🔍 Understanding Dr. Förster's Compass for Ferromagnetic Eddy Current Testing

In the world of eddy current testing (ECT), few visual tools are as foundational as Dr. Friedrich Förster’s impedance plane diagrams. Figure 2, in particular, offers a brilliant distillation of how ferromagnetic cylinders behave under eddy current excitation — and it’s still highly relevant in today’s applications.

What makes Figure 2 special?

Unlike its counterpart Figure 1 (which maps nonferromagnetic behavior), Figure 2 zooms in on cylinders with high relative permeability (μₙ ≫ 1). It simplifies the complex interaction between conductivity (σ), diameter (d), and permeability (μ) into a single curve — assuming full fill factor (η = 1). The axes remain the same: inductive reactance vs. resistance, or imaginary vs. real voltage.

Figure 1 (Non-Ferromagnetic)

Figure 2 (Ferromagnetic)

Why did Förster develop this figure?

He recognized that in ferromagnetic materials, variations in μ and d can’t be easily decoupled. So instead of separating them, he mapped their combined influence as a single arrow on the complex plane. Then he contrasted it with the conductivity arrow, which remains similar to Figure 1. Importantly, crack indications (though not shown) tend to appear perpendicular to the μ/d arrow — providing a powerful basis for signal discrimination.

How does this translate to modern ECT?

Today’s eddy current systems often rely on array probes and automated analysis. But the core challenge remains: how do we distinguish between geometry, material property changes, and flaws? Figure 2 provides a timeless answer — use vector direction. When testing ferromagnetic tubes, you can adjust your frequency to push signal behavior into orthogonal paths:

• Use low f/fg (<10) to separate crack responses from permeability/diameter effects.

• Monitor vector direction and phase to determine root cause without destructive testing.

Even with modern multi-frequency and modeling tools, Figure 2’s concept — that crack signals point in a direction distinct from material geometry changes — remains a guiding principle.

📈 Whether you’re analyzing steam generator tubes in nuclear power or sorting welds in petrochemical plants, this 70-year-old figure still deserves a place in your toolbox.

🔗 Make sure to follow the Dr. Förster Codebreaker Blog Series at eddycurrent.com/blog

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