Olympus EPOCH 650 User Manual page 169

The calibration requires a test block with two known thicknesses made from the
material being measured. Ideally, the two thicknesses should represent thicknesses
that are both below and above the expected thickness of the material to be inspected.
For this example, Olympus standard 5-step steel test block (part number 2214E) is
used. It has steps measuring 0.100 in., 0.200 in., 0.300 in., 0.400 in., and 0.500 in.
If the EPOCH 650 is set to work in metric units, the calibration process is exactly the
same, except that the entries are in millimeters rather than inches.
Because of the acoustic characteristics of dual transducers, nonlinearity in the distance
calibration occurs as the thickness of the material decreases. The point of maximum
sensitivity is determined by the roof angle of the particular dual transducer. The
distance calibration should be performed using a step block that covers the range of
interest. Be careful when interpreting thickness measurement readings made outside
of the calibrated range. The EPOCH 650 does not have v-path correction; possibly
causing some nonlinearity within the calibrated range. The amount of nonlinearity
depends on the minimum thickness used in the calibration process.
The zero offset value of dual element transducers can vary significantly at extreme
temperatures. If the temperature changes more than a few degrees from the
temperature at which the zero offset value was established, recheck its value. If you
intend to make thickness measurements over a wide temperature range, you should
use Olympus D790-SM and D791 dual element transducers. These transducers are
designed for high-temperature applications and have built-in delay lines with a stable
sound velocity that does not change significantly with temperature.
To calibrate using a dual element transducer
1. Follow the initial setup procedure outlined in "Basic Setup" on page 143.
2. Connect the transducer to an appropriate cable, and then connect the cable to the
transducer connectors at the top of the instrument.
3. Select Pulser > Mode = Dual.
4. Press dB, and then increase the gain significantly so that the leading edges of the
back-wall echoes appear as nearly vertical lines on the screen.
5. To use the leading edge when making thickness measurements, set the
measurement gate to the edge detection mode by selecting Gate Setup > G1
Mode = Edge.
DMTA-10055-01EN, Rev. A, February 2015
Calibration 157