Dakota Ultrasonics UMX-2 - Manual (Page 26)

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CHAPTER SIX
SELECTING THE MEASUREMENT MODE
6.1 Which mode & transducer do I use for my application?
High penetration plastics and castings
The most common mode for these types of applications is pulse-echo. The UMX-2
has been optimized for cast materials. Cast iron applications require 1 - 5MHz
frequencies, and cast aluminum requires higher frequencies from 7 -10MHz
frequency. Plastics typically require lower frequencies depending on the thickness
and make-up of the material. Larger diameters offer greater penetration power
because of the crystal size, for difficult to measure materials.
Corrosion & Pit Detection in steel and cast materials
Use pulse-echo mode whenever attempting to locate pits and flaws. Typically a
5MHz transducer, or higher, will be used for these types of applications. Use low
frequencies for greater penetration and use higher frequencies for better resolution.
Measuring Material & Coatings
The pulse-echo coating mode should be used when both material and coating
thicknesses are required, while still requiring the ability to detect flaws and pits. A
special coating style transducer is required for use in this mode. There are a variety
of coating transducers in various frequencies available from Dakota. If a dual
element transducer is selected with the UMX-2 kit, it will be a coating enabled
version.
Thru Paint & Coatings
Often times, users will be faced with applications where the material will be coated
with paint or some other type of epoxy material. Since the velocity of the coating is
approximately 2.5 times slower than that of steel, pulse-echo mode will induce error if
the coating or paint is not completely removed. By using echo-echo mode, the user
is able to successfully measure through both, the coating and steel, and completely
eliminate the thickness of the paint or coating. Therefore, the steel can be measured
without having to remove the coating prior to measuring. Users will often use pulse-
echo mode and echo-echo mode in conjunction when performing inspections on
coated materials.
Thru coating measurements require special high damped transducers. The most
common transducers are the 3.5, 5, and 7.5MHz hi damped transducers. These
transducers are suitable for use in both pulse-echo and echo-echo modes. This
conveniently enables the user to accurately measure overall material thickness using
the thru Coating mode, and then conveniently switch to pit detection mode without
changing transducers. The ¼” 5MHz Hi damped transducer is the most commonly
used transducer for standard thru coating applications.