Design And Operation; Display Modes; Norm" Display Mode - NDT Systems TG-900+ User Manual

1.5 Design and operation

The device uses piezoelectric transducers, which, when a short electric pulse is applied to them, emit
sound waves, the frequency of which significantly exceeds the upper limit of the range perceived by the
human ear (that is, ultrasound). The frequency of the signal emitted by the transducer can be 1.25 MHz
up to 10 MHz. Oscillations of such a frequency do not propagate well in air, so, to exclude an air gap and
ensure acoustic contact between the transducer and the surface of the inspected object, a layer of
contact liquid (glycerin, water, oil, special gels) is applied.
Ultrasonic (US) signals generated by the generator are fed to a piezoelectric emitter. Then they pass
through the outer surface of the object, propagate in the body of the object, and are reflected from its
inner surface (that is, from the border of a significant difference in density "metal-medium"). The
receiver sensor perceives the reflected ultrasonic signal, converts it into an electrical signal, and
transmits it to the meter, in which the time interval is converted into the result of thickness
measurements. The calculated value of the thickness of the product is displayed on the indicator of the
thickness gauge.
Various factors can affect the performance of the device. If the material of the investigated object is
inhomogeneous, contains cavities, cracks, its surface bears traces of corrosion (etc.), then the results
may be distorted.
In addition, to obtain reliable measurements of thickness, it is necessary to use the value of the
propagation velocity of ultrasound in this particular material in the calculation. The actual speed of
ultrasound in different materials differs greatly both from material to material and from those given in
the tables of reference data. The best results can be obtained if the instrument is pre-calibrated on an
object of a known thickness (reference) made of the same material as the object under study. It should
be kept in mind that the speed of sound in one and the same material can be unstable. For example,
heat treatment of material can significantly change the speed of sound propagation in it. This should be
taken into account when evaluating the accuracy of the measurements. It is useful to check the
calibration after measurements to reduce errors.

1.6 Display Modes

The readings may be shown in several modes, to switch between modes, use a "
pressed, the menu for selecting the mode of operation appears: "NORM", "CONTR" and "B SCAN".

1.6.1 "NORM" display mode

The "NORM" mode is intended for carrying out the usual measurement without outputting additional
statistics and information (Fig. 1.3).
7 | P a g e NDT Systems, Inc.
Figure 1.3 – Main measurement mode
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