The detection of long longitudinal defects of hot wires by means of eddy current sensors is a challenge for metrology. To stabilise the sensor the lift off effect has to be controlled and reduced. Also, the sensitivity of the sensor has to be high enough to detect cracks with a depth less than 0.1 mm. This contribution will discuss the frequency response of different eddy current sensors. The results allow the definition of an optimised working point. Different coil arrangements have been investigated. Impedance differential and emission reception modes were investigated to find the best solution for a high sensitivity and robustness. An eddy current sensor array is under development to detect long longitudinal cracks.
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