Multiple frequency eddy current techniques simply
involve collecting data at several different frequencies and then
comparing the data or mixing the data in some way.
Why the need for multiple frequencies?
- Some background information
The impedance of an eddy current probe may be affected by the
- variations in operating frequency
- variations in electrical conductivity and the magnetic permeability
of a object or structure, caused by structural changes such
as grain structure, work hardening, heat treatment, etc.
- changes in liftoff or fill factor resulting from probe wobble,
uneven surfaces, and eccentricity of tubes caused by faulty
manufacture or damage
- the presence of surface defects such as cracks, and subsurface
defects such as voids and nonmetallic inclusions
- dimensional changes, for example, thinning of tube walls due
to corrosion, deposition of metal deposits or sludge, and the
effects of denting
- the presence of supports, walls, and brackets
- the presence of discontinuities such as edges
Several of these factors are often present simultaneously. In
the simple case where interest is confined to detecting defects
or other abrupt changes in geometry, a differential probe can
be used to eliminate unwanted factors, providing they vary in
a gradual manner. For example, variations in electrical conductivity
and tube thinning affect both coils of a differential probe simultaneously.
However, if unwanted parameters that occur abruptly are affecting
the measurements, they can sometimes be negated by mixing signals
collected at several frequencies.
An example of where a multi-frequency eddy current inspection
is used is in heat exchanger tube inspections. Heat exchanger
assemblies are often a collection of tubing that have support
brackets on the outside. When attempting to inspect the full wall
thickness of the tubing, the signal from the mounting bracket
is often troublesome. By collecting a signal at the frequency necessary
to inspect the full thickness of the tube and subtracting a second
signal collected at a lower frequency (which will be more sensitive
to the bracket but less sensitive to features in the tubing),
the effects of the bracket can be reduced.
There are a number of commercially available multi-frequency
eddy current instruments. Most operate at only two frequencies
at a time but some units can collect data at up to four frequencies
simultaneously. Multi-frequency measurements can also be made
using an impedance analyzer but this equipment is generally not
suitable for field measurements. A typical impedance analyzer
system is shown below. The interest in pulsed eddy current instruments
is largely due to their ability to, in essence, perform multi-frequency
measurements very quickly and easily.