t.:NIT UTS · 0° PROBE SCANNING
Ruane & II
T PO'Nelll
0° PROBE SCANNING
:\OTES
10
CALIBRATION
The initial pulse or main bang is a test signal that the · flaw detector creates and has no
significance for calibration. It usually lies just off to the left of a calibrated screen.
C.R. T. - Cathode Ray Tube
Initial
pulse
20
......._
30
40
50
• Dead zone - although
with twin crystal probes no
visible indication exisls on
60
the CRT. the term dead zone
correctly means an area
where indications cannot be
located and is still present
near surface due to the
angle of the crystals used to
focus the beam at the
70
optimum usage range.
-
':.
_,
•
:.
-·
•
._
_1
•
._
Visible area
·/ ofC.R.T.
0 I 2 3 4 5 6 7 8 9 10
When an ultrasonic probe is placed on to a piece of steel, some of the ultrasound in the
probe reflects off the interface between the probe shoe and the steel and some is
transmitted through into the steel. When the transmitted energy strikes the back
surface of the steel it virtually all reflects off the steel to air interface and returns to the
steel to perspex interface. Here some energy transmits into the probe and creates the
first signal (1) and the rest reflects back inside the steel and the process repeats itself,
creating the repeat signals, (2 etc.) until the energy decays away. The spacing between
the echoes represents the thickness of the steel, so if we place the probe on a A2 block,
on the 25mm thickness, then the echoes are ismm apart. Note. If we are using a single
crystal probe then the initial energy that reflected back into the probe will create a
signal at the start of the screen (F) which will be very close to the initial pulse and
there will also be a dead zone visible on the CRT. If we are using a double crystal
probe (separate transmit and receive crystals) then there will be no signal from the
front surface and no dead zone* visible.
Crystal
Probe shoe
Test
Material
F
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0 I 2 3 4 5 6 7 8 9 10
80
To CALIBRATE A 0° PROBE TO A RANGE OF: 0
90
Apply couplant to the A2 block and place the probe on the 25mm thickness to obtain
multiple echoes. We require a range of 100mm on the screen so four echoes would fit
in to this range, so we adjust the coarse range control to give us about four echoes on
the screen. We then adjust the delay control to position the first backwall echo a
quarter of the way along the screen and the fine range control to position the fourth
echo at the end of the screen. This procedure is repeated until all four echoes take up
their respective positions (see sketch). The same basic procedure applies to different
ranges using different thicknesses. By dividing the range by the thickness we can
obtain the number of echoes required and by evenly spacing the echoes on the screen
the desired range is achieved.
There are two methods of
achieving calibration:
1
a)
multi-backwall echo;
b)
delay technique.
TO 100 MM
1
100
© Ru•n• & T P O"Neill
l'9UC 6 0l/0J/�
UTS-1