Measuring Conductivity of Nonferromagnetic Metal Pipe Using Dual-Frequency Impedance Characteristics

The conductivity of metal pipeline is crucial for assessing material properties, ensuring industrial safety, and conducting quality control. In order to accurately measure the conductivity of pipes, this article proposes a dual-frequency eddy current (EC) testing method. The EC analytical model of a cone-shaped solenoid coil outside a conductive pipe is derived using the second-order vector potential (SOVP) of the magnetic field, and the effects of coil parameters on the signal are analyzed. Based on the theoretical model, a quadratic relationship has been found between the logarithm of the probe lift-off and the inductance change of pipe at high frequency, from which the probe lift-off distance can be estimated. Then, the phase signature measured at low frequency is used to determine the conductivity of the pipe through the Newton-Raphson algorithm. Numerical simulations and experiments have been conducted to evaluate the proposed method for conductivity estimation with various pipe materials and probe lift-offs. The method achieves an accurate estimation of pipe conductivity with a relative error lower than 3%.