Thickness gauges ultrasonic: operating principle, instruction, manufacturers, reviews

Ultrasonic thickness measurement is a non-destructive one-sided method for determining the width of a material. It is fast, reliable, versatile and, unlike a micrometer or caliper, does not require access to two sides of the object. The first commercial sensors, using the principle of sonar, appeared in the late 1940s. Small portable devices, optimized for a wide range of applications, became common in the 1970s. And innovations in the field of microprocessor technology made it possible to achieve a new level of accuracy, simplicity and miniaturization.

A large number of well-known companies are engaged in the production of devices. Among them - German company Siemens, American Dakota Ultrasonics, British Cygnus. In Russia devices are produced by such companies as NPF "AKS", NPK "Luch", NPC "MaxProfit", etc.

What can be measured?

Virtually any conventional structural material can be measured by ultrasound. Ultrasonic sensors can be tuned for metals, plastics, composites, fiberglass, ceramics and glass. Also, it is possible to measure extruded plastics and rolled products in the production process - both individual layers or coatings, and multi-layer products, liquids and biological samples. Another operation where an ultrasonic thickness gauge is simply necessary is to determine the thickness of bricks, concrete structures, asphalt and rocks. Such measurements are almost always non-destructive and do not require cutting or disassembling the object.

Materials that are not suitable for conventional ultrasonic measurements due to poor transmission of high frequency waves include wood, paper, concrete and foamed products.

How to measure?

Sound energy can be generated in a wide range of frequencies. Audible sound is in the range of 20 to 20 kHz. The higher the frequency, the higher the perceived tone. The energy of a higher frequency, beyond the limits of human hearing, is called ultrasound. Most often, ultrasonic testing is performed in the frequency range from 500 kHz to 20 MHz, although some specialized instruments reach 50 kHz or 100 MHz. Regardless of frequency, sound energy is mechanical oscillations passing in a certain medium, such as air or steel, in accordance with the basic laws of wave physics.

For measurements use an ultrasonic thickness gauge. The principle of the device is to accurately calculate the time of passage of a pulse from a small probe (converter) through a measured object, reflected by its inner surface or a far wall. Since sound waves are reflected from the boundary between heterogeneous materials, this measurement is usually performed from one side, in the "pulse / echo" mode.

The transducer contains a piezoelectric element that is excited by a short electrical pulse to generate discrete ultrasonic waves. They are sent to the material being measured and pass through it until they collide with the rear wall or other obstacle. The reflected wave returns to the sensor, which converts mechanical vibrations into electrical energy. In essence, ultrasonic thickness gages listen to the echo from the opposite side. Usually the time interval between the sent and reflected signal is only a few millionths of a second. The device records the speed of sound in the material under investigation, from which it can then calculate the thickness using a simple mathematical relationship: d = V t / 2, where:

• D is the thickness of the section;
• V is the speed of sound;
• T is the measured sound transit time.

Important parameter

It is important to note that the speed of sound in the object under study is an essential part of this calculation. Different materials transmit sound waves in different ways. As a rule, in solids it is higher, and in soft substances it is lower. In addition, it can vary considerably with temperature. It is always necessary to calibrate ultrasonic thickness gauges to the speed in the material being measured, which directly affects the accuracy of the instrument readings.

Sound waves in the megahertz range through the air pass poorly, so to improve the transmission of sound between the radiator and the sample is placed a drop of connecting fluid. Usually glycerin, propylene glycol, water, oil and gel are used as a contact fluid. A small amount of liquid is enough to fill an extremely thin air gap.

Measuring modes

Producers of ultrasonic thickness gauges measure the time interval for the passage of energy through the test sample in three ways:

1. The gap between the excitation pulse, which generates a sound wave and the first echo return, minus a small offset value compensating for the delay in the instrument, cable and transducer.
2. The time interval between the returned echo from the sample surface and the first echo reflected.
3. The gap between two consecutive bottom echoes.

The choice of the mode, as a rule, dictates the type of the converter, as well as the specific requirements of the application. The first mode is used with a contact sensor and is recommended for most applications. In the second, there is a delay line or immersion transducers used on convex and concave surfaces, in confined space, for measuring moving material or objects with a high temperature.

The third mode also uses delay lines or submersible sensors and, as a rule, provides high accuracy and the best minimum resolution of the thickness. It is usually used when the quality of measurements in the first or second mode is unsatisfactory. However, the latter mode is suitable only for materials that produce pure multiple echoes, usually with a low attenuation index, as in fine-grained metals, glass, ceramics.

Two types of devices

Thickness meters ultrasonic, as a rule, are divided into two types: corrosive and precision. One of their most important applications is to determine the residual width of the wall of metal pipes, tanks, structural parts and pressure vessels that are susceptible to internal corrosion and can not be seen from the outside. Thickness gages ultrasonic corrosion for this purpose are intended. They use signal processing techniques that are optimized to detect the minimum residual wall width in coarse and rusty samples with specialized two-element sensors.

