According to the ultrasonic pulse reflection principle for thickness measurement, when the ultrasonic pulse emitted by the probe passes through the measured object to reach the material interface, the pulse is reflected back to the probe, and the thickness of the measured material is determined by accurately measuring the time during which the ultrasonic wave propagates in the material. . Various materials that enable ultrasonic waves to propagate inside at a constant speed can be measured using this principle. Designed according to this principle, it can accurately measure various plates and various processed parts. It can also monitor various pipes and pressure vessels in production equipment to monitor the degree of corrosion after corrosion. Can be widely used in petroleum, chemical, metallurgy, shipbuilding, aviation, aerospace and other fields.
skills:
1. General measurement method:
(1) Perform the thickness measurement twice with the probe at one point. In the two measurements, the split faces of the probes should be 90° each other, and the smaller value is the thickness of the workpiece to be tested.
(2) 30mm multi-point measurement method: When the measured value is unstable, a plurality of measurements are made in a circle having a diameter of about 30 mm centering on one measurement point, and the minimum value is the thickness value of the workpiece to be tested.
2. Accurate measurement method: increase the number of measurements around the specified measurement points, and the thickness change is indicated by the thick line.
3. Continuous measurement method: continuous measurement along the specified route by single point measurement method, the interval is not more than 5mm.
4, grid measurement method: draw a grid in the specified area, according to the point thickness record. This method is widely used in corrosion monitoring of high pressure equipment and stainless steel lining.
Factors affecting the indication of the ultrasonic thickness gauge:
[/b] (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 unable to receive the echo signal. For surface rust, in-service equipment, pipes, etc., which have extremely poor coupling effects, can be treated by sand, grinding, and frustration to reduce the roughness, and the oxide and paint layers can be removed to expose the metallic luster. 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 the small diameter tube is thick, because the surface of the common probe is flat, the contact with the curved surface is point contact or line contact, and the sound intensity transmission is low (coupling is not good). A small diameter probe (6mm can be used to accurately measure the surface material such as pipes.
(3) The detection surface is not parallel to the bottom surface, the sound wave encounters the bottom surface to generate scattering, and the probe cannot accept the bottom wave signal.
(4) Castings and austenitic steels are unevenly distributed or coarse grains. When ultrasonic waves pass through them, they cause severe scattering attenuation. The scattered ultrasonic waves propagate along complicated 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 acryl 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 defect detection can be further performed by the ultrasonic flaw detector.
(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 should be used (300-600 °C, do not use ordinary probes.
(10) Laminated materials, composite (heterogeneous) materials. To measure uncoupled systems牟愕
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