Thickness measurement of conductors Introduction

The thickness of conducting materials can be measured because, provided the thickness is less than the effective depth of penetration, the eddy current signal depends on the thickness (see FIG. 5.24). Ultrasonic testing can also be used for thickness measurement, but eddy current thickness measurement is preferred for thin materials because it is easier to obtain accurate readings. The technique can also be applied to measure the thickness of a conducting coating on a nonconductor, for example, metal coatings on plastic.

FIG. 5.24. Impedance diagram showing the conductivity curve and the locus of the operating points for thin red brass (conductivity approximately 40% IACS) at 120 kHz (the thickness curve). The thickness curve meets the conductivity curve when the thickness equals the

Probe and frequency selection

The types of probe most suitable for thickness testing of conductors are the same as the types used for measuring non-conducting coating thickness on conductors. Spot probes, absolute or reflection, with or without spring loading, are normally used, but pencil probes with a probe holder, preferably sprung to minimize lift-off, can also be used. Probes or probe holders in the form of a V-block or with a contoured face should be used on curved surfaces.

The basis for the selection of test frequency is the same as that for the selection of frequency for the detection of subsurface flaws. In fact the detection of corrosion and measurement of its depth is a thickness measurement problem. The frequency should be low enough so that the eddy current intensity at the far surface is relatively high, but low enough to give a relatively high degree of phase separation between signals from different thicknesses. Therefore, the operating frequency should be f₉₀. Normally, t in this formula would equal the maximum thickness to be measured.

FIG. 5.25. Impedance diagram showing the conductivity curve, and the thickness curve for brass at a frequency of 120 kHz, the f₉₀ frequency for a thickness of 0.165 mm. The operating point for this thickness is shown, and lift-off curves for this and various other thicknesses are also shown.

Signals from variations in thickness

If the test frequency is set to f₉₀ for the thickest material to be tested, the operating point is far down the thickness curve, and the lift-off curve from this point is approximately 90⁰ to the thickness curve, as shown in FIG. 5.25. For this diagram, the thickest material to be tested is 0.165 mm, and the frequency is 120 kHz. The eddy current instrument display is normally set up with the lift-off trace horizontal to the left. FIG5.26 shows part of the display shown in FIG. 5.25, rotated so that the lift off trace from the operating point is horizontal (for clarity, the conductivity curve is not shown). It can be seen that, if the probe is applied to thinner material, for example material 0.125 mm thick, the signal will be almost parallel to the lift-off signal for the 0.165 mm thick material, but will be further up the screen.

On the other hand, if material 0.165 is being scanned and the material gradually becomes thinner, the spot will move up the screen along the thickness curve.

Material significantly thicker than that for which f₉₀ was calculated should not be thickness tested because the sensitivity to changes in thickness reduces rapidly below the f90 operating point, as shown in FIG. 5.25. This FIG. also shows that the thickness curve crosses the conductivity curve as the thickness approaches the effective depth of penetration. This means that, when operating in this region of the curve, the same lift-off trace would be obtained from two different thicknesses of material.

FIG. 5.26. Part of the display shown in FIG. 5.25, rotated so that the lift off trace from the operating point is horizontal, as it would appear on an eddy current instrument display. The lift-off traces are shown as straight lines, but they normally appear slightly curved

Reference specimens

Reference specimens should consist of shims or sheets of the material to be tested (or of a material of the same conductivity) which cover the expected range of thickness. As for non-conducting coating thickness measurement, if the measurement is to determine whether or not the test part thickness lies within a given range, one specimen with the thickness of the lower limit and one with the thickness of the higher limit should be used. However, a third specimen, with thickness in the middle of the range, is desirable. If, however, the actual thickness of the material is to be measured, at least three specimen thicknesses are required.

These should have thicknesses equal to the maximum and minimum expected thicknesses, and one approximately midway between these. More shim thicknesses may be required if a wide range of thickness is being measured and the response is notably nonlinear.

Measurement procedure

(a) Connect the probe to be used to the instrument, switch on the test instrument and allow it to warm up for at least 5 minutes.

(b) Ensure the test area is clean and free from dirt or other contaminants.

(c) Select the frequency required, and set the gain to the middle of its range.

(d) Apply the probe to the thickest reference sample, balance the instrument, and adjust the phase so that the lift-off signal is horizontal to the left.

(e) With the probe still on the sample, adjust the spot position to a location towards the lower right hand corner of the display (see FIG. 5.27).

(f) Apply the probe to the thinnest reference sample and check whether a signal is displayed. Do not rebalance. If visible, the spot should lie above the previously obtained signal. Adjust the gain so that the spot is located towards the upper right hand corner of the screen. If the spot is not visible, first reduce the gain until it becomes visible, then further adjust the gain to locate the spot towards the upper right hand corner of the screen, as shown in FIG. 5.27.

(g) For some instruments, changing the gain changes the balance, so steps (4), (5), and (6) should be repeated to ensure that the spot is located towards the lower right hand comer of the display with the probe on the thickest sample, and towards the upper right hand

(h) Locate the probe on the other reference sample(s) in turn and note the signal, but without rebalancing.

(i) If measurement of the thickness is required, draw up a graph of the vertical location of the trace, measured in millimetre or number of scale divisions from top or bottom of the screen along a suitable vertical line on the screen, versus the thickness. If the graph is sharply curved, more reference standards may be required so that an accurate smooth curve can be plotted. Once a satisfactory graph has been obtained, carry out thickness measurement by applying the probe to the test part, measuring the vertical position of the trace from the top or bottom of the screen, then using the graph to determine the thickness.

(j) Alternatively, if the measurement is being carried out to verify whether the thickness lies between acceptable limits, note the positions of the trace from the upper and lower thickness values, then apply the probe to the test parts. The thickness is acceptable if the trace is located between the positions for the upper and lower thickness limits.

(k) If a large number of measurements are required, the calibration should be checked at least every 30 minutes. The calibration should also be checked on completion of testing.

FIG. 5.27. The eddy current instrument display after setting up to verify that the thickness of the test samples lies between acceptable limits.