Eddy Current Tester

What is an Eddy Current Tester

An eddy current tester is a non-destructive testing (NDT) instrument that uses electromagnetic induction to examine electrically conductive materials without cutting, sectioning, or otherwise damaging the part. A coil carrying alternating current generates a primary alternating magnetic field. When the coil is brought close to a conductive test object, that field induces circulating currents, called eddy currents, in the surface of the material. Those eddy currents in turn create a secondary magnetic field that opposes the primary field and changes the electrical impedance of the coil. Anything that disturbs the eddy current flow, such as a crack, a corrosion pit, a change in conductivity, or a change in the gap between probe and part, registers as a measurable change in coil impedance.

Because the method reads impedance rather than a transit time or a radiographic shadow, a single instrument can perform several distinct jobs. Flaw detection finds surface-breaking and near-surface cracks. Conductivity measurement, expressed in percent of the International Annealed Copper Standard (%IACS) or in megasiemens per meter (MS/m), supports aluminum heat-treat verification and alloy sorting. Coating thickness gauging measures a non-conductive layer over a conductive substrate. Material sorting separates mixed alloys or detects heat-treat condition. The defining requirement for all of these is the same: the test object must be electrically conductive, so plastics, ceramics, and dry composites are out of scope.

The physics is well over a century old. Michael Faraday described electromagnetic induction in 1831, and the French physicist Leon Foucault gave his name to the currents (eddy currents are still called courants de Foucault in French) in 1855. Practical NDT instrumentation matured in the mid twentieth century: Friedrich Foerster founded the Institut Dr. Foerster in Germany in 1948 and is widely credited with putting eddy current testing on an industrial footing through impedance-plane analysis and the first commercial crack detectors and conductivity meters. The introduction of the impedance plane display, where lift-off and flaw signals separate by phase angle, turned eddy current testing from a single-number meter into a diagnostic instrument.

Eddy current testing sits inside the broader NDT toolkit alongside the ultrasonic flaw detector, the industrial X-ray system used for radiography, the magnetic particle tester, and the dye penetrant kit. Its strengths are speed, no couplant, no consumables, immediate electronic results, and easy automation, which is why it dominates high-volume inspection such as aircraft fastener-hole scanning, condenser and heat-exchanger tube examination, and in-line tube and bar mill testing. Its principal limitation is depth: eddy current testing is fundamentally a surface and near-surface method, and on ferromagnetic steel the high magnetic permeability shrinks the inspectable depth dramatically. Engineers therefore pair it with ultrasonic or radiographic methods when deep, volumetric flaws are the concern.

Two general signal-display philosophies are used. Older and simpler instruments present a single-frequency impedance-plane dot or a meter reading, suitable for crack detection and conductivity. Modern eddy current array (ECA) systems excite many coils in sequence and assemble a C-scan image, dramatically increasing coverage per pass and reducing operator dependence. The choice among these display modes, the probe, and the test frequency is the heart of eddy current selection, and the chapters below address each in turn.

Instrument and Probe Types

The phrase eddy current tester covers three distinct instrument families that share the same physics but serve different jobs, carry different price points, and are governed by different standards. Buying the wrong family is the most common procurement error: a conductivity meter cannot map a crack network, and a single-channel flaw detector cannot image a weld in one pass. The table below summarizes the families before the text explains each.

Portable flaw detectors are battery-powered handheld instruments that present an impedance-plane display so the operator can separate flaw signals from lift-off by phase angle. They are the everyday tool for aircraft maintenance, weld inspection, and bolt-hole scanning. A representative unit, the Evident (Olympus) NORTEC 600, offers a frequency range of 10 Hz to 12 MHz, gain from 0 dB to 100 dB in 0.1 or 1 dB steps, a 5.7 inch (14.5 cm) 640 by 480 pixel display, a mass of about 1.7 kg including the lithium-ion battery, and an IP66-rated case rated for operation from -10 to +50 degrees C, with up to 10 hours of battery life. Models in the series add dedicated conductivity and coating-thickness measurement, rotary-scanner drive, and dual-frequency mixing.

