GE MFE DMS 2 Operating Manual page 5

Important Notice
When a less-than-complete inspection is to be performed, the
operator must be shown the specific areas to inspect. Infer-
ences about the condition of areas not inspected, based on
data from the evaluated areas, should only be attempted by
personnel fully trained in applicable statistical and probability
techniques. In particular, materials subject to erosion or
corrosion, in which conditions can vary significantly in any given
area, should only be evaluated by fully trained and experienced
operators.
Sound beams reflect from the first interior surface encoun-
tered. Because of part geometry and overlapped flaws or
overlapped surfaces, thickness gauges may measure the
distance to an internal flaw rather than to the back wall of the
material. Operators must take steps to ensure that the entire
thickness of the test material is being examined.
Ultrasonic Thickness Measurement Critical Operating
Procedures
The following operating procedures must be observed by all
users of ultrasonic thickness gauges in order to minimize errors
in test results.
Page iv
1. Calibration of Sound Velocity
The principle of operation of an ultrasonic thickness gauge is
that the instrument measures the time of flight of an ultrasonic
pulse through the test piece and multiplies this time by the
velocity of sound in the material. Thickness measuring error is
minimized by ensuring that the sound velocity to which the
instrument is calibrated is the sound velocity of the material
being tested. Actual sound velocities in materials often vary
significantly from the values found in published tables. In all
cases, best results are obtained if the instrument is calibrated
on a velocity reference block made from the same material as
the test piece; this block should be flat and smooth and as thick
as the maximum thickness of the test piece.
Operators should also be aware that the sound velocity may
not be constant in the material being tested; heat treating, for
example, can cause significant changes in sound velocity. This
must be considered when evaluating the accuracy of the
thickness provided by this instrument. Instruments should
always be calibrated before testing, and the calibration should
be checked after testing, to minimize testing errors.
DMS 2 Operating Manual