8. Application Notes and Optional Features
DMS 2 Operating Manual
Page 179
8. Application Notes and Optional Features
8.1 General Notes
The DMS 2 is easy to use and will produce reliable, consis-
tent measurements when used correctly and with an under-
standing of factors that may affect measurement reliability.
This section explains some of the more common consider-
ations.
8.1.1
Flaws
If, during testing, the DMS 2 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 DMS 2
Measures Thickness, for more information. If this condition
occurs, the A-scan image should be carefully examined to
determine the cause of the suspicious reading. If the cause is
unclear, further examination of the part with an ultrasonic flaw
detection instrument or other suitable NDT method is recom-
mended.
8.1.2
Surface Conditions
A regular pattern on the surface under test, such as machine
grooves, can cause a false thickness reading, particularly 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.
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
grinding the surface until it is smooth enough to permit good
coupling.
8.1.3
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. When using a dual element probe that
has been curved to fit an OD or ID surface, the 2-point
