ISO 2178:2016 pdf free
ISO 2178:2016 pdf free.Non-magnetic coatings on magnetic substrates一Measurement of coating thickness一Magnetic method
The electrical inductivity of a coil changes when an iron core is inserted into the coil or when an iron object, e.g. a plate, approaches the coil. Therefore, the electrical inductivity can be used as a measure of the distance between the coil and a ferromagnetic substrate or as a measure of the coating thickness, if the coil is placed onto a coated magnetizable base metal.
There are many different electronic methods to evaluate changes of the electrical inductivity or the reaction of a coil system to a ferromagnetic substrate. Magnetic induction probes for thickness measurements of coatings on magnetizable materials can consist of one or more coils. Most often two coils are used (see Figure 2): the first (primary coil) to generate a low frequency alternating magnetic field and the second (secondary coil]) to measure the resulting induced voltage U. If the probe is placed on a coated magnetizable material (μr> 1) the magnetic flux density (see Annex A) and the induced voltage of the secondary coil vary as a function of the coating thickness. The function between the induced voltage and the coating thickness is nonlinear and depends on the permeability ur of the base metal. It is usually determined by a calibration. Calibration curves that assign a coating thickness to the induced voltages can be stored in the gauge.
Different designs and geometries of these kind of probes are used. Very often both coils are employed together with a highly magnetizable core in order to increase the sensitivity of the probes and to concentrate the field. In this way, both the coating area, which contributes to the thickness measurement, and the influence of the geometry of the coated component are reduced (see 5.5 and 5.6).
On the contrary, a two pole probe (see Figure 3) has a wide and open field distribution. The two-pole probe has area integrating properties, while a one-pole probe measures locally.ISO 2178 pdf free.
