3.6.1 Monoparameter, Monochannel and Specialized Equipment
In eddy current testing method, the equipment designed for a particular purpose or application sense in a way a change in the test coil impedance which can be caused by various parameters. Among factors related to material properties are conductivity, dimension and the permeability. Dimensional factors that affect eddy current testing are thickness of the material and presence of discontinuities. Similarly permeability is a factor of concern for ferromagnetic materials under test. Geometrical factors of coils such as the geometric relationship between the coil and the suspected discontinuities, the effect of changes in lift off or fill factor and depth of penetration, also affect eddy current testing.
There is thus a strong need of knowing precisely which factor has caused the change in impedance among various parameters of concern. Eddy current equipment classified as monoparameter utilizes certain circuitry and principles by which they indicate exclusively the measurement of variable of interest. Few of such instruments are described in brief.
Crack Detectors
The crack detectors are used to inspect for surface defects. In a typical crack detector, as shown in FIG. 3.24, an oscillator supplies an AC to an AC bridge, containing an eddy current probe coil as one arm of the bridge. A capacitor is connected in parallel with the coil so that L-C inductance-capacitance) circuit is near resonance.
t e c e
FIG. 3.24. Simplified circuit of a crack detector.
When the coil is placed on the test sample, the bridge is unbalanced and the pointer swings off the scale. The bridge can be balanced by adjusting R1. Since the meter works at resonant frequency, the output voltage is maximum for a given change in coil impedance.
Crack detectors have a meter output and three basic controls namely balance, lift-off and sensitivity. Balancing control is performed by adjusting the potentiometer on the adjacent bridge arm, until the bridge output is zero or nearly so. ‘GAIN’ control (sensitivity) adjustment occurs at the bridge output. The signal is then rectified and displayed on the meter.
‘LIFT OFF’ control adjusts the test frequency (by less than 25%) to operate slightly off resonance. The test frequency is chosen to compensate for probe wobble (lift off), not to change the skin depth or phase lag. The meter output is a complex function of signal amplitude, and cannot be used to reliably measure depth or to distinguish between real and false indications such as ferromagnetic inclusions.
Conductivity Testers
In this instance a conductivity tester is defined as a simple instrument designed only to check the conductivity of various types of materials and their alloys. The instrument scale may be (and often is) direct reading in% IACS. Most instruments are equipped with calibration knobs so that the high and low meter readings may be adjusted to agree with the high and low conductivity standards supplied with the equipment. These instruments are most often used in the sorting of material but may be used to determine the thickness of conductive coatings.
They are of fixed frequency and do not give any indication of the voltage current phase relationship.
In using the conductivity tester the operator must be continuously aware of the factors that affect conductivity (thickness of materials, presence of discontinuity, edge effect, lift off and the effect of heat treatment) before reaching any final conclusions. Material sorting or conductivity instruments have a pre-calibrated meter output and a unique way of compensating for lift off. They incorporate AC bridges and normally have two coils (one as a reference). Lift off compensation is normally preset.
Resistance and Reactance Measuring Testers (Impedance Measurements Circuits)
The equipment designed is capable of measuring any variable when used with the impedance plane diagram. Proper test frequency for the proposed test is selected first. The type of material involved and variable to be measured and suppressed, determine the optimum
frequency. In selecting the frequency to be used for a particular test, it is necessary to obtain reference standards and conduct tests at various frequencies and then select frequency that gives the best results. Once the frequency has been selected, the impedance plane curves are plotted. To plot conductivity locus or curve, samples of different materials are needed. For each material a point on conductivity curve is obtained as follows:
(a) The probe is placed on a sample and alternately adjustment of the resistance and reactance controls is made until null (minimum) reading is obtained. Use of scale control is made to keep the readings on scale at all times. Repetition of the above procedure at higher sensitivity settings is made until an absolute null point is obtained. The resistance and reactance values are noted on the control and this point on the graph paper is plotted.
(b) The above procedure is repeated for each material sample. Now if variable lift-off is to be suppressed, lift-off is varied by placing varying thickness of paper between the
Based on single frequency phase discrimination, it becomes increasingly difficult to detect much smaller size tubing discontinuities, especially in the immediate vicinity of signal interfering artefacts such as tube support and tube sheets. Depending on a given operating frequency and severity of the discontinuity, the signal from such intersection may or may not be identified and certainly could not be characterized reliably. This condition is caused by the vectorial summation of several signals combined simultaneously to form a distorted signal.
The equipment employing the multifrequency and multiparameter analysis techniques help in minimizing the effects of undesirable variables for a better and reliable inspection of tubes.
Various multifrequency eddy current testing instruments have been built with two to four frequencies for special applications, especially tube inspection.
All such instruments consist of (1) a detection system giving a real component x and an imaginary component y for each frequency; and (2) the analysis system which is their primary feature.
Multifrequency Test Equipment
A multifrequency test instrument is generally a combination of two or more single frequency instruments.
Various components of a two frequency instrument are listed below:
(a) An oscillator, which generates the sinusoidal voltages required for eddy current generation and demodulation.
(b) A power amplifier, frequently followed by an impedance matching transformer.
(c) A bridge containing the transducer.
(d) A balancing system.
(e) A variable gain signal amplifier.
(f) A demodulator, which extracts the signal resistive and reactive components.
(g) A 0-360 degree phase rotation system which outputs the signals used for analysis (X1
The special feature of this system is that the two channels use only one transducer to induce eddy currents and to receive data from the test object.
There are two basic types of multifrequency systems. They are separated here according to whether multifrequency power is supplied to the probe simultaneously or sequentially for each instrument, the way in which power is supplied to the probe, the way in which the received signals are separated and the type of demodulation, are all important considerations.