What Affects the Indication value of Ultrasonic Thickness Gauge?

Ultrasonic thickness gauges measure thickness according to the principle of ultrasonic pulse reflection. When the ultrasonic pulse emitted by the probe passes through the object to be measured and reaches the interface of the material, the pulse is reflected back to the probe, which is determined by accurately measuring the propagation time of the ultrasonic wave in the material. The thickness of the material being measured. This principle can be used to measure all kinds of materials that can make ultrasonic waves propagate inside them at a constant speed. The thickness gauge designed according to this principle can accurately measure various plates and various processed parts, and can also monitor various pipes and pressure vessels in production equipment to monitor their thinning degree after being corroded during use. Ultrasonic thickness gauges can be widely used in petroleum, chemical, metallurgy, shipbuilding, aviation, aerospace and other fields.

Measurement Methods

• General measurement method.
• Use the probe to measure the thickness twice at one point. In the two measurements, the dividing surfaces of the probe should be 90° from each other, and the smaller value is the thickness of the workpiece to be measured.
• 30mm multi-point measurement method. When the measurement value is unstable, take a measurement point as the center and perform multiple measurements in a circle with a diameter of about 30mm, and take the minimum value as the thickness of the measured workpiece.
• Precise measurement method. increase the number of measurements around the specified measurement point, and the thickness change is represented by a contour line.
• Continuous measurement method. Use single-point measurement method to continuously measure along the specified route, and the interval is not more than 5mm.
• Grid measurement method. Draw a grid in the designated area, and record the thickness by point. This method is widely used in high pressure equipment, stainless steel lining corrosion monitoring.

What Affects the Indication value of Ultrasonic Thickness Gauge?

• 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 unable to receive echo signals. For surface rust, in-service equipment and pipelines with extremely poor coupling effect, the surface can be treated by sanding, grinding, and filing to reduce the roughness. At the same time, the oxide and paint layer can be removed to expose the metallic luster, so that the probe can be removed. It can achieve a good coupling effect with the test object through the coupling agent.
• The radius of curvature of the workpiece is too small, especially when measuring the thickness of small-diameter tubes, because the surface of the commonly used probe is flat, and the contact with the curved surface is point contact or line contact, and the sound intensity transmittance is low (poor coupling). A small diameter special probe (6mm) can be selected, which can accurately measure curved materials such as pipes.
• The detection surface is not parallel to the bottom surface, the sound wave will be scattered when it encounters the bottom surface, and the probe cannot receive the bottom wave signal.
• Due to the uneven structure or coarse grains of castings and austenitic steels, when the ultrasonic waves pass through them, serious scattering attenuation occurs. The scattered ultrasonic waves propagate along complicated paths, which may annihilate the echoes and cause no display.​ A dedicated probe for coarse crystals with a lower frequency (2.5MHz) can be selected.
• The contact surface of the probe has certain wear. The surface of the commonly used thickness probe is made of acrylic resin. Long-term use will increase the surface roughness and reduce the sensitivity, resulting in incorrect display.
• There are a lot of corrosion pits on the back of the tested object. Due to rust spots and corrosion pits on the other side of the measured object, the sound wave is attenuated, resulting in irregular changes in readings, and even no readings in extreme cases.
• There is sediment in the object to be measured (such as a pipeline). When the acoustic impedance of the sediment and the workpiece is not much different, the thickness gauge displays the value of the wall thickness plus the sediment thickness.
• When there are defects in the material (such as inclusions, interlayers, etc.), the displayed value is about 70% of the nominal thickness, and the ultrasonic flaw detector can be used for further defect detection.
• The influence of temperature. Generally, the speed of sound in solid materials decreases with the increase of its temperature. According to experimental data, the speed of sound decreases by 1% for every 100°C increase in hot materials. This is often the case for high temperature in-service equipment. Special probes for high temperature (300-600°C) should be used instead of ordinary probes.
• Laminated materials, composite (heterogeneous) materials. It is impossible to measure uncoupled laminates because ultrasonic waves cannot penetrate uncoupled spaces and cannot travel uniformly in composite (heterogeneous) materials. For equipment made of multiple layers of material (such as urea high-pressure equipment), special attention should be paid to thickness measurement. The indication of the thickness gauge only indicates the thickness of the layer of material in contact with the probe.
• Influence of couplant. The couplant is used to remove the air between the probe and the object to be measured, so that the ultrasonic wave can effectively penetrate the workpiece to achieve the purpose of detection. If you choose the type or use the method improperly, it will cause errors or flickering of the coupling mark, making it impossible to measure. Due to the selection of the appropriate type according to the application, a low-viscosity couplant can be used when used on smooth material surfaces; a high-viscosity couplant should be used when used on rough surfaces, vertical surfaces and top surfaces. High temperature workpiece should use a high temperature couplant. Secondly, the couplant should be used in an appropriate amount and applied evenly. Generally, the couplant should be applied to the surface of the material to be measured, but when the measurement temperature is high, the couplant should be applied to the probe.
• The speed of sound is incorrectly selected. Before measuring the workpiece, preset the sound speed according to the material type or measure the sound speed according to the standard block. When the instrument is calibrated with one material (usually steel) and then used to measure another material, erroneous results will be produced. It is required to correctly identify the material and select the appropriate sound speed before measurement.
• Effects of stress. Most of the equipment and pipelines in service have stress, and the stress state of solid materials has a certain influence on the speed of sound. 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 speed of sound. Otherwise, if the stress is tensile stress, the speed of sound slows down. When the stress and the propagation direction of the wave are not in the same direction, 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, the general stress increases, and the speed of sound increases slowly.
• Influence of metal surface oxide or paint coating. 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, but the propagation speed of sound speed in the two substances is different, which causes errors.