Dakota Ultrasonics DFX-8 plus MANUAL1 - Manual (Page 82)

Dakota Ultrasonics
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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-8. 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