Guide to thickness measurement by ultrasonic

Ultrasonic Thickness Gauges


From the origin to the present day


Ultrasonic thickness gauge is an NDT instrument commonly used to detect  ultrasonic conductive materials thickness. The creation of the first instrument of this kind dates back to 1967 , thanks to brilliant intuition of polish engineer Sobek , he used as parameter the speed of sound in the above sample.


Modern ultrasonic thickness gauges , despite have been evolved , continue to make use of the same physical principle and can reach accuracies in order of hundredth of millimeters , while ones more advanced can "interact" with personal computers and distinguish above coatings thanks to features that use other physical principles.


The operating principle


Instrument based on Sobek ’s prototype determine sample thickness by an accurate measurement   of time taken by an ultrasonic impulse , created by a transmitter, to cross the thickness of a material and return to its source.
The path of return of the sound wave is divided in half and multiplied by the propagation speed of sound for such material.


Areas of application


Ultrasonic thickness gauges can be used like a standard caliber , with the difference of being able to operate in NDT also on problematic surfaces , like boxed , pipes or in the middle of big plates ,simplifying and speeding work in every  productive department or quality control.


An advantage of this instrument is that it’s able to check wear status or corrosion of elements which you can access only one side or difficult to be reached , all without need to disassemble , destroy or take elsewhere samples.


How to choose the right probe


The ultrasonic thickness gauge can measure a wide range of materials such as metals, glass and plastics. Different types of materials will require different probes. This choice is fundamental to do accurate measurements . Below are indicated principal parameters to consider for select the appropriate probe.


Generally the best probe is the one that can send sufficient ultrasonic energy into the material so that the instrument will receive the return echo. Factors that influence propagation of ultrasonic are many:


Output signal’s power: Stronger is the output signal and stronger will be return echo . This parameter depends primarily on the size of probe’s component emitting ultrasonic. A large area of emission will send a greater quantity of energy into material than an area of reduced emission.



Absorption and dispersion: When ultrasonic passes through a material is partially absorbed .
If the sample has a granular structure ultrasonic wave will be subjected to a dispersion effect.
Both phenomena cause an ultrasonic energy reduction , therefore the capability of receiving return echo will be reduced . High frequency ultrasonic are more subjected to dispersion effect from those of lesser intensity . We must point out that these last are less subject to directionality than higher so they aren’t the best to be used. Then , the higher are preferred  if  we want to detect exact position of small cavities or imperfections.



Probe’s geometry: Sometimes probes may have been too big for the space within which to make measurements , in some cases the contact surface of the sample can be so small as to undermine a proper mating probe / material. In these cases probes with a  smaller body and a less extensive contact surface must be chosen .While for curved surfaces will be required a probe with coincident profile.



Material temperature: The propagation speed of sound within a material is inversely proportional to its temperature, so when it ‘s necessary to measure high temperature samples (maximum 300C °) will be equipped  probes designed to detect high temperatures. These kind of sensors are built using special materials and processes that help them resist to high temperatures physical stresses  without damage.


Mating probe / surface


Another important parameter is the mating between inspected area and the probe . A good grip between both surfaces ensures that the instrument operates at its best and provides a realistic  and  reliable measurement . It’s recommended  to ensure that surface and probe are free from dust , residues and dirt  before every measurements. To guarantee a good mating and remove the thin layer of air interposed between  probe and surface is necessary to use a coupling liquid. Modern instruments can make contact even with  pure water, even if it’s more suited to use thicker resources like glycerin or ultrasonic gel.


Limitations of the physical principle used by instrument


One of the main limitations of this instrument is the inability to reliably measure all those materials that contain , even if  minimally , discontinuity. This effect is read by the instrument as an interruption in the material, going to invalidate validity measurement . For example, this instrument will be very accurate in measuring any type of PVC , but it will not create contact with PVC foam due to micro air bubbles in its structure. The flip side of this limitation is that it allows, in front of known thicknesses ,or presumed , to detect imperfections that should not be present. In this case we speak of instruments called imperfections detectors.


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