Jiang Hua Wu
how to Calibrate with Dual-Element (TR) Probes
Dual-element (TR) probes are especially used for wall thickness measurement.
The following peculiarities must be taken into account when using these probes:
Echo Flank. Most dual-element (TR) probes have a roof angle (transducer
elements with inclined orientation toward the test surface). This causes mode
conversions both at beam index (sound entry into the material) and at the
reflection from the backwall, which can result in very jagged echoes.
V-Path Error. Dual-element (TR) probes produce a v-shaped sound path from
the pulser via the reflection from the backwall to the receiver element. This
so-called V-path error affects the measuring accuracy. You should therefore
choose two wall thicknesses that cover the expected thickness measurement
range for the calibration. In this way, the V-path error can be corrected to the
greatest possible extend.
Higher Material Velocity. Due to the V-path error, a higher material velocity
than that of the test material is given during calibration, especially with small
thicknesses. This is typical of dual-element (TR) probes and serves for
compensation of V-path error.
With small wall thicknesses, the above-described effect leads to an echo
amplitude drop which has to be especially taken into account with thicknesses
less than 2 mm.
A stepped reference block having different wall thicknesses is required for
calibration. The wall thicknesses must be selected so that they cover the
We recommend to use the semiautomatic calibration function for the calibration
with T/R probes.
- Set the required test RANGE
- Increase the probe delay until the two calibration echoes selected are
displayed within the range
- Set the pulser and receiver functions according to the probe used and
the test application.
- Select the function group PDELAY.
- Enter the distances of the two calibration echoes in S-REF1 and
- Position one gate on the first calibration echo.
- Position the other gate on the second calibration echo.
- Press the knob while selecting the function CAL to trigger the calibration.
The correct calibration is confirmed by the message Calibration is finished.
The instrument will now automatically determine the sound velocity and the
probe delay and set the corresponding functions accordingly. The value of the
function P-DELAY will be set to the correct value.
If necessary, check the calibration on one or several known calibration lines, e.g.
using the stepped reference block.