Ultrasonic Thickness Gauge is based on the principle of ultrasonic pulse reflection thickness measurement, when the ultrasonic pulse emitted by the probe through the measured object reaches the material interface, the pulse is reflected back to the probe through the accurate measurement of ultrasonic propagation time in the material to be determined The thickness of the material is measured.
Ultrasonic Thickness Gauge is based on the principle of ultrasonic pulse reflection thickness measurement, when the ultrasonic pulse emitted by the probe through the measured object reaches the material interface, the pulse is reflected back to the probe through the accurate measurement of ultrasonic propagation time in the material to be determined The thickness of the material is measured. Any material that allows ultrasound to propagate inside it at a constant speed can be measured using this principle.
Ultrasonic Thickness Gauge uses the latest high-performance, low-power microprocessor technology, based on the principle of ultrasonic measurement, can measure the thickness of metal and other materials, and can measure the sound speed of the material. Thickness measurements can be made on various pipes and pressure vessels in production facilities to monitor the degree of thinning after they are etched during use. It is also possible to make accurate measurements on various plates and various processed parts.
Thickness gauge designed according to ultrasonic pulse reflection principle can accurately measure all kinds of plates and various processed parts. It can also monitor various pipelines and pressure vessels in production equipment and monitor their reduction after corrosion during use. Thin degree. Can be widely used in petroleum, chemical, metallurgy, shipbuilding, aviation, aerospace and other fields.
Techniques for measuring using ultrasonic thickness gauges
First, clean the surface
Before the measurement, all the dust, dirt, and rust on the surface of the measured object should be removed, and the covered material such as paint should be removed.
Second, improve the roughness requirements
Excessively rough surfaces can cause measurement errors and even instrument readings. Before the measurement, the surface of the material to be tested should be as smooth as possible, and it can be smoothed by grinding, polishing, boring, etc. The high-viscosity couplant can also be used, and the coarse grain probe SZ2.5P is selected.
Third, rough machining surface
Regular irregularities caused by rough machined surfaces (such as lathes or planers) can also cause measurement errors. The method of compensation is the same as 2, and the probe crosstalk plate (the thin layer passing through the center of the probe bottom surface) is adjusted and tested. The angle between the material slots,
Make the spacer plate and the slotted slot perpendicular or parallel to each other, and take the minimum value in the reading as the measurement thickness, which can achieve better results.
Fourth, measuring cylindrical surface
For measuring cylindrical materials, such as tubes, oil drums, etc., it is important to select the angle between the probe crosstalk plate and the axis of the material being measured. Simply put, the probe is coupled with the material to be tested, the probe crosstalk plate is parallel or perpendicular to the axis of the material to be measured, and the probe is slowly shaken perpendicular to the axis of the material to be measured, and the reading on the screen will change regularly. Select the minimum value in the reading as the exact thickness of the material.
The criterion for choosing the cross direction of the crosstalk plate of the probe and the axis of the material to be measured depends on the curvature of the material. For a pipe with a larger diameter, the probe crosstalk plate is selected to be perpendicular to the axis of the pipe, and the pipe with a smaller diameter is selected with the pipe. The axis is parallel and perpendicular, and the minimum value of the reading is taken as the thickness of the measurement.
V. Compound appearance
When measuring composite profile materials (such as pipe elbows), the method described in 7.4 can be used. The only difference is that the secondary measurement should be performed, and the two values ​​of the vertical and horizontal axes of the probe's crosstalk plate should be read separately. The smaller one is the thickness of the material at the measuring point.
Six, the temperature effect of the material
Both the thickness of the material and the propagation speed of the ultrasonic wave are affected by the temperature. If the measurement accuracy is required to be high, the test block with the same material can be used to measure the temperature at the same temperature, the temperature measurement error of the material can be calculated, and the parameters can be corrected. For steel, high temperatures will cause large errors and this method can be used to compensate for the correction.
Seven, non-parallel surfaces
In order to obtain a satisfactory ultrasonic response, the other surface of the material to be measured must be parallel or coaxial to the measured surface, otherwise it will cause measurement errors or no readings at all.
The above content is the technique of measuring using an ultrasonic thickness gauge. The thickness gauge designed based on the principle of ultrasonic pulse reflection can accurately measure various plates and various processed parts, and can also be used for various pipelines and pressure vessels in production facilities. Conduct monitoring and monitor the degree of thinning after they are eroded during use.
1, the general measurement method:
(1) Use a probe to measure thickness twice at one point. In the two measurements, the split surface of the probe should be 90° apart, and take the smaller value of the measured workpiece thickness.
(2) 30mm multi-point measurement method: When the measurement value is not stable, take a measurement point as the center, perform multiple measurements in a circle with a diameter of about 30mm, and take the minimum value as the measured workpiece thickness value.
2. Accurate measurement method: Increase the number of measurements around the specified measurement point. Thickness changes are indicated by equal thick lines.
