Ultrasonic thickness gauges

Ultrasonic instruments for thickness measurements

Ultrasonic thickness gauges are used for measure the thickness of materials by accessing from only one side of the wall using ultrasonic waves.

When an ultrasonic wave is sent through the material, this signal is reflected from the back wall of the material and received by the feeler gauge probe. The delay between sending and receiving the signal can be used for calculate the thickness of the material.

To be able to measure the thickness of a wall with an ultrasonic meter, the material must be homogeneous and compact. Almost all metals are suitable for measurement with an ultrasonic thickness gauge, as well as other materials such as glass, plastics and even some types of rubber.

The ultrasonic thickness gauge is used in preventive maintenance, in ordinary maintenance, during non-destructive tests or for the acceptance of materials in the production phase.

The choice of the ultrasonic meter must be based on the application to be addressed. You can choose instruments with a generic probe, suitable for many applications, or instruments with interchangeable probes and that can be adapted to specific applications (high temperature, presence of paints, large measurement area, materials particularly difficult to measure because of medium and low density).

Ultrasonic thickness gauges with numeric display

Ultrasonic thickness gauges with graphic display

Ultrasonic thickness gauges for underwater use

TECHNICAL IN-DEPTH

Typical applications

The most common applications in which ultrasonic thickness gauges are used is corrosion level measurement on metal products (tanks, hulls of ships, cranes, gantries, pipes, tanks and sheets in general).

Corroded metal does not carry ultrasonic waves because it contains air.

Using an ultrasonic thickness gauge, the thickness of the non-corroded part of the metal can be easily measured.

This is particularly useful when the back side of the material is out of reach, this is the case with many ship hulls, pipes and tanks.

Other common applications are measuring the thickness of the walls of plastic and glass bottles, metal cans or plastic containers. 

Ranges of ultrasonic thickness gauges

RODER offers three different ranges of tools:

• Ultrasonic thickness gauges with numeric display (suitable for thickness measurement and corrosion control applications)

• Ultrasonic thickness gauges with graphic display (with A-scan / B-scan functions and graphic display of the ultrasound waveform and relative echoes)

• Thickness gauges for underwater applications

Principle of Operation of ultrasonic thickness gauges

The ultrasonic thickness gauge is a tool used to detect the thickness of the conductive materials of ultrasound in a non-destructive way. The first applications date back to the 60s.

The current ultrasonic measuring instruments, although using more modern acquisition systems and more advanced and complete visual interfaces, exploit the same physical principle as the first measuring instruments built in the last century.

Ultrasonic thickness gauges determine the thickness of a material through an accurate measurement of the time taken by an ultrasonic pulse, generated by a piezoelectric transducer, to cross the thickness of a material and return to its source. The time taken for the round trip of the sound wave is divided in half and then multiplied by the speed of propagation of the sound referred to that particular material.

The transducer contains a piezoelectric element which is excited by a short electrical impulse to generate a train of ultrasonic waves. Sound waves are coupled to the material to be tested and travel through it until they meet a rear wall or another type of material (air, water, rust, enamel, etc). The reflections then travel back to the transducer which converts the sound energy into electrical energy. Basically the transducer intercepts the echo from the opposite side. Typically this time interval is a few millionths of a second. The ultrasonic thickness gauge is programmed with the speed of sound in the material under test, and can therefore calculate the thickness using the simple mathematical report

T = V x (t / 2)

where

T = wall thickness

V = the speed of sound in the test material

t = the transit time of the route

In some cases a zero offset is subtracted to take into account fixed delays of the instrument and the sound path (eg Distance between the ultrasonic translator and the probe-material coupling point).

It is important to note that the speed of sound in the test material is an essential part of this calculation. Different materials transmit sound waves at different speeds, generally faster in hard materials and slower in soft materials. Furthermore, the velocities of sound can change significantly with temperature. It is therefore always necessary to calibrate an ultrasonic thickness gauge for the speed of sound in the material to be measured, and the accuracy can only be as good as this specific calibration. This normally occurs by referring to a sample object whose thickness is known and certified. In the case of high temperature measurements, it is also necessary to remember that the speed of sound decreases with temperature, so for maximum accuracy the reference measurement should be carried out at the same temperature as the "field" test.

