Archive for August, 2014

Thickness Gauges: General Theory of Eddy-Current and Magnetic Gauges

In addition to x-ray fluorescence analyzers for coating thickness, Eastern Applied offers a full range of contact gauges.  The following details the general principal behind eddy-current and magnetic induction coating thickness gauges.  Contact our Application Specialists to discuss your measurement requirements.

EDDY-CURRENT

The eddy-current technique is utilized for measuring non-conductive coatings on nonmagnetic metal substrates, metallic coatings on ferrous substrates, and conductive coatings on non-conductive substrates.

The eddy-current measuring probe contains a coil, which sets up an oscillating electromagnetic field.  When the probe is applied to the surface of the coating being tested, a flow of eddy-currents is induced in both the coating and the substrate of the sample.

Eddy-current flow in closed circular paths and the depth of penetration is inversely proportional to the square root of the probe frequency and also to the square root of the electrical conductivity of the material in which the eddy-currents flow.  The eddy-currents create a counter magnetic field, which in turn reacts upon the probe by reactance and alters its output voltage.  The change in output voltage created is used to calculate the thickness.

In the case of non-conductive films over nonmagnetic metals (lift-off), increasing the thickness of the coating increases the separation from the conductive substrate.  This increased separation causes a decrease in the magnitude of the eddy-currents induced in the base, and therefore a decrease in output voltage of the probe.

In the measurement of metallic coatings on ferrous substrates, the eddy-currents flow in both the coating and the base.  The intensity of the eddy-current reaction depends on the conductivity of both metals involved.  As the coating thickness increases, more of the eddy-current flow in the coating than in the base metal, and the reaction on the probe corresponds more closely to that of the pure coating metal.

MAGNETIC INDUCTION

The magnetic induction method is used for measuring the thickness of any nonmagnetic coating such as zinc, cadmium, paint, etc on a steel substrate.  This measurement method is also called the ‘lift off’ method.  Magnetic principles are also used to measure magnetic nickel on a nonmagnetic substrate.

In this technique, the probe system is essentially a transformer.  The primary winding of the probe is supplied by the basic unit with an alternating current.  The secondary winding forms the output of the probe, which reacts to the presence of a magnetic material.  As the probe is brought closer to the magnetic material, the efficiency steadily increases and progressively more voltage is output.  Thickness readings are based on the output voltage of the probe.  As the coating gets thicker, the distance between the probe and the magnetic substrate increases, decreasing the output voltage.

Thickness Gauge Applications