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What Factors Affect Ultrasonic Measurement?

Sep. 13, 2021

In any ultrasonic measurement application, the choice of strain gauge and transducer depends on the material to be measured, thickness range, geometry, temperature, accuracy requirements, and any special conditions that may exist. As a non-destructive testing equipment manufacturer, zhongke NDT can provide full details for a particular application. Listed below are the main factors that should be considered.



The thickness range will also determine the type of strain gauge and transducer that should be selected. Typically, thin materials are measured at high frequencies and thick or attenuated materials are measured at low frequencies. Delay line transducers are often used for very thin materials, although delay line (and immersion) transducers have a more restricted maximum measurable thickness due to potential interference from multiple interface echoes. In some cases involving a wide range of thicknesses and/or multiple materials, more than one transducer type may be required.

 Phased Array Ultrasonic Automatic Testing Equipment for the Seamless Pipe

 Phased Array Ultrasonic Automatic Testing Equipment for the Seamless Pipe


The type of material and the thickness range to be measured are the most important factors in the selection of gauges and sensors. Many common engineering materials, including most metals, ceramics, and glass, transmit ultrasound very efficiently and can be measured easily over a wide range of thicknesses. Most plastics absorb ultrasonic energy more quickly and therefore have a more limited maximum thickness range, but can still be measured easily in most ultrasonic manufacturing situations. Rubber, glass fiber, and many composites have a higher degree of attenuation and often require a high penetration gauge with a pulse generator/receiver optimized for low-frequency operation.



As the curvature of the part surface increases, the coupling efficiency between the transducer and the specimen decreases, so as the radius of curvature decreases, the size of the transducer should normally also decrease. Measurements on very steep radii, especially concave curves, may require specially profiled delay line transducers or non-contact immersion transducers to achieve the correct acoustic coupling. Delay line and immersion transducers can also be used for measurements in recesses, cavities, and similarly restricted access areas.

 Picture of The Detection Results

    Picture of The Detection Results


Common contact sensors can typically be used on surfaces up to approximately 125° F or 50° C. The use of most contact sensors on hotter materials will result in permanent damage due to thermal expansion effects. In such cases, a delay line transducer with a heat resistant delay line, an immersion transducer, or a high-temperature double crystal transducer should always be used.



Many factors can affect the accuracy of measurements in a given application, including correct instrument calibration, uniformity of material sound velocity, acoustic attenuation and scattering, surface roughness, curvature, poor acoustic coupling and back wall non-parallelism. All these factors should be taken into account when selecting gauges and transducers. With proper calibration, measurements can usually be made to an accuracy of +/- 0.001" or 0.01 mm, and in some cases to an accuracy closer to 0.0001" or 0.001 mm. Accuracy in a given application can best be determined by using a reference standard of precisely known thickness. Typically, gauges using mode 3 measurements with delay lines or immersion sensors are able to determine the thickness of the part most accurately.

For a more detailed discussion of ultrasonic measurement principles, welcome to contact us today or request a quote.

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