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What’s Ultrasonic Thickness Measurement?

A-B Scan Thickness Gauges

 

In the field of ultrasonic testing, ultrasonic thickness measurement is a method of performing non-destructive testing of the local thickness of a solid element (typically made of metal, if using ultrasound testing for industrial purposes) basing on the time taken by the ultrasound wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauge.

 

What material can be measured?

Ultrasonic thickness gages can be set up for metals, plastics, composites, fiberglass, ceramics, and glass. Materials that are generally not suited for conventional ultrasonic measurement include wood, paper, concrete, and foam products.

 

How ultrasonic thickness does gages work?

All ultrasonic thickness gages work by very precisely measuring how long it takes for a sound pulse that has been generated by a probe called an ultrasonic transducer to travel through a test piece. Because sound waves reflect from boundaries between dissimilar materials, this measurement is normally made from one side in a “pulse/echo” mode, where the gage measures the round trip transit time of a pulse that reflects off the far side or back wall of the test piece.

The ultrasonic transducer contains a piezoelectric element which is excited by a short electrical impulse to generate a burst of ultrasonic waves. The sound waves are coupled into the test material and travels through it until they encounter a back wall or other boundary. The reflections then travel back to the transducer, which converts the sound energy back into electrical energy. In essence, the gage listens for the echo from the opposite side. Typically this time interval is only a few millionths of a second. The gage is programmed with the speed of sound in the test material, from which it can then calculate thickness using the simple mathematical relationship

T = (V) x (t/2)
T = the thickness of the part
V = the velocity of sound in the test material
t = the measured round-trip transit time

It is important to note that the velocity of sound in the test material is an essential part of this calculation. Different materials transmit sound waves at different velocities, generally faster in hard materials and slower in soft materials, and sound velocity can change significantly with temperature. Thus it is always necessary to calibrate an ultrasonic thickness gage to the speed of sound in the material being measured, and accuracy can be only as good as this calibration.

Sound waves in the megahertz range do not travel efficiently through air, so a drop of coupling liquid is used between the transducer and the test piece in order to achieve 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 air gap that would otherwise exist between the transducer and the target.

There are three common ways of measuring the time interval that represents the sound wave’s travel through the test piece. Mode 1 is the most common approach, simply measuring the time interval between the excitation pulse that generates the sound wave and the first returning echo and subtracting a small zero offset value that compensates for fixed instrument, cable, and transducer delays. Mode 2 involves measuring the time interval between an echo returned from the surface of the test piece and the first backwall echo. Mode 3 involves measuring the time interval between two successive backwall echoes. The type of transducer and specific application requirements will usually dictate the choice of mode.

 

Advantages of ultrasonic thickness measurement:

  • Quick measurements with direct results.
  • High precision measurements.
  • The inspected object can be in-service.
  • No particular surface preparation required.
  • Measurements can be made without removal of the paint.
  • Measurements can be made in inaccessible regions using suitable probes.
  • In many cases there is the possibility of measuring corroded surfaces without the need of cleaning the surface (using special probes).
  • Corrosion rate calculations can be made with thickness reduction observations with repeated measurements in the same points.
  • Measurements on high temperature surfaces possible.
  • Display of corrosion profile on B-Scan form.