Guide · Paint-thickness gauges
Magnetic vs eddy-current paint gauges
Fe gauges read paint over steel, NFe gauges read paint over aluminum, and neither reads plastic. Which one a used-car check needs, and how makers label them.
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Short answer
A magnetic (Fe) gauge reads paint over steel and iron. An eddy-current (NFe) gauge reads paint over aluminum and other non-magnetic metals.[1] Neither reads paint over plastic or fiberglass; that takes an ultrasonic gauge.[2] For a used car, a combination Fe/NFe gauge covers both metals, and AUTO BILD notes that tin-filled repairs on a steel body can only be found with a gauge that also has NFe capability.[3]
What Fe and NFe mean
The letters on a gauge name the metal underneath the paint, not the paint.
- Fe (ferrous): magnetic metals such as iron and steel. The gauge uses a magnetic principle, either magnetic induction or a Hall-effect sensor.
- NFe (non-ferrous): non-magnetic metals such as aluminum and copper. The gauge uses eddy currents induced in the metal.
UNI-T explains it this way and adds that the object must be metal with a non-conductive coating.[1] DeFelsko uses the same letters: F for a magnetic principle on ferrous metal, N or NFe for eddy current on non-ferrous metal.[4]
Both methods measure the same thing: the distance from the probe tip to the metal. Everything in between counts as coating.
Which method reads which panel
| Panel material | Method | Gauge mode |
|---|---|---|
| Steel | Magnetic | Fe |
| Aluminum | Eddy current | NFe |
| Plastic bumper or mirror | Ultrasonic | Neither Fe nor NFe |
| Fiberglass | Ultrasonic | Neither Fe nor NFe |
DeFelsko assigns paint over steel to magnetic gauges, paint over aluminum and other non-ferrous metals to eddy-current gauges, and paint over plastic and fiberglass to ultrasonic gauges.[2] See the steel panels, aluminum panels and plastic bumpers pages for each surface.
Why most used-car buyers need both
Mixed bodies. DeFelsko notes that recent vehicles can combine steel doors and fenders, aluminum hoods and roofs, and plastic bumpers and mirrors.[2] A steel-only gauge leaves the aluminum panels unmeasured. Actual materials vary by vehicle, so do not assume a layout.
Tin filler. AUTO BILD notes that tin-filled repairs on a steel body can only be found with a gauge that also has NFe (eddy-current) capability.[3] See tin-filled repairs.
That is why a combination gauge is the usual choice for inspection. Some switch modes automatically, others let you pick.
How makers label the two
| Maker and model | Fe / NFe labeling |
|---|---|
| DeFelsko PosiTector 6000 | Probe families F (ferrous), N (non-ferrous) and FN (both)[5] |
| DeFelsko PosiTest DFT | DFT-Ferrous for steel; DFT-Combo for steel and aluminum[4] |
| Elcometer 456 | Ferrous (F), Non-Ferrous (N) and Dual (FNF) versions[6] |
| Elcometer 311 | The FNF version switches automatically between steel and aluminum[7] |
| UNI-T UT343E | Ferrous and non-ferrous, with automatic or manual substrate identification[8] |
| Linshang LS221 | Hall effect for ferrous, eddy current for non-ferrous[9] |
A model name alone does not tell you which version a listing sells. Check for the F, N or FN code before you order.
What neither method tells you
A magnetic or eddy-current reading is the full distance from the probe to the base metal: primer, base color, clear coat, any repaint, filler and dirt, with no distinction between layers.[4] Neither method isolates the clear coat. See can a paint gauge measure clear coat.
The industrial practice for these gauges is ASTM D7091. It covers magnetic gauges for nonmagnetic coatings on ferrous metals and eddy-current gauges for nonconductive coatings on non-ferrous metals.[10] It supplements the gauge maker's instructions rather than replacing them.
Next
Questions
Can an eddy-current gauge measure paint on steel?
Not as the makers set them up. UNI-T and DeFelsko assign steel to the Fe (magnetic) mode and aluminum to the NFe (eddy-current) mode, so use Fe mode on steel panels.[1][4]
Do I need a combination Fe/NFe gauge for a used car?
Usually. DeFelsko notes recent cars can have aluminum hoods and roofs alongside steel doors, and AUTO BILD notes tin-filled repairs on steel can only be found with a gauge that also has NFe capability.[2][3]
Is a Hall-effect gauge different from a magnetic gauge?
Hall effect is one way of sensing the magnetic field, so it is still a magnetic method for steel. Linshang, for example, uses Hall effect for its LS221's ferrous mode and eddy current for its non-ferrous mode.[9]
Will either type read a plastic bumper?
No. DeFelsko says paint over plastic and fiberglass needs an ultrasonic gauge. See plastic bumpers and fiberglass.[2]