3, continuous measurement method: using a single point measurement method along the specified route of continuous measurement, the interval is not greater than 5mm.
4. Mesh measurement method: Mark the grid in the designated area and record the thickness according to the point. This method is widely used in high pressure equipment, stainless steel lining corrosion monitoring.
5, the factors that affect the ultrasonic thickness gauge indication:
(1) The surface roughness of the workpiece is too large, which results in poor coupling between the probe and the contact surface, low reflection echo, and even failure to receive echo signals. For surface corrosion, the in-service equipment and pipelines with poor coupling effect can be treated with sand, grinding, and contusion to reduce the roughness. At the same time, the oxides and paint layers can be removed to expose the metallic luster. Coupling agent can achieve a good coupling effect with the test object.
(2) The radius of curvature of the workpiece is too small, especially when the small-diameter tube measures thickness, because the surface of the commonly used probe is a plane, contact with the surface is point contact or line contact, and the sound intensity transmittance is low (coupling is not good). Optional small diameter probe (6mm) can be used to accurately measure curved surface materials such as pipes.
(3) The detection surface is not parallel to the bottom surface, and the acoustic wave encounters scattering on the bottom surface, and the probe cannot receive the final wave signal.
(4) castings, austenitic steel due to uneven tissue or coarse grain, ultrasonic scattering in it when there is a serious scattering attenuation, scattered ultrasonic waves along a complex path, may cause echoes annihilation, causing no display . The coarse crystal special probe (2.5MHz) with lower frequency can be used.
(5) The probe contact surface has a certain amount of wear. The commonly used thickness measurement probe surface is acrylic resin, and its long-term use will increase the surface roughness, which will result in a decrease in sensitivity, which may result in incorrect display. Optional sanding with 500# sandpaper makes it smooth and parallel. If it is still unstable, consider replacing the probe.
(6) There are a large number of corrosion pits on the back of the measured object. Due to rust and corrosion pits on the other side of the test object, sound waves are attenuated, resulting in random readings and no reading in extreme cases.
(7) There are deposits in the measured object (such as pipelines). When the acoustic impedance of the deposits and the workpiece are not much different, the thickness gauge shows the wall thickness plus the thickness of the sediment.
(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, flaw detection can be further performed with an ultrasonic flaw detector.
(9) The effect of temperature. The sound velocity in general solid materials decreases as their temperature increases, and experimental data shows that the sound speed drops by 1% for every 100°C increase in hot material. This is often the case for equipment that is in service at high temperatures. Should use high-temperature special probe (300-600 °C), do not use ordinary probe.
(10) Laminated and composite (heterogeneous) materials. It is not possible to measure uncoupled laminates because ultrasonic waves cannot penetrate the uncoupled space and cannot propagate uniformly in composite (heterogeneous) materials. For devices made of multi-layered materials (such as urea high-pressure equipment), special attention should be paid to the thickness measurement, and the value indicated by the thickness gage only indicates the thickness of the material in contact with the probe.
(12) Coupling agent effects. Coupling agent is used to eliminate the air between the probe and the measured object, so that the ultrasonic energy can effectively penetrate the workpiece to achieve the purpose of detection. If you choose a type or use it incorrectly, it will cause the error or coupling symbol to flicker and cannot be measured. Depending on the type of application, a low viscosity couplant can be used when used on a smooth material surface, and a high viscosity couplant should be used on rough, vertical, and top surfaces. High temperature parts should use high temperature couplant. Secondly, the couplant should be applied in proper amount and evenly applied. Generally, the couplant should be coated on the surface of the tested material, but when the measuring temperature is high, the couplant should be coated on the probe.
(13) The speed of sound is incorrect. Before measuring the workpiece, preset its sound speed according to the type of material or reversely measure the speed of sound according to the standard block. When a material is used to calibrate the instrument (usually the test block is steel) and another material is measured, it will produce erroneous results. Requirements must be correctly identified before the measurement of materials, select the appropriate speed of sound.
(14) The effect of stress. Most of the equipment and pipelines in service have stress, and the stress state of the solid material has a certain influence on the sound speed. When the stress direction and the propagation direction are the same, if the stress is compressive stress, the stress will increase the elasticity of the workpiece and accelerate the sound speed. If the stress is tensile stress, the speed of sound will slow down. When the propagation direction of stress and wave is not the same, the vibration track of the particle in the wave process is disturbed by the stress and the wave propagation direction is deviated. According to the data, the general stress increases and the speed of sound increases slowly.
(15) Ultrasonic Thickness Gauge Effects of oxide or paint overlay on metal surfaces. Although the dense oxide or paint coating produced on the metal surface is tightly bound to the base material and has no obvious interface, the speed of sound propagation in the two materials is different, resulting in errors, and the error size varies with the thickness of the cover. Also different.
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