Conductivity and sorting meters are single-purpose instruments optimized to read electrical conductivity precisely on non-ferromagnetic metals such as aluminum, copper, titanium, and brass. Conductivity correlates with alloy, temper, and heat-treat condition, so these meters verify aluminum aircraft skins and forgings, sort scrap, and detect fire damage or over-aging. The Foerster SIGMATEST 2.069 is a reference example: it measures from 0.5 to 65 MS/m (1 to 112 %IACS), with absolute instrument accuracy of plus-or-minus 0.5 percent of the measured value and resolution of plus-or-minus 0.1 percent, using five excitation frequencies of 60, 120, 240, 480, and 960 kHz, with temperature compensation that standardizes readings to 20 degrees C.

Tube and array systems are rack or field instruments built for production-rate inspection. Heat-exchanger and condenser tube testing uses bobbin probes pulled through the tube bore, with multi-channel acquisition that produces strip-chart and Lissajous (impedance-plane loop) displays. Eddy current array systems drive many small coils in a controlled firing sequence to build a C-scan image of welds, fastener rows, or flat surfaces in a single pass. These systems pair a data-acquisition unit, such as the Eddyfi Ectane or Zetec MIZ-200, with application-specific probes and analysis software, and they are the only practical option when coverage area and throughput dominate the requirement.

A separate but related product is the eddy current coating thickness gauge, which uses the same induction principle in a deliberately inverted way: instead of treating lift-off as noise, it treats the probe-to-substrate gap as the measurement. The eddy current method gauges a non-conductive coating over a non-ferrous conductive substrate such as paint or anodize over aluminum, while the magnetic-induction method gauges a non-magnetic coating over steel. Both are standardized by ASTM E376 and ISO 2360, and the gauges are sold by Fischer, Elcometer, and DeFelsko.

Probe Operating Modes and Configurations

The probe is where eddy current testing succeeds or fails, and probe choice is governed by two independent decisions: the operating mode of the coils and the physical configuration relative to the part. The operating mode determines what kind of flaw the probe responds to, and the configuration determines how the probe couples to the geometry. The table below compares the four operating modes.

Absolute probes use one active coil whose impedance is read directly, sometimes balanced against a reference coil placed far from the part. Because the probe reports the total impedance state, it responds to gradual, long-wavelength changes such as overall wall thinning, bulk conductivity variation, and alloy or heat-treat sorting. The price is sensitivity to drift: a change in temperature, in probe-to-part gap, or in ambient conditions shifts the reading even with no flaw present, so absolute probes demand stable setup and frequent rebalancing.

Differential probes place two active coils a small distance apart and subtract their outputs, so the probe responds only to the difference between two adjacent points on the part. This rejects slow drift and uniform conditions while staying very sensitive to abrupt local features such as short cracks, pits, and tube dents. The classic differential signature is a figure-eight or Lissajous loop as each coil passes the flaw in turn. The trade-off is that a long, gradual flaw aligned with the scan can pass under both coils equally and produce little or no signal, so differential probes are paired with absolute channels when both flaw classes matter.

Reflection probes, also called driver-pickup or send-receive probes, separate the function of generating eddy currents from the function of sensing them: a large driver coil establishes the field while one or more smaller pickup coils read the response. Decoupling the two roles improves signal-to-noise ratio and lets the designer optimize each coil independently, which is why most high-performance surface arrays and many tube probes use the reflection arrangement.

