Dakota Ultrasonics MMX-7 - Manual (Page 26)

Dakota Ultrasonics
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through air efficiently. By using a liquid couplant between the transducer and test
piece the amount of ultrasound entering the test piece is much greater.
4.5 Temperature
Temperature has an effect on sound velocity. The higher the temperature, the slower
sound travels in a material. High temperatures can also damage transducers and
present a problem for various liquid couplants.
Since the sound velocity varies with temperature it is important to calibrate at the
same temperature as the material being measured.
Normal temperature range
Most standard transducers will operate from 0
°
F to 180
°
F.
High temperature measurements
Special transducers and couplants are a vailable for temperatures above 180
°
F up to
650
°
F with intermittent contact. It is necessary to cool the transducer, by submerging
the transducer in water between readings, when measuring high temperatures.
Modes and temperature errors
In addition to errors caused by velocity changing with temperature, some modes
(measurement techniques) are affected more than others. For example, dual
element mode has larger errors due to changes in temperature of the delay line.
However, multi-echo techniques help to minimize these errors.
4.6 Measurement Modes
In this section we will discuss the different measurements modes the MMX-7 is
capable of operating in, the transducers required, and the reasons for using specific
modes:
Pulse-Echo Mode (Flaw & Pit detection)
Pulse-echo mode measures from the initial pulse (sometimes referred to as an
artificial zero) to the first echo (reflection). In this mode, the transducer is placed on a
reference disk, located on top of the MMX-7, and a key is pressed to establish a zero
point for the particular transducer.
In this mode errors result from surface coatings and temperature variations.
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