Dry Film Thickness Gauge, Eddy Current (NDT, ECT)

Definition - What does Dry Film Thickness Gauge, Eddy Current (NDT, ECT) mean?

A dry film thickness gauge, eddy current is an electronic tool used to measure the thickness of non-conductive coatings on conductive but nonferrous or nonmagnetic substrates like brass, aluminium, copper or aluminium.

Eddy current dry film thickness gauges are used in eddy current testing (ECT), a non-destructive test (NDT) method that determines dry film thickness based on the strength or level of eddy currents induced in a metal by a probe containing a conducting wire coil. The distance between the probe and the substrate, which is equivalent to the coating thickness, inversely affects the eddy current strength level. So, the strength of the eddy current deceases as the coating thickness increases.

Corrosionpedia explains Dry Film Thickness Gauge, Eddy Current (NDT, ECT)

An eddy current dry film thickness gauge is a widely used non-destructive coating thickness measurement tool because of its high sensitivity and wide range of thickness capability.

However, like other measuring instruments, its accuracy depends largely on regular calibration, the right operating conditions, its proper use according to the manufacturer’s instructions and other factors that include but is not limited to the following:

  1. Coating thickness – The user should check the thicknesses ranges the instrument is designed for and the number of measurements to be taken to arrive at an accurate result.
  2. Thickness and electrical properties of the base metal or substrate – Eddy current-type thickness gauges can only be used if the electrical conductivity of the coating differs significantly from that of the substrate. Eddy current dry film thickness gauges are usually designed for various ranges of substrate thicknesses.
  3. Surface contour and the location of the test specimen – Areas near the edges or which exhibit curvature should be avoided unless the instrument is designed for such measurements.
  4. Surface topography and foreign particles – The probe should be clean. Foreign particles and the roughness of the measured surface may affect the probe's contact with the test surface.
  5. User’s pressure and positioning of the probe – An appropriate jig to hold the probe may prevent this problem.

The user should read and understand the instrument’s manual to ensure that it is used as intended by the manufacturer.

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