Tacit Skills in Nondestructive Testing: Why Eddy Current Testing Still Depends on Human Judgment

Nondestructive testing (NDT) is often described in terms of equipment, procedures, codes, and certifications. We talk about frequencies, probe designs, calibration standards, probability of detection (POD), and acceptance criteria. All of those things matter — but they do not tell the whole story.

Behind every reliable inspection result is something far less tangible and much harder to teach: tacit skill.

Nowhere is this more evident than in eddy current testing (ECT).

What Are Tacit Skills?

Tacit skills are abilities that are:

• Learned through experience rather than formal instruction

• Difficult to fully articulate or write into a procedure

• Developed over time through repetition, pattern recognition, and judgment

In simple terms, tacit skills are the things a competent professional knows how to do but may struggle to explain step-by-step.

You can read about them.
You can observe them.
But you truly acquire them only by doing the work.

Tacit Skills vs. Explicit Skills in NDT

In NDT, we often focus on explicit skills, such as:

• Knowing which probe to use

• Setting frequencies and gains

• Following a written procedure

• Passing a Level I, II, or III exam

These are measurable and teachable.

Tacit skills, on the other hand, include:

• Recognizing when a signal “doesn’t feel right”

• Knowing when a lift-off signal is masking a flaw

• Sensing when an impedance plane rotation is meaningful — or misleading

• Understanding when material variability, geometry, or permeability is influencing the response

• Adjusting technique instinctively when conditions change

These skills rarely appear in procedures, but they are what separate competent analysts from exceptional ones.

Why Eddy Current Testing Is Especially Dependent on Tacit Skill

Eddy current testing is uniquely sensitive to a wide range of variables:

• Conductivity

• Magnetic permeability

• Geometry

• Surface condition

• Probe coupling and orientation

• Frequency selection

• Phase relationships in impedance space

Because of this, two inspectors following the same procedure can arrive at very different conclusions.

The difference is not usually the instrument — it’s the analyst.

Experienced eddy current professionals develop an internal “library” of signal behaviors. They recognize patterns quickly and intuitively:

• What a crack should look like

• What a permeability change usually looks like

• When a signal is repeatable — and when it isn’t

This pattern recognition is a classic example of tacit knowledge.

Why Procedures and Automation Can’t Replace Tacit Skill

Modern eddy current systems include:

• Automated data acquisition

• Signal screening algorithms

• Auto-analysis tools

• Historical data overlays

These tools are incredibly powerful — but they still rely on human interpretation.

Automation can flag signals.
It cannot fully understand context.

For example:

• Is a signal caused by material variation or degradation?

• Is a phase shift meaningful, or an artifact of probe wobble?

• Does the signal behave consistently across multiple frequencies?

These decisions still depend on human judgment, built from years of experience.

How Tacit Skills Are Actually Developed

Tacit skills in eddy current testing are developed through:

• Repeated exposure to real inspection data

• Comparing known flaws to false indications

• Reviewing historical inspections over multiple years

• Making mistakes — and learning from them

• Working with mentors who explain why they made certain calls

This is why hands-on training, data review, and long-term experience are irreplaceable.

You cannot shortcut this process with a textbook alone.

Why This Matters for Managers and Decision-Makers

From a management perspective, tacit skill explains why:

• Two Level II analysts are not interchangeable

• Years of experience matter beyond certification hours

• Removing experienced personnel can increase risk — even if procedures remain unchanged

• Training programs that focus only on passing exams often fall short

Eddy current testing is not “black magic” — but it does require respect for the human element.

The Role of eddycurrent.com in Preserving Tacit Knowledge

One of the goals of eddycurrent.com is to help bridge the gap between formal knowledge and tacit understanding by:

• Preserving historical and modern technical literature

• Providing real-world context around eddy current theory

• Encouraging practitioners to study applications across industries

• Highlighting the human factors that influence inspection quality

By studying how eddy current testing evolved — from early analog instruments to modern digital systems — practitioners gain insight into why techniques work, not just how to perform them.

Final Thoughts

Tacit skills are the invisible backbone of reliable nondestructive testing.

In eddy current testing, they are not optional — they are essential.

Instruments will continue to improve.
Automation will continue to advance.
But the quality of an inspection will always depend on the person interpreting the data.

Understanding, developing, and respecting tacit skill is one of the most important steps toward better eddy current testing.

Call to Action

If you want to deepen your understanding of eddy current testing beyond formulas and procedures — including the human factors that influence real inspection outcomes — explore the educational resources, historical archives, and training materials available at eddycurrent.com.

You’ll find that mastering eddy current testing isn’t just about learning the rules — it’s about learning how to think like an eddy current professional.