Thickness and Flaw Gauges & Detectors
AGS-TECH Inc. offers ULTRASONIC FLAW DETECTORS and a number of different THICKNESS GAUGES with different principles of operation. One of the popular types are the ULTRASONIC THICKNESS GAUGES ( also referred to as UTM ) which are measuring instruments for the NON-DESTRUCTIVE TESTING & investigation of a material's thickness using ultrasonic waves. Another type is HALL EFFECT THICKNESS GAUGE ( also referred to as MAGNETIC BOTTLE THICKNESS GAUGE ). The Hall Effect thickness gauges offer the advantage of accuracy not being affected by the shape of samples. A third common type of NON-DESTRUCTIVE TESTING ( NDT ) instruments are EDDY CURRENT THICKNESS GAUGES. Eddy-current-type thickness gauges are electronic instruments that measure variations in impedance of an eddy-current inducing coil caused by coating thickness variations. They can only be used if the electrical conductivity of the coating differs significantly from that of the substrate. Yet a classical type of instruments are the DIGITAL THICKNESS GAUGES. They come in a variety of forms and capabilities. Most of them are relatively inexpensive instruments that rely on contacting two opposing surfaces of the specimen to measure thickness. Some of the brand name thickness gauges and ultrasonic flaw detectors we sell are SADT, SINOAGE and MITECH.
ULTRASONIC THICKNESS GAUGES : What makes ultrasonic measurements so attractive is their ability to gauge thickness without a need for accessing both sides of the test specimen. Various versions of these instruments such as ultrasonic coating thickness gauge, paint thickness gauge and digital thickness gauge are commercially available. A variety of materials including metals, ceramics, glasses and plastics can be tested. The instrument measures the amount of time it takes sound waves to traverse from the transducer through the material to the back end of the part and then the time which the reflection takes to get back to the transducer. From the time measured, the instrument calculates the thickness based on the speed of sound through the specimen. The transducer sensors are generally piezoelectric or EMAT. Thickness gauges with both a predetermined frequency as well as some with tunable frequencies are available. The tunable ones allow inspection of a wider range of materials. Typical ultrasonic thickness gauge frequencies are 5 mHz. Our thickness gauges offer the capability to save data and to output it to data logging devices. Ultrasonic thickness gauges are non-destructive testers, they do not require access to both sides of the test specimens, some models can be used on coatings and linings, accuracies less than 0.1mm can be obtained, easy to use on the field and no need for lab environment. Some disadvantages are the requirement of calibration for each material, need for good contact with the material which sometimes requires special coupling gels or petroleum jelly to be used at the device/sample contact interface. Popular application areas of portable ultrasonic thickness gauges are shipbuilding, construction industries, pipelines and pipe manufacturing, container and tank manufacturing....etc. The technicians can easily remove dirt and corrosion from the surfaces and then apply the coupling gel and press the probe against the metal to measure thickness. Hall Effect gages measure total wall thicknesses only, while ultrasonic gages are capable to measure individual layers in multilayer plastic products.
In HALL EFFECT THICKNESS GAUGES the measurement accuracy will not be affected by the shape of samples. These devices are based on the theory of Hall Effect. For testing, the steel ball is placed on one side of the sample and the probe on the other side. The Hall Effect sensor on the probe measures the distance from the probe tip to the steel ball. The calculator will display the real thickness readings. As you can imagine, this non-destructive test method offers quick measurement for spot thickness on area where accurate measurement of corners, small radii, or complex shapes are required. In nondestructive testing, Hall Effect gages employ a probe containing a strong permanent magnet and a Hall semiconductor connected to a voltage measurement circuit. If a ferromagnetic target such as a steel ball of known mass is placed in the magnetic field, it bends the field, and this changes the voltage across the Hall sensor. As the target is moved away from the magnet, the magnetic field and hence the Hall voltage, change in a predictable manner. Plotting these changes, an instrument can generate a calibration curve that compares the measured Hall voltage to the distance of the target from the probe. The information entered into the instrument during the calibration allows the gage to establish a lookup table, in effect plotting a curve of voltage changes. During measurements, the gage checks the measured values against the lookup table and displays thickness on a digital screen. Users only need to key in known values during calibration and let the gage do the comparing and calculating. The calibration process is automatic. Advanced equipment versions offer display of the real time thickness readings and automatically captures the minimum thickness. Hall Effect thickness gauges are widely used in plastic packaging industry with rapid measurement ability, up to 16 times per second and accuracies of about ±1%. They can store thousands of thickness readings in memory. Resolutions of 0.01 mm or 0.001 mm (equivalent to 0.001” or 0.0001”) are possible.
