Fe vs NFe vs Dual Mode: A Quick Guide to Substrate Identification

The bottom line

A thickness gauge is not a universal instrument. Choosing the wrong principle does not give you a slightly high or low number — it gives you a meaningless number. The order is always: substrate, then coating, then instrument.

Step one: is the substrate ferromagnetic?

The most practical field method is to carry a magnet:

• The magnet sticks → ferromagnetic substrate (carbon steel, low-alloy steel, cast iron, ferritic stainless steel) → magnetic induction, Fe;
• The magnet does not stick, but the part is metal → non-magnetic conductive substrate (aluminum, copper, brass, austenitic stainless steel) → eddy current, NFe;
• Neither metal nor conductive → plastic, wood, glass, concrete, fibreglass → both magnetic induction and eddy current fail, and only ultrasonic works.

Step two: is the coating conductive?

Both principles require the coating to block a physical quantity: magnetic induction needs a non-magnetic coating, eddy current needs a non-conductive one. The common non-magnetic, non-conductive coatings are paint, varnish, powder coating, anodising, conformal coating, plastic coatings and enamel.

If the coating itself conducts — some conductive paints, thermally sprayed metal layers — both methods fail, and you need a different technique such as coulometric or X-ray methods.

Dual mode: two purchases rolled into one

For contractors who handle both steel and aluminum parts, a dual-mode gauge is the efficiency choice. Dongru Fe/NFe models include DR-A5 (0-2000 μm, automatic recognition), DR130 (0-1500 μm), DR230 (0-2000 μm), DR280A (0-1250 μm), DR330 (0-2000 μm), DR3000 (0-3000 μm) and DR380A (0-2000 μm).

A dual-mode probe decides by first applying a magnetic field: a response means magnetic induction, no response means it switches to eddy current. That is also why it fails on non-metallic substrates — by design, that combination lies outside both principles.

Substrate families and the models that cover them

The one-line rule

Magnet sticks → Fe. Magnet does not stick but the part conducts → NFe. Does not conduct → ultrasonic.

Common misconceptions

• “A dual-mode gauge is less accurate than a single-principle one” — accuracy depends on the probe and the calibration, not on integration. What a dual-mode gauge costs you is range at the top end and purchase price, not reading quality.
• “Buy the magnetic model now and add an eddy current one later” — for a contractor who must accept work across substrates, two instruments cost more in calibration and record-keeping than one dual-mode gauge.
• “A dual-mode gauge will limp through non-metallic substrates” — it will not. There is no response in principle.

Frequently asked questions

Why is substrate identification the first step?

Because the substrate decides the measuring principle, and the principle decides the gauge model. Get this wrong and the calibration and verification that follow are pointless.

What happens if I use a magnetic gauge on a non-metallic substrate?

The gauge gives no response or errors out — measurement simply cannot proceed. The instrument has no fault; it lacks the physical quantity to measure.

Why is my reading on stainless steel unstable?

Austenitic stainless is not ferrous. Treating it as ferrous gives a reading that swings with position — not a steady systematic bias — so it cannot be corrected away.

Why is my reading consistently low?

Check the minimum substrate thickness. Below it, the reading is systematically low and the bias grows as the substrate gets thinner — easily mistaken for “insufficient coating thickness.”

How should I confirm the substrate reliably?

In three steps: a magnet test, material confirmation, and a geometry check. Never judge by whether the reading looks stable — the displayed numbers can look normal even when the call is wrong.

Related instruments: DONGRU DR-A5 Automotive Paint Thickness Gauge · Request a quote