NDT
INTERNATIONAL, INC.
NDT-710
Thickness
Gauge
4
Materials
Material is the most important determinant in final selection of gauge and transducer. Certain
materials, including most metals, glass, and ceramics, are excellent for sound propagation and
lend themselves to a wide range of measurement modes and transducer frequencies.
Other materials, such as plastics, absorb ultrasound more quickly and have a limited
measurable maximum thickness range. They are generally measured with gauges that utilize
contact type transducers. Rubber, fiberglass, and composites are even more attenuating and
often require gauges with special high penetration pulser/receivers and low frequency
transducers.
Thickness
Thickness ranges will also dictate the type of gauge and transducer to be selected. In general,
thin materials require high frequency transducers and thick or attenuating materials require
lower frequencies. Very thin material may not be within the range of a gauge utilizing contact
transducers; a delay line transducer may then be the answer. Similarly, gauges with delay line
and immersion transducers have limited maximum thickness capabilities primarily due to
potential interference from a multiple of the interface echo.
Geometry
A contract transducer is preferred for most ultrasonic measurements, unless sharp curvature or
small part size makes contact measurements impractical. As the surface curvature of the test
piece increases, the coupling efficiency from the transducer to the test piece is reduced. In
general, as the surface curvature increases, the size of the contact transducer should be
reduced. Extreme curvature or inaccessibility of the test surface requires a system with a delay
line or an immersion transducer.
Temperature
Contact transducers can be safely used on material surfaces up to 120 F (50 C). Thickness
measurements with contact transducers on material surfaces in excess of these temperatures
will result in transducer failure. Transducers with special heat resistant delay lines are
recommended on hot or warm surfaces above +180 F.
Accuracy
It should be considered that many factors may affect accuracy: sound attenuation and
scattering, sound velocity variations, poor coupling, surface roughness, non-parallelism,
curvature, echo polarity, etc. Selection of the best possible combination of gauge and
transducer should take into account all these factors. With proper calibration, measurements
can usually be made to an accuracy of 0.001 inch or 0.01 mm.