EDDY CURRENT TYPE THICKNESS GAUGES are electronic instruments that measure variations in impedance of an eddy-current inducing coil caused by coating thickness variations. They can only be used if the electrical conductivity of the coating differs significantly from that of the substrate. Eddy current techniques can be used for a number of dimensional measurements. The ability to make rapid measurements without the need for couplant or, in some cases even without the need for surface contact, makes eddy current techniques very useful. The type of measurements that can be made include thickness of thin metal sheet and foil, and of metallic coatings on metallic and nonmetallic substrate, cross-sectional dimensions of cylindrical tubes and rods, thickness of nonmetallic coatings on metallic substrates. One application where the eddy current technique is commonly used to measure material thickness is in the detection and characterization of corrosion damage & thinning on the skins of aircraft. Eddy current testing can be used to do spot checks or scanners can be used to inspect small areas. Eddy current inspection has an advantage over ultrasound in this application because no mechanical coupling is required to get the energy into the structure. Therefore, in multi-layered areas of the structure like lap splices, eddy current can often determine if corrosion thinning is present in buried layers. Eddy current inspection has an advantage over radiography for this application because only single sided access is required to perform the inspection. To get a piece of radiographic film on the back side of the aircraft skin might require uninstalling interior furnishings, panels, and insulation which could be very costly and damaging. Eddy current techniques are also used to measure the thickness of hot sheet, strip and foil in rolling mills. An important application of tube-wall thickness measurement is the detection and assessment of external and internal corrosion. Internal probes must be used when the external surfaces are not accessible, such as when testing pipes that are buried or supported by brackets. Success has been achieved in measuring thickness variations in ferromagnetic metal pipes with the remote field technique. Dimensions of cylindrical tubes and rods can be measured with either outer diameter coils or internal axial coils, whichever is appropriate. The relationship between change in impedance and change in diameter is fairly constant, with the exception at very low frequencies. Eddy current techniques can determine thickness changes down to about three percent of the skin thickness. It is also possible to measure the thicknesses of thin layers of metal on metallic substrates, provided the two metals have widely differing electrical conductivities. A frequency must be selected such that there is complete eddy current penetration of the layer, but not of the substrate itself. The method has also been used successfully for measuring thickness of very thin protective coatings of ferromagnetic metals (such as chromium and nickel) on non-ferromagnetic metal bases. On the other hand, the thickness of nonmetallic coatings on metal substrates can be determined simply from the effect of liftoff on impedance. This method is used for measuring the thickness of paint and plastic coatings. The coating serves as a spacer between the probe and the conductive surface. As the distance between the probe and the conductive base metal increases, the eddy current field strength decreases because less of the probe's magnetic field can interact with the base metal. Thicknesses between 0.5 and 25 µm can be measured with an accuracy between 10% for lower values and 4% for higher values.
DIGITAL THICKNESS GAUGES : They rely on contacting two opposing surfaces of the specimen to measure thickness. Most digital thickness gauges are switchable from metric reading to inch reading. They are limited in their capabilities because proper contacting is needed to make accurate measurements. They are also more prone to operator error due to variations from user to user’s specimen handling differences as well as the wide differences in specimen properties such as hardness, elasticity….etc. They may be however sufficient for some applications and their prices are lower compared to the other types of thickness testers. The MITUTOYO brand is well recognized for its digital thickness gauges.
Our PORTABLE ULTRASONIC THICKNESS GAUGES from SADT are:
SADT Models SA40 / SA40EZ / SA50 : SA40 / SA40EZ are the miniaturized ultrasonic thickness gauges that can measure wall thickness and velocity. These intelligent gauges are designed to measure the thickness of both metallic and nonmetallic materials such as steel, aluminum, copper, brass, silver and etc. These versatile models can easily be equipped with the low & high frequency probes, high temperature probe for demanding application environments. The SA50 ultrasonic thickness meter is micro-processor controlled and is based on the ultrasonic measurement principle. It is capable of measuring the thickness and acoustic speed of ultrasound transmitted through various materials. The SA50 is designed to measure the thickness of standard metal materials and metal materials covered with coating. Download our SADT product brochure from above link to see differences in measuring range, resolution, accuracy, memory capacity, ….etc between these three models.
