Temperature; Measurement Modes - DAKOTA ULTRASONICS MMX-7 Operation Manual

MMX-7 A/B Scan Thickness Gauge
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 MMX-7 is
capable of operating in, the transducers required, and the reasons for using specific
modes:
Pulse-Echo Mode (Flaw & Pit detection) – (P-E)
Pulse-echo mode measures from the initial pulse (sometimes referred to as an
artificial zero) to the first echo (reflection). A manual probe zero can be performed
while in this mode by placing the transducer on the reference disk located at the top
end cap of the MMX-7 and selecting the 'zero transducer' option in the "PRB" menu,
or the 'zero probe' option in the hot menu items.
Errors can result from surface coatings and temperature gradients in pulse-echo
mode. Therefore, the zero process should be performed on a frequent basis.
This mode is commonly used to detect flaws and pits, as it only requires one
reflection and adjustable gate to improve sensitivity when measuring heavily
corroded metals.
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