't penetrate the uncoupled space, and can't propagate uniformly in composite (heterogeneous) materials. For equipment made of multilayer materials (such as urea high-pressure equipment), special attention should be paid to thickness measurement. The indicated value of the thickness gauge only indicates the thickness of the material layer in contact with the probe.
The function of (12) coupling agent. The coupling agent is used to remove the air between the probe and the measured object, so that the ultrasonic wave can effectively penetrate the workpiece and achieve the purpose of detection. If you choose the type or use it improperly, it will cause errors or the coupling sign will flash, which will make it impossible to measure. Because the appropriate type is selected according to the use situation, the coupling agent with low viscosity can be used when it is used on the surface of smooth materials; When used on rough surface, vertical surface and top surface, high viscosity coupling agent should be used. High temperature coupling agent should be selected for high temperature workpieces. Secondly, the coupling agent should be used in proper amount and spread evenly. In general, the coupling agent should be coated on the surface of the measured material, but when the measuring temperature is high, the coupling agent should be coated on the probe.
(13) Wrong sound speed selection. Before measuring the workpiece, preset the sound speed according to the material type or measure the echo speed according to the standard block. When one material is used to calibrate the instrument (the common test block is steel), and then another material is used to measure it, it will produce wrong results. Before measurement, it is necessary to correctly identify the material and select the appropriate speed of sound.
(14) stress. Most in-service equipment and pipelines are stressed, and the stress state of solid materials has a certain influence on the speed of sound. When the stress direction coincides with the propagation direction, if the stress is pressure,
Stress, stress will increase the elasticity of the workpiece and accelerate the speed of sound; On the contrary, if the stress is tensile stress, the speed of sound will slow down. When the stress and the propagation direction of the wave are different, the vibration trajectory of the particle is disturbed by the stress, and the propagation direction of the wave deviates. The data show that the general stress increases and the sound speed increases slowly.
(15) Influence of oxide or paint coating on metal surface. Although the dense oxide or paint coating produced on the metal surface is closely combined with the matrix material, the propagation speed of sound in the two substances is different, resulting in errors, and the error varies with the thickness of the covering.
Yaze (Shanghai) Instrument Technology Co., Ltd. is a professional instrument and equipment company integrating R&D, production and sales. This principle can be used to measure all kinds of materials, such as metal, plastic, ceramics, glass, etc., which spread uniformly in the company's ultrasonic thickness gauge equipment. On the one hand, it can accurately measure all kinds of plates and workpieces, on the other hand, it can monitor all kinds of pipes and pressure vessels in production equipment and monitor their thinning degree after corrosion in use. Widely used in petroleum, chemical industry, metallurgy, shipbuilding, aviation, aerospace and other fields. Suitable for measuring the thickness of metals (such as steel, cast iron, aluminum, copper, etc.). ), plastics, ceramics, glass, glass fiber and any other good conductor of ultrasonic waves.