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ignored during evaluation. On the A-scan image, an expectancy domain is shown as a blank
rectangle (see fig. above). Its association to the respective layer is determined by its position in
direction of the y-axis and corresponds to the column of layer thickness on the left edge of screen.
For layer samples of a know structure the use of expectancy domains is highly effective. Major
advantage of this function: processing adapts to the variance in layer thickness by “following” the
layer interface echoes. However, please note that not only the nominal thickness values must be
known but also the tolerances limits. At least, the latter must be estimated as good as possible. If a
layer thickness is beyond the set tolerance range, as a consequence, no echo will be assigned to
the expectancy domain concerned. As the system will consider this situation to be an error,
processing is stopped at this point. In such case, you have to correct the nominal value or
tolerance limits concerned and repeat processing accordingly. It will also be a problem if several
echoes can be found in one expectancy domain as you cannot be sure that the largest echo
actually represents the right one. In the ideal case, the expectancy domains should be set in a way
to make sure exactly the echo to be detected will actually be found in the respective domain. Any
adjacent unwanted echoes should be located beyond this domain.
Expectancy domains should not be mixed up with the upper and lower limits from the statistics
function. Expectancy domains serve for suppression of unwanted echoes. They are not meant to
provide a base for the statistical evaluation of product quality (pass/fail recognition). For that
reason, when setting expectancy domains, the focus should be put on reliable suppression of
unwanted echoes as found on the A-scan image. Any settings for statistical evaluation can be
made via the statistics module (see section 12).
The parameters for defining expectancy domains can be made numerically (section 12.1.4.3) or
graphically via the A-scan image (section 12.2.4).
8.5.6
Number criteria
8.5.6.1
Number of layers
In this menu you can set the number of layers to be evaluated. Generally, this number is known to
the user.
The number you have set in this menu will be used
by the criteria “First echoes”, “Last echoes” and
“Largest echoes” as described later in the paragraph.