Dakota Ultrasonics UMX-2 - Manual (Page 41)

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CHAPTER NINE
THRU PAINT MEASUREMENT TECHNIQUE
9.1 Introduction to Thru Paint Measurement (Dual Element Probes)
The principle behind thru paint measurement is by measuring the time between two
back wall echoes returning from the test material. Since both of these back wall
echoes travel the same path through the paint or coating, the thickness of the coating
is subtracted out of the measurement so that only the actual material thickness can
be measured. This feature saves the user a great deal of time scraping and
removing the coating from tanks and pipes during the inspection process.
The primary purpose of thru paint measurement is to determine actual or overall
material thickness by eliminating the coating thickness. Thru paint mode cannot be
used for flaw or pit detection. Therefore, inspectors may need to use echo-echo thru
paint mode in conjunction with the standard pulse-echo flaw detection mode for some
applications. Pipe and tube inspectors will use the echo-echo mode for pipes with
powder coatings, and pulse-echo mode for pipes without coating. The combination
of using both modes is ideal for the advanced inspectors needs.
Note: The procedure to select the measurement mode in the UMX-2 is located at
the end of this chapter.
9.2 Triple Echo (TCG) – Single Element Probes
The TCG mode was added to include single element membrane style transducers,
and operates using three individual return echoes. Each echo is then adjusted, in
terms of ‘gain’, to provide sufficient signal amplitude for each individual echo. This is
needed because, if you’re standing at the edge of a canyon and scream hello there,
each return echo becomes weaker than the first until the sound in gone.
Similar to echo-echo mode, the measurement is made between the first two return
echoes, eliminating the paint/coating and measuring only the based material.
However, a second measurement is made between the second and third echoes,
and compared to the first measurement. If both measurements are the same, the
stability indicator will fully light up, and a stable measurement will be displayed.
Advantages:
The primary advantage using a single element transducer is linearity. Since only one
crystal is used, and perfectly flat, a correction curve is not needed to compensate for
focusing issues. A second advantage would be the comparison made between two
different sets of echoes.