Specifications

Page 26 DM4E, DM4, DM4 DL Operating Manual
6. Application
6.1 General Notes
The DM4 is easy to use and will produce reliable, consistent
measurements when used correctly and with an understand-
ing of factors that may affect measurement reliability. This
section explains some of the more common considerations.
6.1.1 Flaws
If, during testing, the DM4 suddenly reads a value which is
much thinner than the apparent thickness of the part, it may
be reading the distance to a flaw in the test piece, rather than
the distance to the backwall. See Section 1.3, How the DM4
Measures Thickness, for more information. If the cause is
unclear, further examination of the part with an ultrasonic flaw
detection instrument or other suitable NDT method is recom-
mended.
6.1.2 Surface Conditions
A regular pattern on the surface under test, such as machine
grooves, may cause a false thickness reading when using a
dual element probe. Higher frequency
probes, such as the DA312, are especially sensitive to this
condition. The problem can usually be corrected by rotating
the probe so that the crosstalk barrier is at a right angle to the
grooves, or using the Dual Multi measurement mode.
It is possible for the surface of a test piece to be too rough to
permit a good reading. Excess couplant could be trapped
between the probe and surface under test, causing a false
reading. A very rough surface may prevent coupling altogether
(no coupling indicator). This problem can be corrected by grind-
ing the surface until it is smooth enough to permit good cou-
pling.
6.1.3 Curved Surfaces
When measuring on curved surfaces, such as tubes or pipes,
be sure to keep the probe centered on the part and as stable
as possible.
As a rule, smaller diameter probes improve coupling and
minimize “rocking” on curved parts. In some cases, special
probes with contoured faces to match surface curvature may
be needed. Practice may be helpful to develop the proper
technique.
When using a flat dual probe, position the crosstalk barrier at
a right angle to the long axis of the part, as shown in the
following figure
6.2 Thickness Measurement on Hot Test Pieces
The instrument can be used to measure the thickness of
materials with surface temperatures of up to 1,000°F (538°C)
with special, high temperature, dual element probe Model
HT400. A special couplant, Krautkramer ZGM, and duty
cycle are also required for high temperature use. The
following procedure is recommended
Calibrate the DM4 according to the appropriate procedure in
Section 2.5 or 2.6.
Thoroughly clean the surface to be tested, using a wire
Knead the ZGM high temperature couplant in the tube before
using. Then place a drop, about the size of a pea, on the
contact face of the probe, not on the test piece .
Carefully couple the probe to the surface under test. To pre-
vent damage to the probe face, do not twist the probe while it is
in contact with the test surface. On a curved surface with a
dual probe, orient the crosstalk barrier as described in Section
6.1.3. Allow 2 or 3 seconds for the ZGM couplant to melt and
provide good coupling. At temperatures above 1020°F (550°C),
the ZGM couplant may ignite spontaneously. This will not
affect thequality of the coupling.
Do not allow the probe to be coupled for more than 5 seconds.
If a thickness reading does not appear within 5 seconds, re-
move the probe and cool it in air. On curved surfaces, gently
rocking the probe may help achieve good coupling.
While coupled, readings tend to drift upward as the probe
warms. Using MIN (minimum capture) mode (Section 2.9)
may be helpful in minimizing this problem.
Crosstalk Barrier Orientation