SADT Models ST5900 / ST5900+ : These instruments are the miniaturized ultrasonic thickness gauges that can measure wall thicknesses. The ST5900 has a fixed velocity of 5900 m/s, which is used only for measuring the wall thickness of steel. On the other hand, the model ST5900+ is capable to adjust velocity between 1000~9990m/s so that it can measure the thickness of both metallic and nonmetallic materials like steel, aluminum, brass, silver,…. etc. For details on various probes please download product brochure from the above link.
Our PORTABLE ULTRASONIC THICKNESS GAUGES from MITECH are:
Multi-Mode Ultrasonic Thickness Gauge MITECH MT180 / MT190 : These are multi-mode ultrasonic thickness gauges based on the same operating principles as SONAR. The instrument is capable of measuring the thickness of various materials with accuracies as high as 0.1/0.01 millimeters. The multi-mode feature of the gauge allows the user to toggle between pulse-echo mode (flaw and pit detection), and echo-echo mode (filtering paint or coating thickness). Multi-mode: Pulse-Echo mode and Echo-Echo mode. The MITECH MT180 / MT190 models are capable of performing measurements on a wide range of materials, including metals, plastic, ceramics, composites, epoxies, glass and other ultrasonic wave conducting materials. Various transducer models are available for special applications such as coarse grain materials and high temperature environments. The instruments offer Probe-Zero function, Sound-Velocity-Calibration function, Two-Point Calibration function, Single Point Mode and Scan Mode. The MITECH MT180 / MT190 models are capable of seven measurement readings per second in the single point mode, and sixteen per second in the scan mode. They have coupling status indicator, option for Metric/Imperial unit selection, battery information indicator for the remaining capacity of the battery, auto sleep and auto power off function to conserve battery life, optional software to process the memory data on the PC. For details on various probes and transducers please download product brochure from the above link.
ULTRASONIC FLAW DETECTORS : Modern versions are small, portable, microprocessor-based instruments suitable for plant and field use. High frequency sound waves are used to detect hidden cracks, porosity, voids, flaws and discontinuities in solids such as ceramic, plastic, metal, alloys…etc. These ultrasonic waves reflect from or transmit through such flaws in the material or product in predictable ways and produce distinctive echo patterns. Ultrasonic flaw detectors are nondestructive test instruments (NDT testing). They are popular in testing of welded structures, structural materials, manufacturing materials. The majority of ultrasonic flaw detectors operate at frequencies between 500,000 and 10,000,000 cycles per second (500 KHz to 10 MHz), far beyond the audible frequencies our ears can detect. In ultrasonic flaw detection, generally the lower limit of detection for a small flaw is one-half wavelength and anything smaller than that will be invisible to the test instrument. The expression summarizing a sound wave is:
Wavelength = Speed of Sound / Frequency
Sound waves in solids exhibit various modes of propagation:
- A longitudinal or compression wave is characterized by particle motion in the same direction as wave propagation. In other words the waves travel as a result of compressions and rarefactions in the medium.
- A shear / transverse wave exhibits particle motion perpendicular to the direction of wave propagation.
- A surface or Rayleigh wave has an elliptical particle motion and travels across the surface of a material, penetrating to a depth of approximately one wavelength. Seismic waves in earthquakes are also Rayleigh waves.
- A plate or Lamb wave is a complex mode of vibration observed in thin plates where material thickness is less than one wavelength and the wave fills the entire cross-section of the medium.
Sound waves may be converted from one form to another.