The second decision is physical configuration. Surface (pencil or pancake) probes sit on a flat or gently curved face for crack detection and conductivity. Bobbin probes are coaxial coils pulled through the bore of a tube to inspect from the inside out, the standard for heat-exchanger and condenser tube examination. Encircling (feed-through) probes surround a bar, wire, or tube so the part passes through the coil, the standard for in-line mill inspection. Rotating-scanner probes spin a small coil inside a bolt hole or around a tube to map circumferential cracks. Array probes arrange many coils into a flexible pad or tape that conforms to a weld cap, blade root, or fastener row and images the whole footprint in one pass, with surface tape arrays covering scan widths up to roughly 224 mm (8.8 in) on some systems. Matching configuration to geometry is as important as matching frequency to depth.

Physics, Standards and Calibration

Three physical quantities govern every eddy current test: test frequency, the electrical conductivity of the material, and its magnetic permeability. They combine into a single design parameter called the standard depth of penetration, also called skin depth and written as the Greek letter delta. The standard depth of penetration is the depth at which the eddy current density has fallen to 1/e, about 37 percent, of its value at the surface. It is given by delta = 1 / sqrt(pi times f times mu times sigma), where f is the test frequency, mu is the magnetic permeability, and sigma is the electrical conductivity.

The exponential decay of current with depth has three practical waypoints that engineers memorize. At one standard depth of penetration the current density is about 37 percent of the surface value, at two standard depths it is about 13.5 percent, and at three standard depths it is only about 5 percent. Useful flaw detection is therefore practically limited to roughly one to three standard depths below the surface, which is why eddy current testing is classed as a surface and near-surface method rather than a volumetric one. The equation also explains the levers an operator controls: raising frequency reduces penetration and sharpens surface sensitivity, while lowering frequency increases penetration to reach deeper or far-side features.

Conductivity and permeability shape the result before the operator touches the dial. Higher conductivity reduces penetration, which is why eddy currents reach deeper into low-conductivity titanium alloy than into high-conductivity copper at the same frequency. Magnetic permeability is the dominant variable for ferromagnetic steel: with relative permeability in the hundreds or thousands, penetration collapses to a fraction of a millimeter and large magnetic noise masks flaw signals, so steel is tested with magnetic saturation, low-frequency or pulsed eddy current, or replaced by magnetic particle testing for surface work. A further consequence of depth is phase: eddy currents at increasing depth lag the surface current in phase, and this phase lag is what lets impedance-plane analysis separate a deep flaw from a shallow one and separate both from lift-off.

Eddy current testing is one of the most thoroughly standardized NDT methods, with parallel ISO and ASTM frameworks. The table below lists the standards engineers cite most often in specifications and purchase orders.

Calibration ties the instrument back to these standards through reference standards (calibration blocks) with manufactured artificial flaws such as electrical-discharge-machined notches, drilled holes, or flat-bottom holes of known size, and through conductivity reference coupons certified traceable to a national metrology institute. ASME Boiler and Pressure Vessel Code Section V, Article 8, governs eddy current examination of tubular products in the United States, and aerospace primes typically add their own process specifications on top of the ASTM and ISO base. Before any purchase, confirm that the instrument and probe combination is verified against the specific standard named in the inspection procedure, because a meter that reads conductivity perfectly may not satisfy a tube-flaw acceptance standard.

Key Specification Parameters

Eddy current instrument data sheets list many figures, but only a handful drive selection. The most important parameters are test frequency range, number of channels and frequencies, gain and dynamic range, the impedance-plane and display capability, conductivity range and accuracy where relevant, environmental rating, and connectivity. The table below shows representative values across the three instrument families, then the text decodes each parameter.

Frequency range is the single most consequential flaw-detector spec because it sets which materials and depths are reachable. A wide range, such as the 10 Hz to 12 MHz of the NORTEC 600, lets one instrument cover deep low-frequency sub-surface work and high-frequency fine surface-crack work. Conductivity meters instead use a small fixed set of frequencies (the SIGMATEST uses 60, 120, 240, 480, and 960 kHz) chosen to span thin and thick parts, because conductivity, not flaw depth, is the target.

