Dakota Ultrasonics UMX-2 - Manual (Page 24)

UMX-2 Underwater Material & Coating Thickness Gauge
20
Since pulse-echo only requires one reflection, it is the most sensitive mode for
measuring weak reflections (flaws) typically found when measuring heavily corroded
metals.
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. This is very useful during corrosion inspections overall. The dual element
style transducer will find wall deterioration, pits, and any porosity pockets during tank
and pipeline inspections.
Echo-Echo Mode – Thru-Paint (E-E)
The echo-echo mode measures between two reflections. This technique is
commonly used to eliminate errors from surface coatings and also to make
measurements in multiple layered materials. The disadvantage is that two echoes
are needed which requires a much stronger echo (reflection).