Definition
Ultrasonic thickness gauge is a non-destructive testing instrument that uses the principle of ultrasonic pulse reflection to measure the thickness of materials. It calculates the thickness of the material by measuring the time it takes for ultrasonic waves to travel inside the material, combined with the known speed of sound. This technology does not need to destroy the object to be measured and is widely used in thickness measurement of various homogeneous materials such as metals, plastics, glass, and composites.
Measurement principle
The working principle of an ultrasonic thickness gauge is based on the physical properties of ultrasonic waves propagating through a medium. The piezoelectric wafer in the instrument probe generates a high-frequency ultrasonic pulse that passes through a couplant into the material under test. When ultrasonic waves propagate to the underside of the material or encounter defect interfaces inside the material, reflections occur, and the reflected echoes are received by the same probe. The instrument accurately measures the time interval between the transmission of ultrasonic waves and the receiving echo, which is the propagation time of sound waves once and there is a round trip through the material.
The material thickness (T) can be calculated by the following formula: T = (v × t) / 2. Where v represents the propagation velocity (velocity of sound) of ultrasound in the material being measured, and t represents the measured propagation time. Sound velocity is an intrinsic property of materials and is often determined by calibrating a homogeneous material test block of known thickness before measurement.
Measurement method
Common measurement methods include pulse echo and resonance. The pulsed echo method is the most commonly used method and is suitable for most homogeneous materials. During operation, it is necessary to use a suitable couplant (e.g., glycerin, coupling slurry) between the probe and the measured surface to ensure the effective transmission of sound waves. Before measuring, the correct sound velocity must be set according to the material being measured or calibrated using a standard test block. For thin-walled or special materials, the multi-echo technique is sometimes used to improve measurement accuracy. When measuring, the probe should be kept perpendicular to the surface being measured and multiple measurements should be taken at different locations to obtain representative data.
Influencing factors
Measurement accuracy is influenced by various factors. The properties of the material itself, such as grain size, anisotropy, internal defects, or inclusions, can scatter or attenuate sound waves, affecting the echo signal. The roughness, curvature and cleanliness of the measured surface will affect the coupling efficiency of sound waves. Rough surfaces can cause sound energy scattering, while curved surfaces can cause sound beam dispersion. Environmental factors such as temperature changes can alter the sound velocity of a material. Operational factors, including the type and amount of couplant, the pressure and angle of the probe, and the calibration status of the instrument, all have a direct impact on the measurement results. Therefore, standardized operating procedures and application-specific conditional control are necessary.
Applications
The application of ultrasonic thickness gauges covers many industrial fields. In manufacturing, it is used to monitor wall thickness corrosion or erosion during operation of equipment such as pressure vessels, pipelines, storage tanks, boilers, etc. In the aerospace field, it is used to inspect the thickness of aircraft skins and structural parts. In the automotive industry, it is used to measure the thickness of body panels, engine components. In shipbuilding, petrochemical, electric power and other industries, it is a tool for regular safety inspections and life assessment of critical equipment. In addition, it is also commonly used for quality control of plastic products, glass products and composite products.
Instrument selection considerations
When choosing an ultrasonic thickness gauge, it is necessary to comprehensively consider the measurement needs and technical parameters. The measurement range should cover the expected thickness of the workpiece being measured. For high-temperature materials, consider the probe's ability to withstand high temperatures or use a high-temperature couplant. The acoustic properties of the material determine whether special probe frequencies or functions are required, for example, low-frequency probes are preferable for coarse-grained materials. The application scenario determines the portability, protection level, and battery life requirements of the instrument. The functions of the instrument, such as whether it supports high temperature measurement, minimum pipe diameter measurement, data storage and output, etc., should be selected according to the actual inspection process. Finally, on the basis of clarifying your own application conditions, referring to the requirements of relevant industry standards (such as ASTM E797, GB/T 11344) for measurement methods can help you make an appropriate choice.