14
7.3.2 Known Velocity Calibration
If the material velocity is known, you can enter the velocity value directly into the gauge. For a list
of the sound velocities of common materials see “Sound Velocities of Common Materials” on
page 27
1.
Select MENU/CAL/VELOCITY and press ENTER to display the DIGITS EDIT BOX.
2.
Adjust to match the material velocity using the LEFT, RIGHT, UP and DOWN arrows then
press OK.
3.
Press MEAS to return to the measurement screen.
7.3.3 Known Thickness Calibration
If the sound velocity of a material is unknown, a sample with one or two known thicknesses can be
used to determine the sound velocity.
Note: Although the gauge has a Thru-Paint™/coating feature, known thickness calibration must be
performed on material with the paint or coating removed.
One-point Calibration:
The one point calibration option is most suited for linearity over large
ranges. You should always calibrate on high side of the intended measurement range. For example,
if the measurement range is 2.54 mm to 25.4 mm (.100" to 1.0"), you should calibrate on a known
thickness sample close to 25.4 mm (1.0").
Note: Before you start this calibration procedure, perform a probe zero.
1.
Apply a drop of couplant on the transducer and place the transducer in steady contact with the
sample or the material being tested. Be sure that the reading is stable and the repeatability
indicator, in the top left corner of the display, is fully lit and stable.
2.
Select MENU/CAL/ONE POINT and press ENTER to display the DIGITS EDIT BOX.
3.
Adjust to match the material thickness using the LEFT, RIGHT, UP and DOWN arrows.
4.
Press OK to calculate the velocity and return to the measurement screen, or ESC to cancel.
Note: Place the transducer back on the calibration point. The thickness reading should now match
the known thickness but if outside tolerance, repeat the steps above.
Two-point Calibration:
For improved accuracy over a smaller measurement range, conduct
one-point calibration followed by a two-point calibration. For example, if the measurement range is
2.03 mm to 6.35 mm (.080" to 0.250"), perform a one point calibration on a known thickness sample
close to 6.35 mm (.250"), followed by a two-point calibration close to 2.03 mm (.080").
1.
Conduct a one point calibration routine as described previously in this section. You should
conduct this routine at the high end of the measurement range
2.
Repeat the routine with another sample at the low end of the measurement range, except this
time enter the thickness into the TWO POINT area in the CAL menu.
Note: CHECK YOUR CALIBRATION. Place the transducer back on both calibration points. If the
thickness is out of tolerance repeat the steps above.
8 MEASUREMENT - TAKING READINGS
Disclaimer:
Inherent in ultrasonic thickness measurement is the possibility that the instrument will
use the second rather than the first echo from the back surface of the material being measured. This
may result in a thickness reading that is TWICE what it should be.
Responsibility for proper use of the instrument and recognition of this phenomenon rests solely with
the user of the instrument.
Other errors may occur from measuring coated materials where the coating is insufficiently bonded
to the material surface. Irregular and inaccurate readings may result. Again, the user is responsible
for proper use and interpretation of the measurements acquired.
