Eddy Current Instruments

Eddy current instruments are nondestructive testing (NDT) devices that induce detectable eddy currents in conductive materials. They are used to detect flaws, determine thickness, inspect welds, measure conductivity, and sort alloys. Eddy current instruments include a straight or angled magnetic probe and an analog or digital meter with a zero reference point. Moving the magnetic probe over the surface of a conductive material such as a metal tube induces circulating currents (eddies) of electrons that oppose the externally applied magnetic field from the probe. Surface irregularities such as cracks and corrosion interrupt the surface flow of eddy currents and are detected. Eddy current instruments are well-suited for NDT because they can determine material or part characteristics without permanently altering the test subject. They vary widely in terms of capabilities, but are suitable for a variety of application and industries. 

Important specifications for eddy current instruments include transducer frequency range, maximum gain, number of channels or probes, and form factor. Transducers are components that convert one form of energy (such as sound) into another form of energy (such as electricity). For eddy current instruments that include a transducer, the frequency range is the transducer’s operating range. Otherwise, the frequency range is the operating range of the entire instrument. Typically, frequency range is expressed in megahertz (MHz). Maximum gain, the largest amount of amplification available, is usually expressed in decibels (dB). The number of channels or probes is variable. Some eddy current instruments can be mounted on a rack, in a cabinet, or on a printed circuit board (PCB). Others are designed to be held by hand or operated from a benchtop. Complete monitoring systems can continuously detect or measure flaws, thickness, or corrosion in plant or field applications.

Eddy current instruments are available with a variety of special features. Some devices include a drive system that can be used to rotate the probe radially in order to scan the inner diameter of pipes, tubes, cylinders, liners, and other internal surfaces for flaws. Other devices include magnetic induction probes that are suitable for scientific measurement, navigation, and industrial applications. Eddy current instruments with data logging capabilities can store and retrieve data for future processing, typically by a computer. Devices with sorting gates or an alarm mode notify users when a reading is outside a range of specified values.

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