PVX-B
Precision Visual Thickness Gauge
21
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 mode has larger errors due to changes in temperature of the delay line.
However, multi-echo techniques help to minimize these errors. The PVX-B operates
in multi-echo modes for a variety of transducer types.
4.6 Measurement Modes
In this section we will discuss the different measurements modes the
PVX-B
is
capable of operating in, the transducers required, and the reasons for using specific
modes:
Pulse-Echo Mode
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
PVX-B
, and a key is pressed to establish a zero
point for the particular transducer.
In pulse-echo mode errors result from surface coatings and temperature variations.
This mode is only used with single element contact style transducers.
Echo-Echo (Interface-Echo) Modes
The echo-echo modes measure between two reflections. This technique is
commonly used to eliminate errors from surface coatings in echo-echo mode, as well
as make measurements in multiple layered materials. The disadvantage is that two
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