Dakota Ultrasonics CMX - Manual (Page 35)

CMX High Performance Material & Coating Thickness Gauge
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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 available 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 pulse-echo mode has larger errors due to changes in the temperature of the
delay line. However, multi-echo techniques offer temperature compensation help to
minimize these errors.
4.6 Measurement Modes
In this section we will discuss the different measurements modes the CMX is capable
of operating in, the transducers required, and the reasons for using specific modes:
Pulse-Echo Mode (Flaw & Pit detection) – Coating Off (P-E)
Pulse-echo mode measures from the initial pulse (sometimes referred to as an
artificial zero) to the first echo (reflection). In this mode, either an automatic or
manual zero can be performed depending on the zero probe function setting. If the
manual mode has been selected, the transducer is placed on a reference disk,
located on top of the CMX, and a key is pressed to establish a zero point for the
particular transducer. If the Auto Zero feature is enabled, a simple key press will
perform an electronic zero to establish the same zero point.
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.