When sound travels through a material and encounters a boundary of another material, a portion of the energy will be reflected back and a portion transmitted through. The amount of energy reflected, or reflection coefficient, is related to the relative acoustic impedance of the two materials. Acoustic impedance in turn is a material property defined as density multiplied by the speed of sound in a given material. For two materials, the reflection coefficient as a percentage of incident energy pressure is:
R = (Z2 - Z1) / (Z2 + Z1)
R = reflection coefficient (e.g. percentage of energy reflected)
Z1 = acoustic impedance of first material
Z2 = acoustic impedance of second material
In ultrasonic flaw detection, the reflection coefficient approaches 100% for metal / air boundaries, which can be interpreted as all of the sound energy being reflected from a crack or discontinuity in the path of the wave. This makes ultrasonic flaw detection possible. When it comes to reflection and refraction of sound waves, the situation is similar to that of light waves. Sound energy at ultrasonic frequencies is highly directional and the sound beams used for flaw detection are well defined. When sound reflects off a boundary, the angle of reflection equals the angle of incidence. A sound beam that hits a surface at perpendicular incidence will reflect straight back. Sound waves that are transmitted from one material to another bend in accordance to Snell's Law of refraction. Sound waves hitting a boundary at an angle will be bent according to the formula:
Sin Ø1/Sin Ø2 = V1/V2
Ø1 = Incident angle in first material
Ø2= Refracted angle in second material
V1 = Velocity of sound in the first material
V2 = Velocity of sound in the second material
Transducers of ultrasonic flaw detectors have an active element made of a piezoelectric material. When this element is vibrated by an incoming sound wave, it generates an electrical pulse. When it is excited by a high voltage electrical pulse, it vibrates across a specific spectrum of frequencies and generates sound waves. Because sound energy at ultrasonic frequencies does not travel efficiently through gasses, a thin layer of coupling gel is used between the transducer and the test piece.
Ultrasonic transducers used in flaw detection applications are:
- Contact Transducers: These are used in direct contact with the test piece. They send sound energy perpendicular to the surface and are typically used for locating voids, porosity, cracks, delaminations parallel to the outside surface of a part, as well as for measuring thickness.
- Angle Beam Transducers: They are used in conjunction with plastic or epoxy wedges (angle beams) to introduce shear waves or longitudinal waves into a test piece at a designated angle with respect to the surface. They are popular in weld inspection.
- Delay Line Transducers: These incorporate a short plastic waveguide or delay line between the active element and the test piece. They are used to improve near surface resolution. They are suitable for high temperature testing, where the delay line protects the active element from thermal damage.
- Immersion Transducers: These are designed to couple sound energy into the test piece through a water column or water bath. They are used in automated scanning applications and also in situations where a sharply focused beam is needed for improved flaw resolution.
- Dual Element Transducers: These utilize separate transmitter and receiver elements in a single assembly. They are often used in applications involving rough surfaces, coarse grained materials, detection of pitting or porosity.
Ultrasonic flaw detectors generate and display an ultrasonic waveform interpreted with the aid of analysis software, to locate flaws in materials and finished products. Modern devices include an ultrasonic pulse emitter & receiver, hardware and software for signal capture and analysis, a waveform display, and a data logging module. Digital signal processing is used for stability and precision. The pulse emitter & receiver section provides an excitation pulse to drive the transducer, and amplification and filtering for the returning echoes. Pulse amplitude, shape, and damping can be controlled to optimize transducer performance, and receiver gain and bandwidth can be adjusted to optimize signal-to-noise ratios. Advanced version flaw detectors capture a waveform digitally and then perform various measurement and analysis on it. A clock or timer is used to synchronize transducer pulses and provide distance calibration. Signal processing generates a waveform display that shows signal amplitude versus time on a calibrated scale, digital processing algorithms incorporate distance & amplitude correction and trigonometric calculations for angled sound paths. Alarm gates monitor signal levels at selected points in the wave train and flag echoes from flaws. Screens with multicolor displays are calibrated in units of depth or distance. Internal data loggers record full waveform and setup information associated with each test, information like echo amplitude, depth or distance readings, presence or absence of alarm conditions. Ultrasonic flaw detection is basically a comparative technique. Using appropriate reference standards along with a knowledge of sound wave propagation and generally accepted test procedures, a trained operator identifies specific echo patterns corresponding to the echo response from good parts and from representative flaws. The echo pattern from a tested material or product may then be compared to the patterns from these calibration standards to determine its condition. An echo that precedes the backwall echo implies the presence of a laminar crack or void. Analysis of the reflected echo reveals the depth, size, and shape of the structure. In some cases testing is performed in a through transmission mode. In such a case the sound energy travels between two transducers placed on opposite sides of the test piece. If a large flaw is present in the sound path, the beam will be blocked and the sound will not reach the receiver. Cracks and flaws perpendicular to the surface of a test piece, or tilted with respect to that surface, are usually invisible with straight beam test techniques because of their orientation with respect to the sound beam. In such cases which are common in welded structures, angle beam techniques are used, employing either common angle beam transducer assemblies or immersion transducers aligned so as to direct sound energy into the test piece at a selected angle. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material. If the angle is high enough, all of the energy in the second material will be in the form of shear waves. The energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials. In addition, the minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave. The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece and, after bouncing off the far side it is highly sensitive to cracks perpendicular to the coupling surface.
