21
21
Now couple the transducer to the thinnest step. If thickness readout does not agree with the
known thickness, reposition the cursor at ZERO and use the Up or Down arrow keys to produce
the known thickness on the thickness readout.
Then, again check the thickest step. If thickness readout still does not agree with known
thickness, reposition the cursor at VEL (velocity) and use the Up or Down arrow keys to obtain
the known thickness.
Continued adjustments between ZERO and VELOCITY will produce correct thickness readout
on both the thickest and thinnest steps. At this point, the steps in between will read correctly
[within the resolution, ±0.001 inch]. The velocity value below VEL is the velocity of the material
of the specific material of block under test.
A short-cut in the calibration routine for metals other than steel is to scroll the value at VEL to
the nominal velocity for that material. Velocity values are tabulated in a variety of publications,
including NDT Systems, Inc.' OPTIMA transducer catalog.
ECHO - ECHO
Setup Using 3/8" 5MHz Contact Transducer as
above.
22.
To examine ECHO to ECHO function, place the transducer on the
0.500 inch thickness step. ESC back to MAIN Menu using the
MENU/ESC keys. Cursor to and set RANGE to 2.00 inch. Scroll
back to GATES Menu. Cursor to MULT. Use right arrow key to
select “1-2". ECHO allows the selection of a multiple echo from
which the thickness measurement (Thickness gate) will begin. Up to
the 3rd multiple echo can be selected using the Up/Dn arrow keys.
This function is useful when using single element transducers with
thin elastomeric membranes or on coated test objects.
23.
Cursor to ECHO BLOCKING GATE (ECHO-BLK). Note the thick
horizontal bar on the Thickness gate. Use the right arrow key to select the length of the echo
blocking gate that is appropriate to block out any unwanted signals between multiple echos in
the thickness mode. The FACTORY default value is .200 inch. ECHO-BLK is actively displayed
when MULT is set at 1-2 or 2-3. Look at the upper, Main Trace window. There are 2 multiple
echos in it. One at about half screen and another at about 80% across the screen. However, if
you look at the Tracking (lower window) you will notice the gage is currently triggered between a
single echo’s half cycle producing a reading of 0.044" which is wrong. Note the short blocking
bar is about half way between. Now, look at the following image. In the Main Trace window
you’ll note the ECHO-BLK gate is set just beyond the echo half cycle set. With the gate set in
this manner note the Thickness Measurement bar extends to the echo set at the 80% screen
position which, is the correct position. The track window now displays the echo being measured
to and where the gate terminates.
For clarification regarding echo half cycles refer to the following RF based images. The one at
0.029 is triggered on successive positive half cycles. Note the position of the ECHO-BLK Gate.
Then look the image displaying 0.500" and note the position of the ECHO-BLK gate.
NOTE: Tabulated velocity values have been obtained from a variety of sources that do
not always agree. It is strongly recommended that accurately known thicknesses of the
same material as the test object be used to calibrate for accurate thickness
measurement
