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Ultrasonically, the plastic is about ½ the ultrasonic speed or velocity as
metal under test. Therefore, once the metal approaches 0.600" thick it will
start to run into the 2
nd
delay line multiple. Once the material exceeds 0.600"
in this case, no reading or in some cases an erroneous reading will be made.
The example with the water path is generally known as a “bubbler” type of
transducer whereby the column or cone is filled with a slow flowing water
path. It reacts much the same way as the plastic delay but generally permits
the ability to focus the sound beam as well as conform easier to irregular
surfaces. The same ruses as above apply.
Returning to the oscilloscope pattern lets discuss the traces as shown and
briefly discuss how to use them. Again, if the optional data port module were
installed at the time of order, this nomenclature would match the actual wires
exiting the data port adaptor.
Sync. Sync is the reference timing pulse that is the start of the thickness
measurement cycle. An output for this signal os provided to synchronize an
oscilloscope time base.
Video. The video is the waveform representation of the sound as it travels
through its path. In the previous oscilloscope example we are looking at a
delay line transducer coupled into a metal test sample. At this point we are
working with the concept here so the thickness is not important. On the
Video trace you will not the Initial Pulse is synchronized with the falling edge
of the sync trace. Time measurement for thickness starts here. This point is
also known as T=0. From this point, we have incorporated a “dead” zone
known as an IP Blocking gate where no measurements are possible but time
continues to count. You can then note that the IF or interface appears next.
In the delay mode, this is where the start of measurement occurs. In this
example, the signal is measuring the 1
st
to 2
nd
echo.
Measurement Gate: An example of this can be observed by looking at the
“Measurement Gate” in the oscilloscope display where a step indicates the
start and stop of the measurement interval. The TG930 then calculates the
time from the start to the stop of an echo period and presents the results as
a thickness value on the units digital display.
Measurement Time: This is the actual area of time that the instrument is
looking for an echo. It usually starts very shortly after the initial pulse in the
contact mode or very shortly after the interface in delay mode If for example,
the interface blocking gate (IFBlk on TG930 display) were set to short or
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within the interface time zone, the gage will think it is reading the interface
itself and trigger on a secondary half cycle within the interface echo
envelope. This will have the effect of appearing to “lock up” the instrument
since no other echos will be seen.
The lower example is typical of a standard contact transducer test. Although
the wearplate is indeed a form of delay, it is usually so thin (on the order of
0.005" - 0.020" thick) as not to be considered a delay line but rather creates
a very minor offset which we will compensate with a ZERO or Lo Cal offset
as discussed in section 4.4.