Our ultrasonic flaw detectors from SADT / SINOAGE are:
Ultrasonic Flaw Detector SADT SUD10 and SUD20 : SUD10 is a portable, microprocessor-based instrument used widely in manufacturing plants and in the field. SADT SUD10, is a smart digital device with new EL display technology. SUD10 offers almost all functions of a professional nondestructive test instrument. The SADT SUD20 model has the same functions as SUD10, but is smaller and lighter. Here are some features of these devices:
-High-speed capture and very low noise
-DAC, AVG, B Scan
-Solid metal housing (IP65)
-Automated video of test process and play
-High contrast viewing of the waveform at bright, direct sunlight as well as complete darkness. Easy reading from all angles.
-Powerful PC software & data can be exported to Excel
-Automated calibration of transducer Zero, Offset and/or Velocity
-Automated gain, peak hold and peak memory functions
-Automated display of precise flaw location (Depth d, level p, distance s, amplitude, sz dB, Ø)
-Automated switch for three gauges (Depth d, level p, distance s)
-Ten independent setup functions, any criteria can be input freely, can work in the field without test block
-Big memory of 300 A graph and 30000 thickness values
-RS232/USB port, communication with PC is easy
-The embedded software can be updated online
-Li battery, continuous working time of up to 8 hours
-Display freezing function
-Automatic echo degree
-Angles and K-value
-Lock and unlock function of system parameters
-Dormancy and screen savers
-Electronic clock calendar
-Two gates setting and alarm indication
For details download our SADT / SINOAGE brochure from the link above.
Some of our ultrasonic detectors from MITECH are:
MFD620C Portable Ultrasonic Flaw Detector with hi-resolution color TFT LCD display.
The background color and the wave color can be selectable according to the environment.
LCD brightness can be manually set. Continue working for over 8 hours with high
performance lithium-ion battery module (with large capacity lithium-ion battery option),
easy to be dismantled and the battery module can be charged independently outside the
device. It is light and portable, easily to be taken by one hand; easy operation; superior
reliability guarantees long lifetime.
0~6000mm (at steel velocity); range selectable in fixed steps or continuously variable.
Spike excitation with low, middle and high choices of the pulse energy.
Pulse Repetition Rate: manually adjustable from 10 to 1000 Hz.
Pulse width: Adjustable in a certain range to match different probes.
Damping: 200, 300, 400, 500, 600 selectable to meet different resolution and
Probe working mode: Single element, dual element and through transmission;
Real-time sampling at 160MHz high speed, enough to record the defect information.
Rectification: Positive half wave, negative half wave, full wave, and RF :
DB Step: 0dB, 0.1 dB, 2dB, 6dB step value as well as auto-gain mode
Alarm with sound and light
Total 1000 configuration channels, all instrument operating parameters plus DAC/AVG
curve can be stored; stored configuration data can be easily previewed and recalled for
quick, repeatable instrument setup. Total 1000 datasets store all instrument operating
parameters plus A-scan. All the configuration channels and datasets can be transferred to
PC via USB port.
Automatically searches the peak wave inside the gate and holds it on the display.
Equivalent diameter calculation: find out the peak echo and calculate its equivalent
Continuous Record: Record the display continuously and save it to the memory inside the
Defect Localization: Localize the defect position, including the distance, the depth and its
plane projection distance.
Defect Sizing: Calculate the defect size
Defect Evaluation: Evaluate the defect by echo envelope.
DAC: Distance Amplitude Correction
AVG: Distance Gain Size curve function
Crack measure: Measure and calculate the crack depth
B-Scan: Display the cross-section of the test block.
Real time clock for tracking the time.
USB2.0 high-speed communication port
For details and other similar equipment, please visit our equipment website: http://www.sourceindustrialsupply.com