The Six Probes That Predicted the Future of Eddy Current Testing

By Edward Korkowski - eddycurrent.com

When we think of eddy current arrays today, we imagine colorful C-scans, real-time imaging, and multi-channel data streaming from a high-tech instrument. But rewind the clock to 1993, and the groundwork for all of this was being quietly laid out in a Materials Evaluation paper by W.G. Clark, Jr.

In that paper, six prototype probes were introduced—not just variations of the old bobbin coil, but a glimpse into the future of eddy current testing.

🛠️ What Was the Problem with Bobbin Probes?

Bobbin probes were—and still are—a workhorse in many NDT applications. They're great at detecting general degradation like wall thinning or volumetric flaws in tubing. But they have limitations:

• They can’t size axial cracks very well

• They're directionally biased (not effective at detecting circumferential cracks)

• They can’t pinpoint (or determine how many) flaws are present circumferentially around the tube for any given axial position of the probe.

Enter multi-element, software-assisted probes—the ancestors of today’s ECAs.

🔬 The Six Prototypes That Changed Everything

Here’s what Clark and the Westinghouse team came up with:

1. The Ruler Probe

A stacked set of bobbin coils that could be selectively activated. Instead of rotating a pancake coil to size a flaw, the Ruler could do it in milliseconds—measuring flaw length without moving.

2. The Balloon Probe

Imagine a ring of pancake coils pressed against the inside of a tube wall by an inflatable bladder. The Balloon probe gave the spatial resolution of a rotating probe but with straight-pull deployment. Think of it as a rotating probe without the rotation.

3. The Criss-Cross Probe

A pair of diagonal coil pairs crossed in opposing directions. This design was sensitive to cracks in any orientation, solving the age-old “crack missed because of coil orientation problem."

4. The Bullseye Probe

Concentric coils of different diameters. Small coils for surface flaws, big coils for deeper defects. Together, they created a depth profile without moving the probe in or out of the tube.

5. The 8×1 Probe

Eight pancake coils arranged around the circumference. It scanned the full tube in one axial plane, catching axial and circumferential flaws without rotation.

6. Shaped-Field Sensor

A field geometry and current configuration engineered to produce a specific field shape—an early version of what we now call shaped-field eddy current (SFEC).

🤯 Sidebar: ECA Before It Had a Name

These probes weren’t called “arrays” yet. But their DNA is in every modern ECA system:

• Modular probe design

• Coil multiplexing

• Real-time digital signal processing

• Software-defined test plans

• Imaging and flaw classification

The engineers behind these six designs weren’t just fixing old problems—they were inventing a new way to think about flaw detection.

🧪 Lab Validation, Real-World Impact

Each of these designs was:

• Built and tested in lab mockups

• Compared against bobbin and rotating pancake probes

• Evaluated for lift-off tolerance, flaw sensitivity, sizing capability, and user friendliness

The verdict? These were the future. And 30 years later, they still are.

🚀 Final Thought: More Than Just Better Probes

What these probes really did was unlock the potential of data. By going from single-coil measurements to spatially distributed, programmable, multi-element systems, ECT became more than flaw detection—it became flaw characterization and eventually, imaging.

If you’re using ECAs today, tip your hat to the Balloon, the Ruler, and the Criss-Cross. They walked so modern systems could run.

🧠 Want to learn more about shaped-field probes, ECA imaging, and how to interpret complex signal patterns?

Explore the archive and deep-dive resources at👉 eddycurrent.com — The only one-stop shop for all things eddy current.