What’s the Deal with All These 3-Letter Codes used in Eddy Current Testing of Tubing?

If you’ve ever looked at an eddy current tubing inspection report, you’ve probably seen entries like: SGA, R1 C2, 2.5v, 37°, M1, TSH+0.75”, TEHTEC, and a mysterious three-letter code like "WAR" or "PLP" in the %TW column.

To the untrained eye, those report entries may seem a lot like eddy current squiggly lines- a bit confusing. But to trained analysts and regulators, these codes are the backbone of consistent, auditable, and meaningful eddy current documentation—especially in highly regulated industries like nuclear power generation.

Let’s demystify what these codes mean, where they come from, and how they’re used in practice.

Start with the Basics: What’s Being Reported?

Every time a signal is detected during an eddy current examination (typically with a bobbin coil), that information needs to be entered into a standardized inspection report. Here’s a breakdown of what that usually includes:

- Component ID – Identifies the heat exchanger (e.g., "SGA")

- Row / Column – Tube position in the bundle (e.g., Row 1, Column 9)

- Signal Amplitude (Volts) – Indicates signal amplitude

- Channel - Indicates the test configuration channel used to make the report entry

- %TW – This column is reserved for a numerical depth estimate, or descriptive 3-letter reporting code

- Phase - This column provides the signal phase in degrees, and indicates signal phase angle

- Location + Distance – Often referenced from structural features (e.g., "TSH+0.75”)

- Test Extent – Range of inspection (e.g., "TEHTEC" for Tube End Hot-leg to Tube End Cold-leg

- Utility Columns – Notes, codes, or sizing technique identification.

Sample Eddy Current Report Entry

Comp Row Col %TW Chan. Volts Loc. Distance Extent Util 1

SGA 1 15 39 M1 2.90 TSH +1.52” TEHTEC 96004.1

Wait, What’s the Deal with That “Util 1” Column?

In this case, `96004.1` isn’t a voltage reading or a tube ID—it’s most likely a reference to the qualified sizing technique # used to estimate the flaw depth.

Why is that important?

Because in industries like nuclear power, it’s not enough to say a flaw is 39% through-wall. You need to back up that claim with an industry-qualified or site-validated technique—one that accounts for things like:

- Probe type and coil configuration

- Coil drive voltage and gains

- Tube material type

- Test Instrument

- Method of voltage normalization

- Flaw morphology (e.g., single-sided vs. double-sided wear)

- Structural location (e.g., free span vs. tube support)

- Signal calibration curves backed by destructive testing data

If the wrong technique is used—or if none is documented—your depth estimate might be meaningless in the eyes of regulators. That’s why the "Util 1" column is more than a throwaway field; it’s often where traceability lives.

So… Why Not Just Always Report the Depth?

Great Question!

It’s true that the first thing most engineers and regulators want to know is, “How deep is it?” But that’s only allowed when the flaw type has a qualified sizing technique.

For example, wear at tube support plates is a well-understood mechanism, often reported using validated amplitude-vs-depth curves. So sure—report the %TW.

But what about a dent, permeability variation, or an indication that is flaw-like but cannot be easily quantified in terms of severity?

That’s when %TW gets replaced with something more appropriate...

Enter the Three-Letter Code

When the signal doesn’t fit neatly into a box—or doesn’t have a qualified sizing technique—three-letter reporting codes are commonly used. These codes act as shorthand for describing what the signal is (or might be) without making unsupported assumptions. Some codes signal specific flaws. Others flag the need for further analysis.

Common Eddy Current Three-Letter Reporting Codes:

Code Meaning Follow-Up Required?

WAR- Wear (qualified sizing technique used) No – if proper technique applied and documented

DNT - Dent May require follow-up based on voltage and location

NQI - Non-Quantifiable Indication Yes – diagnostic probe required

DSI- Distorted Support Indication Yes – diagnostic probe required

DDI- Distorted Dent with Indication Yes – diagnostic probe required

RST- Retest: Restricted Tube Yes – evaluate with small- diameter or camera inspection

PVN- Permeability Variation Yes – often requires advanced probe or engineering disposition

SAI- Axial Indication (possible crack) Yes – typically results in tube plugging

OBS- Obstructed Tube Yes – smallest probe could not pass; often removed from service

RBD- Retest: Bad Data Yes – re-inspection required

NDD- No Degradation Detected No – disposition complete, no further action required

PLP- Possible Loose Part Yes – foreign object concern; investigate with camera or tools .

Codes that Usually Trigger Diagnostic Testing

Bobbin probes are excellent for quickly screening tubing data, but certain types of signals need more information to be dispositioned correctly.

These codes (and there are many more) typically trigger follow-up testing using rotating coil probes (RPC), array probes, or cameras:

- NQI – Non-Quantifiable Indication

- DDI – Distorted Dent with Indication

- DSI – Distorted Support Indication

- RST – Retest: Restriction

- PVN – Permeability Variation

The goal of follow-up testing is to confirm or rule out degradation, provide more detailed characterization, and guide decisions such as plugging or continued service.

Special Note: PLP – Possible Loose Part

Arguably one of the most important three-letter codes is “PLP”. Even though advanced test materials may be highly resistant to corrosion, they may be susceptible to other forces.

PLP stands for “Possible Loose Part”, and it’s the bobbin probe’s way of saying:

“HEY! There might be foreign material here.”

Even in perfectly maintained heat exchangers, foreign material is often the Achilles heel that can take a component offline. And in the real world, and unplanned tube failure translates to additional expenditures in time, money, and if the heat exchanger is operating in a radiological environment- additional radiological dose to workers. One stray object inside the tube bundle can cause:

- Blocked probe access

- Tube wear from vibration or movement

- Forced shutdowns

- Millions in unplanned inspections and repairs.

When a PLP is reported, the follow-up often involves visual inspection using cameras or foreign object retrieval tools. Think of it like covering a jet engine while it’s parked—you don’t want anything inside that doesn’t belong. Tubes in the vicinity of the possible loose part signals or confirmed foreign material may be inspected with enhanced probes like rotating or array probes, to "bound" the area.

How Do You Know Which Codes Go in the ETSS?

That’s where the Level III’s judgment and experience come in.

Three-letter codes aren’t just invented on the spot—they’re selected and defined in the Examination Technique Specification Sheet (ETSS). To decide which ones to include, the Level III typically:

1. Reviews raw eddy current data from previous outages

2. Examines historical reports to identify recurring conditions

3. Consults the Degradation Assessment, if one has been created by a materials engineer.

But what happens when a condition is encountered that doesn’t have a predefined code?

- Use “LAR” (Lead Analyst Review) as a temporary placeholder

- Or, create a new code, formally define it, and issue a revision to the ETSS.

This keeps the process standardized, transparent, and defensible—even as new signal types are discovered.

Final Thoughts

Three-letter codes might look cryptic, but they’re essential for clarity, consistency, and safety in eddy current testing. Especially in industries like nuclear power, where traceability and technical rigor are non-negotiable, these codes help translate complex signals into clear engineering decisions.

So next time you see PLP, DDI, or NQI, don’t just skim past it. That little trio of letters might be the reason someone launches a visual inspection, plugs a tube, or rewrites an outage plan.

Still decoding your own ECT report? Want to learn more about ETSSs, diagnostic probes, or signal interpretation?

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