Number of frequencies and channels distinguishes simple and capable instruments. A single-frequency unit detects flaws but cannot suppress an unwanted variable. A dual-frequency unit with mixing can subtract a known interferer, for example removing a tube support plate signal to reveal a crack underneath it. Array systems multiply channels: driving 32 or more coils in sequence is what turns a point measurement into a single-pass C-scan image.

Gain and dynamic range, quoted in decibels (for example 0 to 100 dB in 0.1 dB increments), set how small a flaw signal the instrument can amplify into the display without clipping the large lift-off and edge signals. Fine gain steps matter when balancing a weak flaw indication against a strong background.

Conductivity range and accuracy govern sorting and heat-treat work. A range of about 0.9 to 110 %IACS (roughly 0.5 to 64 MS/m) covers the practical span of engineering metals, and an absolute accuracy near plus-or-minus 0.5 percent of reading is required to separate adjacent aluminum tempers. Conductivity readings must be temperature-compensated to a reference of 20 degrees C, because conductivity changes measurably with temperature.

Display, environmental rating, and connectivity finish the list. An impedance-plane (vectorscope) display is mandatory for serious flaw work, and a C-scan display is mandatory for array inspection. Field instruments need an ingress rating such as IP66 and a wide operating-temperature band (for example -10 to +50 degrees C). Modern units add USB, VGA output, and encoder inputs so scans can be position-tagged and archived for audit, which standards increasingly require.

Selection Decision Factors

To turn the preceding chapters into a specific purchase, work through the ordered sequence below. Most selection mistakes come not from one wrong value but from skipping the early framing questions, so resolve material and job first and leave the brand decision for last.

1. Material and its magnetic class: First confirm the part is conductive at all, then determine whether it is non-ferromagnetic (aluminum, copper, titanium, austenitic stainless) or ferromagnetic (carbon and low-alloy steel). Ferromagnetic parts need magnetic saturation, low-frequency or pulsed eddy current, or a switch to magnetic particle testing for surface cracks.
2. Inspection job: Decide which of the four jobs you are buying for: surface crack detection, sub-surface or far-side flaw detection, conductivity and alloy or heat-treat sorting, or coating thickness. The job selects the instrument family from Chapter 2.
3. Flaw type, size, and depth: Specify the smallest flaw that must be found and its depth below the surface. This sets the test frequency through the standard depth of penetration and chooses absolute versus differential mode.
4. Part geometry: Flat face, tube bore, bar outside diameter, bolt hole, or weld cap. Geometry selects the probe configuration: surface, bobbin, encircling, rotating, or array.
5. Throughput and coverage: A few manual spot checks favor a single-channel portable detector, while full-surface or full-length coverage at production rate favors an array or encircling system with encoders and C-scan imaging.
6. Governing standard and acceptance criteria: Identify the controlling standard (for example ASTM E243 for copper-alloy tube, ASTM E309 for saturated steel tube, ISO 17643 for welds, ASTM E1004 for conductivity) and confirm the instrument and probe are verified against it, including the required calibration blocks and reference coupons.
7. Environment and certifications: Set the ingress rating (for example IP66), operating-temperature band, and any explosion-proof, aerospace, or nuclear qualification the site demands. Add data-archiving and traceability features where audits require them.
8. Total cost of ownership: Sum instrument, probes (which wear and are often the recurring cost), calibration blocks and conductivity coupons, software licenses, training and operator certification (such as ISO 9712 or SNT-TC-1A Level II), and annual recalibration. Probes and consumable reference standards frequently outweigh the instrument price over a multi-year program.

One dimension that buyers consistently underestimate is serviceability and ecosystem: local availability of replacement probes and calibration standards, software update support, the breadth of the probe catalog for future jobs, and the depth of local applications support. Evident (Olympus), Eddyfi Technologies, Zetec, and Institut Dr. Foerster all maintain probe lines, calibration services, and training in major industrial regions, which is why they remain default choices for large or long-lived inspection programs. A cheaper instrument with no local probe supply or calibration support can cost far more once a critical inspection is held up waiting for a replacement coil.