지식

Single Element Transducer Modes

Single Element Transducers: Modes 1, 2, and 3

Once a sound pulse has been generated and echoes have been received, there are a variety of ways that timing can be performed. There are three common ways of measuring the time interval that represents the sound wave's travel through the test piece when using common contact, delay line, and immersion transducers. The type of transducer and the specific application requirements will usually dictate the choice of mode.

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 wear plate delays.

Mode 1 is the normal measurement mode when testing with contact transducers. It has the advantage of typically offering the greatest maximum thickness capability, and since only a single backwall echo is required, it has the best penetration capability in challenging materials like castings, low density plastics, and rubber. The disadvantages of Mode 1 are that the minimum measurable thickness will be higher than in other modes, and accuracy may be slightly lower due to coupling variations. Also, the contact transducers associated with Mode 1 can be used only on materials whose surface temperature is below 50° C or 125° F, so high temperature measurement is not possible.

Mode 2 involves measuring the time interval between an interface echo returned from the near surface of the test piece and the first backwall echo, which represents one round trip in the test piece. This mode normally requires delay line or immersion transducers.

Mode 2 is most often use to optimize near-surface resolution in plastics and composites, for high temperature measurements with high temperature delay line transducers, measurements on sharp radiuses using focused immersion transducers and focused or radiused delay line transducers, and for on-line measurement of moving material using immersion transducers. The primary disadvantage of Mode 2 measurement is that maximum thickness is limited by delay line length.

Mode 3 involves measuring the time interval between two successive backwall echoes, representing one round trip in the test piece, using delay line or immersion transducers.

Mode 3 typically offers the highest measurement accuracy and best minimum thickness resolution in a given application, although the maximum thickness will be limited. Mode 3 requires two or more clean multiple backwall echoes, which typically limits its use to materials of relatively low attenuation and high acoustic impedance such as fine-grained metals, ceramics, and glass. Measurements can be made at high temperatures with appropriate high temperature delay lines. Mode 3 also offers the advantage of ignoring thin nonmetallic coatings like paint when measuring the thickness of coated metals.

Continue on to
Dual Element Transducers >>