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A constant alternating current (A.C) is passed via two of the pins into the work piece; the two other pins measure the voltage drop across the crack, where the crack depth is derived from. The A.C. of the instrument utilizes the skin effect, which forces the current flow to the surface of the conductor and therefore follows the contour of the crack.

The operation and measured value formation are monitored by the microprocessor of the gauge. Incorrect handling or faulty measurements caused by wrong attachment or imperfect contacts therefore are out of question.

If there is a crack between these two points, the electrical resistance is higher than for a crack-free surface. The resistance will grow with the unknown crack depth. In this new approach, a four-pole technique is used:

Where :

f : Frequency

μ : Relative Permeability

μo : Permeability Constant

σ : Specific Electric Conductivity

Equation 1 gives the penetration depth in terms of the frequency and the material properties.

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