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Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Eddy Current Chapter 5 and 6

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(g)<br />

(h)<br />

(i)<br />

(j)<br />

For some instruments, changing the gain changes the balance point, so<br />

steps (d), (e), and (f) should be repeated to ensure that the spot is<br />

located on the horizontal centre line, towards the right hand edge of the<br />

screen with the probe on the thinnest shim or bare metal, and towards<br />

the left hand edge of the screen with the probe on the thickest shim.<br />

The position of the spot when the probe is located on the thickest shim<br />

can be marked.<br />

Locate the probe on the other shims in turn, marking the position of the<br />

spot.<br />

If measurement of the thickness is required, draw up a graph of the<br />

horizontal position of the spot, measured in millimetre or number of scale<br />

divisions from one edge of the screen, versus the coating thickness. If<br />

the graph is sharply curved, different frequencies should be tried to<br />

determine if a more linear response can be achieved. Alternatively, use<br />

more reference shims so that an accurate smooth curve can be plotted.<br />

Once a satisfactory graph has been obtained, carry out thickness<br />

measurement by applying the probe to the test part, measuring the<br />

horizontal position of the spot from the edge of the screen, then using<br />

the graph to determine the thickness.<br />

Charlie Chong/ Fion Zhang

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