Table of Contents
Advertisement
Dakota Ultrasonics
V-Path Correction
Dual element delay line transducers have two piezoelectric elements mounted at an
angle on one end of the delay line. One element is used for transmitting sound, while
the other element only receives sound. The two elements and their delay lines are
packaged in a single housing but acoustically isolated from each other with a sound
barrier. This allows the transducer the ability to achieve very high sensitivity for
detecting small defects. Also, the surface of the test material does not have to be as
flat in order to obtain good measurements.
Dual element transducers are normally used in pulse-echo mode for finding defects,
and in echo-echo mode for through coating measurements.
Dual element delay line transducers are usable over a range of 0.025 inches to 20
inches depending on the material, frequency, and diameter.
A limitation of dual element delay-line transducers is the V shaped sound path.
Because the sound travels from one element to another, the time versus thickness
relationship is non-linear. Therefore, a correction table in the instruments software is
used to compensate for this error.
Dual Element Transducer showing V-path of signal
Searching for small defects
Dual element delay line transducers are especially useful in searching for small
defects. In the pulse-echo mode with high amplifier gain, very small defects can be
measured, and is useful during corrosion inspections to locate pits, porosity and
cracks during tank and pipeline inspections, for example.
Echo-Echo Mode – Thru Coat (E-E)
The echo-echo mode measures between two reflections. This technique is
commonly used to eliminate errors from surface coatings and applications with
temperature gradients. The disadvantage is that two echoes are required for
measurement. Certain materials can be difficult to pass sound through, making it
hard to successfully achieve multiple echoes.
24
Advertisement
Table of Contents
