PSEC vs. FSEC — What's the Difference?
Two framings exist in the industry — we use the physically honest one.
Technical Note · DELTA TEST ME LLC
TL;DR
PSEC means we drove the tube wall partially into magnetic saturation. FSEC means we drove it fully into saturation. The name describes the achieved state of the material, not the size of the magnet on the probe. Counter-intuitively, this means PSEC is what we use on strongly ferromagnetic alloys (CS, P91, T22) — because we can never fully saturate them in practice — and FSEC is what we use on slightly ferromagnetic ones (duplex, austenitic with cold work, Monel), where full saturation is actually reachable.
The Physics
Every ferromagnetic material has a saturation flux density Bs — about 2 T for carbon steel, slightly less for ferritic stainless, lower again for low-alloy variants. To reach it, the applied magnetizing field H must exceed the saturation threshold Hs. Above the saturation "knee" of the B-H curve, the differential permeability µdiff collapses towards 1 — and the AC eddy-current behavior becomes predictable, as if the material were paramagnetic.
Whether the bobbin-probe magnet pushes a given tube wall over the knee depends on the magnetic circuit: material × wall thickness × magnet strength. A modest magnet on a thin duplex wall reaches the plateau easily. The same magnet (or even a stronger one) on a 4 mm carbon-steel wall stops well inside the partial-saturation region — there is simply too much ferromagnetic material in the magnetic path.
The same probe magnet, applied to two different tube walls, lands at two different operating points on the B-H curve. The amber region (below the knee) is partial saturation territory — we still see permeability variation, phase analysis is harder. The green region (plateau, above the knee) is full saturation — the AC behavior becomes predictable and phase-based sizing is straightforward.
Why There Are Two Names In Industry
Both vendors and inspectors call DC-biased eddy-current testing PSEC or FSEC — but the two camps define them differently:
Framing A · our position
Achievement-based
We name the technique after what saturation we actually achieved in the wall, measured on the impedance plane.
- PSEC
- µdiff still > 1. The wall was driven part-way up the B-H curve but not over the knee. Strongly ferromagnetic + heavy wall.
- FSEC
- µdiff → 1. The wall is fully past the saturation knee. Slightly ferromagnetic + thin wall, or favorable geometry.
Why we use this
- + Physically honest
- + Predicts inspection performance from material + WT
- + Matches what the impedance plane actually shows
Framing B · common vendor naming
Probe-class / marketing-based
The name follows the SKU on the probe shelf — magnet strength, not the saturation state on the actual tube.
- PSEC
- Probe carries a "moderate" magnet — marketed for slightly ferromagnetic alloys.
- FSEC
- Probe carries a "strong" magnet — marketed for CS / heavy-wall ferromagnetic tubing.
Why it's misleading
- − A "strong magnet" on thick CS still produces partial saturation
- − Customer expects FSEC but reads a PSEC signature on the impedance plane
- − Naming describes the catalog page, not the measurement
Our position at DELTA TEST ME
We name the method after what we actually achieve on your tube wall — not after the SKU on the probe shelf. If µdiff on the impedance plane is collapsed to ~1, we call it FSEC. If the µ-noise is suppressed but not flat, we call it PSEC. This means our reports tell you the truth about what was measured, and it explains why the same probe can be FSEC on one tube material and PSEC on another.
Decision Matrix — Material × Wall Thickness
| Material | Typical wall | Method |
|---|---|---|
| Carbon steel (CS) — heat exchanger | 2 – 5 mm | PSEC |
| Low-alloy steel (LAS) | 2 – 5 mm | PSEC |
| P91 / T22 / T11 (creep-resistant ferritic) | 3 – 8 mm | PSEC |
| Ferritic stainless steel | ≥ 3 mm | PSEC |
| Ferritic stainless steel | < 3 mm | FSEC |
| Duplex / Super-duplex SS | 1.5 – 3 mm | FSEC |
| Austenitic SS with δ-ferrite / strain-induced martensite | 1 – 3 mm | FSEC |
| Monel 400 | 1.5 – 3 mm | FSEC |
Borderline materials (e.g. ferritic stainless at 3 mm wall) sit on the knee — small wall-thickness or composition differences flip the method. We confirm the actual saturation state on a sample tube before the production run.
What This Means for Your Inspection
- ▸PSEC and FSEC reports look similar on the impedance plane — but sensitivity, sizing accuracy and crack-detection capability differ. Knowing which one applies to your bundle matters for fitness-for-service decisions.
- ▸When a vendor offers "FSEC on heavy-wall CS", ask whether the wall is actually past the saturation knee. In our experience the answer is usually no — the method is technically PSEC. That is perfectly fine; it just needs to be named honestly.
- ▸Borderline cases get calibrated against a sample tube. We confirm the achieved saturation state before committing to PSEC or FSEC reporting.
- ▸In all cases we cross-check ambiguous indications with IRIS for absolute wall thickness, or with the magnet-probe technique for metallurgical false-indication discrimination.
A note on standards
The applicable standard, ASTM E2884 — Standard Guide for Eddy Current Examination of Tubing Using Partial Saturation — uses "partial saturation" as the umbrella term and does not strictly separate PSEC from FSEC. The vendor-marketing names came later, which is one reason the industry now has two conventions in use. Our framing keeps the standard's umbrella ("saturation-assisted EC") and adds an honest qualifier (achieved partial vs. achieved full) on the report.