High translator oscillation frequencies have a shorter wavelength thus allowing the measurement of thinner materials. Lower frequencies with a larger wavelength penetrate farther and are used to test very thick samples, or materials that are more difficult to pass through such as fiberglass and coarse-grained molten metals (e.g. cast iron) where sound waves have a less efficient transit. Selecting an optimal test frequency often involves balancing these two requirements (resolution and penetration capability).

Sound waves in the megahertz range do not travel efficiently through the air, so a drop of coupling liquid is used between the transducer and the specimen in order to obtain good sound transmission. Common couplants are glycerin, propylene glycol, water, oil and gel. Only a small amount is needed, just enough to fill the extremely thin space that forms between the transducer and the material to be measured.

Advantages of ultrasonic measurement

Measure on one side of the material

Ultrasonic thickness gauges are often used in situations where the operator has access to only one side of the material, such as in the case of pipes or conduits, or in those cases where simple mechanical measurement is impossible or impractical for other reasons such as size excessive construction, access restrictions or mechanical impracticability (e.g. in the center of large sheets or on sheet coils where the turns are wound one on top of the other). The simple fact that thickness measurements with ultrasound technology can be easily and quickly done on one side, without the need to cut parts, is one of the main advantages of this technology.

Non destructive measure

No cutting or sectioning of parts is required, saving the costs of scrap and preparation of the specimen.

Highly reliable

Modern digital ultrasound meters are very precise, repeatable and reliable and in many cases suitable for use even by unskilled personnel.

Versatile

Almost all common engineering materials can be measured with the appropriate configurations: metals, many plastics, composites, fiberglass, glass, carbon fiber, ceramic and rubber. 
Most ultrasonic thickness gauges can be pre-programmed with multiple purposes of use

Wide measuring range

Ultrasonic gauges are available for measuring ranges from 0,2 mm up to 500 mm depending on the material and type of transducer. Resolutions down to 0,001mm can be achieved.

Easy to use

The vast majority of applications using ultrasonic thickness gauges require simple pre-programmed configurations and only a small part of operator interaction.

Immediate response

The ultrasound measurement is usually performed in just one or two seconds for each measurement point and the numerical results are immediately displayed through as a digital reading of the display.

Compatible with data logging and statistical analysis programs

Most modern portable ultrasonic thickness gauges offer both a local datalogger for measurement data, and any USB or RS232 ports for transferring the measurements to an external computer for storage and further analysis.

The choice of probe and instrument

For each ultrasonic measurement application, the choice of a suitable instrument and transducer is fundamental, based on the type of test material, its thickness range, the degree of accuracy required by the measurement. It is also necessary to consider part geometry, temperature, and any other special circumstances that may affect the test setup.

In general, the best probe for each type of measurement is the one that manages to send sufficient ultrasonic energy into the material, considering that the instrument must receive an adequate return echo. The factors that influence the propagation of ultrasound are manifold.

Strength of the output signal

The stronger the output signal, the stronger the return echo to be detected and processed. This parameter basically depends on the size of the component of the probe emitting the ultrasound and on the resonance frequency of the transducer.

A large emission surface, combined with a large coupling surface with the material under test, will send a greater quantity of energy into the material than a smaller emission area.

Absorption and dispersion

When an ultrasound passes through a material, part of the emitted energy is absorbed by the material itself. If the sample material has a granular structure, the ultrasonic wave will undergo a dispersion and attenuation effect. Both phenomena cause a reduction of ultrasonic energy and consequently the ability of the instrument to perceive the return echo. High frequency ultrasounds suffer more from dispersion effects than lower frequency waves.   

Temperature of the material

The speed of propagation of sound within a material is inversely proportional to its temperature. When it is necessary to measure samples with a high surface temperature, up to a maximum of 350 ° C, probes designed specifically for high temperature measurements must be used. These particular probes are built using special processes and materials, which allow them to resist the physical stress of high temperatures without being damaged.

Probe / surface coupling

Another very important parameter is the coupling between the surface under test and the tip of the probe. A good adherence between the two surfaces ensures that the instrument works at its best and provides a reliable and realistic measurement. For this reason it is recommended to make sure before each measurement that the surface and the probe are free of dust, residues and dirt.

To guarantee an excellent coupling and to eliminate the thin layer of air between the probe and the surface, it is necessary to use a coupling liquid.

Type of probe

All transducers that are commonly used with ultrasonic feeler gauges incorporate a resonant ceramic element and differ in the way this translator is coupled to the material under test.

