FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

2.2 Primary Factors
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Computer simulation results verified that vectorial addition was valid for
signals due to gap and wall thickness changes, thus allowing these effects to
be studied independently. A number of sets of data were collected, each at a
different fixed wall thickness and containing the response of the eddy current
probe to a range of gap values. One such set is shown in Figure 4 where the
non-linear response of the probe is evident. In order to relate these
independent data sets and determine the effect of wall thickness changes, it
is only necessary to determine the response of the probe to wall thickness at
one fixed gap. This was done by performing a rotational scan on a pressure
tube sample with a large wall thickness variation around the circumference and
no calandria tube (infinite gap). The result, shown in Figure 5, was a good
linear fit with a regression coefficient of 0.9887. The slope of this line
was used to shift the sets of previously collected data so that they all
referenced a common balance point at infinite gap and 5.0 mm wall thickness.
Straight lines were fitted to the points of common gap with a great deal of
success, the smallest regression coefficient being 0.9960. This overview,
shown in Figure 6, formed the data base of the probe's response which was to
be used during field inspections.
2.3 Pressure Tube Resistivity
The calculated complex plane response of the probe indicated that pressure
tube resistivity could significantly influence the eddy current signal if
large enough variations were encountered during the inspections. This
possibility was explored by measuring the resistivity and wall thickness
around the circumference of a pressure tube sample and then scanning the same
section with the gap probe in order to compare both influences.
The resistivity scan in Figure 7c shows a well defined change of about 1
just after the 180° mark. Using Figure 3, it was calculated that a
resistivity change of this magnitude should cause approximately the same
effect in the Y component of the probe response as a 0.17 mm wall thickness
change. Figure 7a shows that this is about half of the total wall thickness
variation measured in the pressure tube sample. However, the response of the
gap probe shown in Figure 7b does not agree with this prediction as the wall
thickness influence was followed very closely with almost no sign of the
resistivity influence.
The apparent insensitivity of the probe to resistivity variations was
encouraging for the immediate goal of the Pickering 1 and 2 inspections,
however, further investigations are required for the long term development of
the gap measurement system.
2.4 Temperature
The temperature sensitivity of the probe in its operating environment and in
isolation are shown in Figures 8 and 9 respectively. The response in Figure 9
is somewhat unexpected since send-receive probes are generally expected to be
insensitive to temperature variations due to the small effect that variations
in coil resistance have on the output signal. In this case the response is
thought to be caused by a physical deformation of the probe and is currently
under investigation. The linear response of -0.036V/°C obtained from the