DFX-7
Ultrasonic Flaw Detector
73
Considerations:
Should only be used with single element probes.
The effective diameter must be estimated for standard transducers not
certified for
DGS
.
Center frequency clarity is required. Highly damped transducers can
sometimes produce a broadband frequency range, and should not be used.
10.2 DGS Diagram
In order to get familiar with the
DGS
diagram, let’s take a look at some of the general
concepts. The near and far fields are referenced at point
(A)
. In the near field the
wave front is not as uniform or consistent as the far field. As a result, the evaluation
of flaws/defects are optimized in the far field when the wave front becomes uniform.
Evaluating defects less than .7 NFL’s is marginal at best.
The
DGS
diagram uses a logarithmic scale for both the Gain (dB) y axis, and near
field length (NFL) distance at
(B)
. The curve at
(C)
is the back wall reflector. It’s
important to note that when the distance is doubled, the sound energy decreases by
1/2 (-6dB). However, a defect/reflector that’s less than or equal to the effective
diameter of the transducer, doubling the distance decreases the sound energy by 1/4
(-12dB), at
(D)
.
The curve at
(E)
represents a defect that is equal to 100% the effective diameter of
the transducer, while 5% the effective diameter at
(F)
. As an example for using this
diagram to determine a reflector size, consider the following: If I set my gauge for
80% screen height on a back wall reflector at 5 NFL’s, and found a defect also at 5
NFL’s, but at 80% (1/10
th
= 20dB), the ERS would be 30% of the effective diameter.
10.3 Estimating Effective Diameter
There are a number of transducers that are specially documented as
DGS/AVG
transducers. What this typically infers is that the effective diameter, near field length,
and center frequency have been determined and documented on the transducer
certificate. If
DGS/AVG
transducers are being used, the values for effective diameter
and frequency can simply be entered into the
DFX-7
. However, if a standard
transducer will be used and has a standard transducer certificate, the effective
diameter must be estimated in order to determine the near field length (NFL).
Calculating NFL
