Magnetic Film Thickness
Magnetic film gages are used to nondestructively measure the thickness
of a nonmagnetic coating on ferrous substrates. Most coatings on steel
and iron are measured this way. Magnetic gages use one of two principles
of operation: magnetic pull-off or magnetic/electromagnetic
Related: Eddy Current
Magnetic pull-off gages use a permanent magnet, a calibrated spring, and
a graduated scale. The attraction between the magnet and magnetic steel
pulls the two together. As the coating thickness separating the two increases,
it becomes easier to pull the magnet away. Coating thickness is determined
by measuring this pull-off force. Thinner coatings will have stronger
magnetic attraction while thicker films will have comparatively less magnetic
attraction. Testing with magnetic gages is sensitive to surface roughness,
curvature, substrate thickness, and the make up of the metal alloy.
Related: Magnetic and Electromagnetic Induction
Magnetic and Electromagnetic Induction
Magnetic induction instruments use a permanent magnet as
the source of the magnetic field. A Hall-effect generator or magneto-resistor
is used to sense the magnetic flux density at a pole of the magnet. Electromagnetic
induction instruments use an alternating magnetic field. A soft, ferromagnetic
rod wound with a coil of fine wire is used to produce a magnetic field.
A second coil of wire is used to detect changes in magnetic flux.
These electronic instruments measure the change in magnetic
flux density at the surface of a magnetic probe as it nears a steel surface.
The magnitude of the flux density at the probe surface is directly related
to the distance from the steel substrate. By measuring flux density the
coating thickness can be determined.
Related: Magnetic Pull-off
Eddy current techniques are used to nondestructively measure the thickness
of nonconductive coatings on nonferrous metal substrates. A coil of fine
wire conducting a high-frequency alternating current (above 1 MHz) is
used to set up an alternating magnetic field at the surface of the instrument's
probe. When the probe is brought near a conductive surface, the alternating
magnetic field will set up eddy currents on the surface. The substrate
characteristics and the distance of the probe from the substrate (the
coating thickness) affect the magnitude of the eddy currents. The eddy
currents create their own opposing electromagnetic field that can be sensed
by the exciting coil or by a second, adjacent coil.
Related: Magnetic Film Thickness Gages
The ultrasonic pulse-echo technique of ultrasonic gages (e.g. PosiTector
200) is used to measure the thickness of coatings on non-metal substrates
(wood, concrete, plastics, composites, etc.) without damaging the coating.
An ultrasonic transducer emits a high frequency sound pulse that travels
into the coating via a coupling gel and reflects from ANY surface that
is different in density. Coating thickness readings are obtained by measuring
the time taken for the ultrasonic signal to propagate from the probe to
the coating/substrate interface and back. The travel time is divided by
two and multiplied by the velocity of sound in the coating to obtain the
thickness of the coating. In some circumstances, individual layers in
a multi-layer system can be measured.