In other cases it is recommended to use high-precision instruments with single converters, for metals, plastics, fiberglass, composites, rubber and ceramics. A lot of different sensors of precision devices have been created that can be measured with an accuracy of ± 0.025 mm and higher, which exceeds the values of corrosion meters.

GOST Thickness gauges ultrasonic classifies according to purpose, degree of automation, protection from environmental influences, resistance to mechanical influences, and also determines their main indicators.

Types of transducers

• Contact sensors are used for direct contact with the test sample. Measurements with their help are simple, therefore they are used most often.
• Converters with a delay line contain a plastic, epoxy or quartz cylinder as an intermediate link between the active element and the object under study. The main reason for their use is the measurement of thin objects, where it is important to separate the excitation pulses from the bottom echo signals. The delay line can serve as a heat insulator, protecting the sensor's temperature sensitive element from direct contact with hot materials. It can also be shaped to improve grip with sharply concave or curved surfaces.
• Submersible transducers for supplying sound energy to the measured element use a water column or bath. They are used to measure moving objects, to scan or optimize the adhesion in the presence of sharp radii, grooves or channels.
• Transducers with two elements are used in corrosion gauges to determine the width of objects with a coarse, corroded surface. They consist of a separate transmitting and receiving element, installed at a small angle to the delay line, to focus the energy at a selected distance below the surface of the sample being measured. Although such measurements are not as accurate as those of other types of sensors, they tend to provide significantly higher performance.

Thickness gauge ultrasonic: instruction

To prepare for the measurements, you must connect the inverter to the instrument, turn it on, set the sound speed and calibrate. To do this, apply a little contact material to the calibration standard, attach the sensor and turn on the calibration mode. This procedure must be carried out after replacing the converter or batteries. Possible calibration options for the known thickness and speed of sound.

To carry out the measurements, it is necessary to apply a contact substance to the surface of the object and attach a sensor. The result is displayed on the display. It is possible to use the device in scanning mode, for example, to search for the smallest material thickness. You can also set the signal to detect a place with a wall size that is less than the set value.

To measure the speed of sound, you need to measure the object with a caliper or micrometer, attach the transducer and wait for the result. After setting the previously measured value, press the button to store the data in the instrument memory. Some devices allow you to transfer the results to a PC.

Ultrasonic thickness gauge: reviews

Users appreciate the compact size, ease of use, reliability, ease of calibration of modern devices. Experts note the lack of alternatives to devices of this type in assessing the condition of cars, the quality of performance of bodywork. The device allows you to determine whether the vehicle was repainted and whether it was involved in an accident. Thickness gauges, which do not require contact liquid for operation, and also capable of self-calibration, are most popular.

Material and range

The ultrasonic thickness gauge, the principle of operation of which is selected depending on the composition, range of measurements, geometry, temperature, accuracy requirements and other possible conditions, is sometimes irreplaceable.

The material type and measurement limits are the most important factors when choosing a device and a converter. Many substances, including most metals, ceramics and glass, perform ultrasound very effectively and allow measurements to be made in a wide range. Most plastics absorb energy faster and therefore have a more limited maximum thickness range, but in most production situations, problems do not cause problems. Rubber, fiberglass and many composite materials absorb much more and require large transmitters and receivers optimized for operation at low frequencies.

The thickness determines the type of the converter. Thin objects are measured at high frequencies, and thick or damped at low frequencies. For very thin materials, a delay line is used, although they, as well as submersible transducers, are limited in thickness due to interference from a multiple echo. In the case of wide objects or objects consisting of several materials, sensors of different types may be needed.

Curvature of the surface

With increasing curvature of the surface, the contact efficiency between the transducer and the measured object decreases, therefore, as the radius of curvature decreases, the sensor size must be reduced. Measuring very small radii may require the use of delay lines or contactless immersion transducers. They can also be used for measurements in grooves, cavities and other places with limited access.

Temperature

Contact inverters are generally applicable at an object temperature of up to 50 ° C. Hotter materials can damage the sensor due to the effect of thermal expansion. In such cases, always use transducers with a heat-resistant delay line, immersion or high temperature sensors with two elements.

In some cases, an object with a low acoustic impedance (density multiplied by the speed of sound) is connected to a material with a higher acoustic impedance. Typical examples are plastic, rubber and glass coatings of steel or other metals, as well as a polymer coating of fiberglass. In this case, the echo from the boundary between the two materials will be phase-inverted - inverted in relation to the echo from the boundary with the air. This can be corrected simply by changing the setting of the instrument, but if nothing is done, the readings will be inaccurate.

Error

The accuracy of measurements is influenced by many factors, including verification of ultrasonic thickness gauges, their calibration, uniformity of velocity in matter, attenuation and scattering of sound, roughness and curvature of the surface, poor coupling and bottom nonparallelism. Accuracy is best achieved by using standards of known size. With a correct calibration, the error of the ultrasonic thickness gauge is ± 0.01 mm and even ± 0.001 mm. Lines of delay or immersion sensors in the third mode also increase the accuracy of measurements.