Contact Transducers: Contact transducers are used in direct contact with the specimen. A thin "wear plate" protects the active element from damage during normal use. Contact transducer measurements are often the simplest to make and are usually the first way to go for most thickness or corrosion measurement applications.

DELAY LINE transducers: Delay line transducers incorporate a plastic cylinder, usually of epoxy or fused silica, used as a delay line between the active element and the test piece. One of the main reasons for their use is for measurements of thin materials, where it is important to separate the excitation pulses from “backwall” echoes. Additionally, a delay line can be used as a thermal insulator, protecting the heat-sensitive transducer element from direct contact with the hot material. Finally, delay lines can be shaped to improve ultrasound coupling in confined spaces.

Immersion Transducers: Immersion transducers use a column or a water bath for coupling to the material. They can be used for online measurements directly on the production line or to measure moving products

Double element transducers: double element transducers, or simply "dual", are mainly used for measurements made on rough or corroded surfaces. They incorporate separate transmission and reception, with two elements mounted on a delay line with a small angle to concentrate the sound energy a precise distance below the surface of a test piece. Although measurements with dual transducers are sometimes less accurate than those made with other types of transducers, they usually provide significantly better performance in corrosion control applications and where there are many irregularities in the surfaces of the material.

Limits of the ultrasonic thickness gauges

One of the main limitations of ultrasonic thickness gauges lies in the inability to measure materials that are not compact or are not homogeneous.

The presence of micro-bubbles (such as for example in expanded materials or in some types of cast iron castings) or micro-discontinuities can lead to a significant attenuation of the return echo and therefore the impossibility of accurately determining the measurement thick. In some cases the return echo is not even present because it is completely dispersed in the "micro-cavities" of the material.

Furthermore, the measurement in non-homogeneous materials (multiple laminates, bituminous agglomerates, resins loaded with glass fibers, concrete, wood, granites), while presenting the possibility of determining the transit time of the ultrasonic echo, does not allow to determine the thickness of the material in a unique way due to the presence of multiple materials that contribute in different ways to the propagation of the echo.

Advanced use of ultrasound measurement and analysis technologies

Some types of ultrasonic measuring instruments, in particular those equipped with a graphic display, are able to perform detailed analysis of the waveform of the ultrasound received and therefore allow greater control of the parameters involved in the thickness measurement with ultrasound (amplification , gain, threshold).

Here are the details of some graphical and numerical representations of the data obtained by an instrument with advanced analysis characteristics of the received ultrasound.

A-SCAN - RF mode

RF mode displays the waveform in a similar way to an oscilloscope. Displays both positive and negative peaks. The peak (both positive and negative) selected for the measurement is shown in the upper part of the display. This is the preferred mode for precise measurement of thin objects using a pencil transducer. It is important to note that the measurement must be within the visible display in order to be able to see the waveform. However, even if the waveform is out of the visible display, a measurement can still be made and viewed in digital mode. If the wave is out of the display, you can change the range manually by adjusting the delay and width values ​​or use the Auto Find feature located in the UTIL menu.

The following is a list of the features visible on the display: 

A) Stability of the reading indicator : indicates the stability of the return echo on a scale from 1 to 6 - the bar shown in the image above indicates the repeatability signal. If the instrument is displaying a reading from memory, the repeatability indicator will be replaced by the text MEM

B) Battery level indicator : the fully colored battery symbol means that the battery is fully charged. Note: in the image above the battery is at 50%

C) Thickness reading : digital thickness reading (in inches or millimeters)

D) Detection indicator : the vertical dashed line displays the zero crossing detection point on the waveform where the measurement was obtained. Note that the digital thickness reading is the same as the location of the bearing indicator according to the F values ​​shown in the image

E) Echo signal : Graphic representation of the echo waveform drawn on the Y axis with reference to the amplitude and on the X axis with reference to the time.

F) Measurement Labels : The measurement labels are calculated based on the delay set (left side of the screen) and based on the Width parameter set (width value for each reference mark)

G) Measure unit : Displays the current unit of measurement.

H) Hot Menu: Each location displayed under the waveform is called a "hot menu". These locations allow a quick view of all the significant parameters of the instrument.

