When an eddy current probe is placed near a conductive material, the alternating magnetic field from the probe causes electrical currents to flow in the material. These currents have associated with them their own magnetic field, which opposes the original magnetic field from the coil. The result is that the impedance of the probe is greatly reduced by the presence of the conducting material. If the conductor is also magnetic, as is normal steel, the situation is similar though slightly more complicated. Here, the impedance of the probe may be either increased or decreased depending on the permeability...
When an eddy current probe is placed near a conductive material, the alternating magnetic field from the probe causes electrical currents to flow in the material. These currents have associated with them their own magnetic field, which opposes the original magnetic field from the coil. The result is that the impedance of the probe is greatly reduced by the presence of the conducting material. If the conductor is also magnetic, as is normal steel, the situation is similar though slightly more complicated. Here, the impedance of the probe may be either increased or decreased depending on the permeability of the material and the frequency of the alternating field. Anything that affects the flow of current in the conductive material will also affect the impedance of the eddy current probe. For example, the electrical currents cannot flow through a crack but must flow around it. The alteration in the currents also changes the magnetic field produced by the currents and, consequently, the impedance of the probe. Normally, the impedance change caused by a defect is much smaller than that caused by the presence of the material in the first place, and measuring this small change requires a bridge circuit much like the balanced bridge used with strain gauges. The balanced bridge allows one to amplify the small changes in impedance caused by defects in the presence of the much larger change caused by the presence of the conductive and magnetic pipeline steel. The LFEC instrument is constructed using an IBM-AT compatible portable computer. Inside the PAC-386 are two plug-in circuit cards that turn the PAC-386 into an eddy current instrument. The first, also commercially available, is a Spectrum DSP56000 digital signal processing card, while the second is a specially-built interface card for the eddy current probe. The PAC-386 is a standard MS-DOS machine and will operate most MS-DOS software. In the discussion below, it is assumed that the user is familiar with the MSDOS operating system.