4
Use of Couplants
Operators must be familiar with the use of ultrasonic couplants. Testing skills must be developed so that couplant is
used and applied in a consistent manner to minimize variations in couplant layer thickness and errors in test results.
Calibration and actual testing should be performed under similar coupling conditions, using a minimum of couplant
and applying consistent pressure on the transducer.
Doubling
Ultrasonic thickness gauges will, under certain conditions, display readings which are twice (or in some cases, three
times) the actual material thickness being measured. This effect, commonly known as “doubling,” can occur below
the minimum specified range of the transducer. If the transducer being used is worn, doubling is possible at
thicknesses greater that the minimum specified range.
When using a new transducer, any reading which is less than twice the minimum specified range of the transducer
may be a “doubled” reading, and the thicknesses of the material being tested should be verified by the use of other
methods. If the transducer shows any sign of wear, doubling may occur at thicknesses greater than twice the
minimum specific range. This thickness should be determined by calibrating the instrument/transducer combination
on reference blocks that represent the complete range of possible thicknesses that may be encountered in testing.
This is particularly important when the test piece is being ultrasonically measured for the first time or in any case
where the history of thickness of the specimen in unknown.
To Next Page
Loading...