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A-SCAN - Rectified mode

Adjusted A-Scan mode displays half waveform. Both positive and negative peaks are displayed based on the selected polarity. This is the best display view for error detection applications. It is important to note that the measurement must be within the visible display in order to be able to see the waveform. However, even if the waveform is out of the visible display, a measurement can still be made and viewed in digital mode. If the wave is out of the display, you can change the range manually by adjusting the delay and width values ​​or use the Auto Find feature located in the UTIL menu.

The following is a list of the features visible on the display: 

A) Stability of the reading indicator: indicates the stability of the return echo on a scale from 1 to 6 - the bar displayed in the image above indicates the repeatability signal. If the PVX is displaying a reading from the memory the repeatability indicator will be replaced by the text MEM

B) Battery level indicator: the fully colored battery symbol means that the battery is fully charged. Note: in the image above the battery is at 50%

C) Thickness reading: digital reading of the thickness (in inches or millimeters)

D) Bearing indicator: the vertical dashed line displays the zero crossing detection point on the waveform where the measurement was obtained. Note that the digital thickness reading is the same as the location of the bearing indicator according to the F values ​​shown in the image

E) Echo signal: Graphic representation of the waveform of the echo drawn on the Y axis with reference to the amplitude and on the X axis with reference to time.

F) Measurement Labels : The measurement labels are calculated based on the delay set (left side of the screen) and based on the Width parameter set (width value for each reference mark)

G) Measure unit : Displays the current unit of measurement.

H) Hot Menu: Each location displayed under the waveform is called a "hot menu". These locations allow a quick view of all the significant parameters of the instrument.

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B-scan

The B-Scan mode displays a cross view of the section of the material to be measured. This view is commonly used to visualize the bottom or blind contour of the material surface. It is very similar to the fish finder. If a defect is located during a scan, the B-Scan will draw the defect on the screen. The rectangle (E) represents the cross section of the material. You will notice that the overall thickness of the material will be .500 "and the display range from 0.00" to 1.00 "respectively. The images are displayed at a rate of 15 seconds per screen from right to left - Note also that at point J the thickness has a sudden drop.

It is important to set the measurement range on the display so that the maximum thickness of the material can be seen. 

The following is a list of the features visible on the display: 

A) Stability of the reading indicator : indicates the stability of the return echo on a scale from 1 to 6 - the bar displayed in the image above indicates the repeatability signal. If the PVX is displaying a reading from the memory the repeatability indicator will be replaced by the text MEM

B) Battery level indicator : the fully colored battery symbol means that the battery is fully charged. Note: in the image above the battery is at 50%

C) Thickness reading : digital thickness reading (in inches or millimeters)

D) B-SCAN display area: This is the area where the B-scan scan is displayed

E) B-scan chart : B-scan graph display area The B-scan scan is displayed from right to left at a rate of 15 seconds per scan.

F) Measurement Labels : The measurement labels are calculated based on the delay set (left side of the screen) and based on the Width parameter set (width value for each reference mark)

G) Measure unit : Displays the current unit of measurement.

H) Hot Menu: Each location displayed under the waveform is called a "hot menu". These locations allow a quick view of all the significant parameters of the instrument.

 I) Scan Bar: The scan bar graphically represents the thickness value measured and represented in the B-scan graph. It is very useful for finding defects with direct scans on the material.

J) Contour: The B-scan view allows you to see the profile of the material from the opposite side to the measurement side.

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DIGITS

The DIGIT display allows you to view the current thickness value using large and easily visible characters. The scan bar has been added to allow the operator to detect defects and irregularities during scanning operations.

This is the list of features of the display in digits function.

A) Stability of the reading indicator : indicates the stability of the return echo on a scale from 1 to 6 - the bar displayed in the image above indicates the repeatability signal. If the PVX is displaying a reading from the memory the repeatability indicator will be replaced by the text MEM

B) Battery level indicator : the fully colored battery symbol means that the battery is fully charged. Note: in the image above the battery is at 50%

C) Thickness reading : digital thickness reading (in inches or millimeters)

D) DIGITS display area: This is the area where the thickness is displayed

F) Measurement Labels : The measurement labels are calculated based on the delay set (left side of the screen) and based on the Width parameter set (width value for each reference mark)

G) Scan bar : The scan bar corresponds directly to the thickness value. This screen is widely used for scanning material with the B-SCAN function. It is very easy to observe the presence of defects using the scan bar.
H) Hot Menu: Each location displayed under the waveform is called a "hot menu". These locations allow a quick view of all the significant parameters of the instrument.