The thickness of the paint is directly related to its performance and quality. In order to accurately assess the thickness of the paint coating, it is necessary to use professional technology and instruments to detect it. In industry communication, I am often asked how to measure the thickness of wet film, plastic paint film, layered film, and the advantages and disadvantages of various measurement methods. Below we will introduce the common methods and principles of paint coating thickness measurement, to tell you which coating thickness gauge is better for detecting the thickness of paint coating.
There are many methods for testing the thickness of the paint coating, which can be divided into weighing method, magnetic method, eddy current method, measuring tool measurement method, microscope method, non-destructive instrument measurement method, etc. according to their principles. But the most commonly used are the following two. Their advantages are that they are affordable, portable and easy to operate, and have high accuracy.
The first type: magnetic measurement method
It is suitable for measuring the thickness of non-permeable layer on permeable material. Magnetically conductive materials are generally: steel, iron, silver, and nickel. This method has high measurement accuracy.
Among them, the magnetic measurement method market can be subdivided into two categories: magnetic attraction and magnetic induction.
1. The principle of magnetic attraction thickness gauge
The thickness of the coating is measured by using the suction force between the permanent magnet probe and the magnetically conductive steel to be proportional to the distance between the two. This distance is the thickness of the coating, so as long as the coating and the The difference in the permeability of the substrate is large enough to be measured.
2. The principle of magnetic induction thickness gauge
The thickness of the coating is measured by the size of the magnetic flux that the probe passes through the non-ferromagnetic coating and flowing into the iron substrate. The thicker the coating, the smaller the magnetic flux. When the probe with a coil on the soft iron core is placed on the test object, the instrument automatically outputs the test current. The magnitude of the magnetic flux affects the magnitude of the induced electromotive force. The instrument amplifies the signal and then indicates the thickness of the coating.
The second type: eddy current measurement method
It is suitable for measuring the thickness of non-conductive layer on conductive metal. This method has a slightly lower accuracy than the magnetic thickness measurement method.
The eddy current thickness gauge uses high-frequency alternating current to generate an electromagnetic field in the coil as the measuring head. When the probe is close to a conductive metal body, an eddy current is formed in the metal material. This eddy current decreases as the distance from the metal body decreases. Increase, and will affect the magnetic flux of the probe coil, this feedback effect is a measure of the distance between the probe and the base metal.
The relevant standards that may be involved in measuring the thickness of paint coatings are as follows:
GB1764-79 "Method for Measuring Thickness of Paint Film";
ISO2808 "Paints and varnishes-Determination of paint film thickness";
ASTMD1400-81 "Determination of the dry film thickness of non-metallic coatings on non-magnetic metal substrates for colored paints, varnishes, spray paints and related products";
ASTME376-69 "Determination of coating thickness by magnetic method or eddy current method (electric field method)";
ASTMD2691-70 "Microscopic measurement of dry film thickness on the surface of wood products";
ASTMG12-83 "Non-destructive measurement of steel pipe coating thickness";
ASTMD1212-79 "Determination of Wet Film Thickness of Organic Coatings";
JTGF80/1-2017 "Highway Engineering Quality Inspection and Evaluation Standard Volume 1 Civil Engineering";
GB/T18226-2015 "Technical Conditions for Anticorrosion of Steel Components in Highway Traffic Engineering" and so on.
Matters needing attention when using coating thickness gauge to measure paint coating thickness
1. When the environmental temperature and humidity change greatly, or the electromagnetic properties and thickness of the substrate to be tested are greatly different, please zero-calibrate first;
2. When measuring, pay attention to the influence of the curvature of the test piece on the measurement. Therefore, it is unreliable when measuring on a curved specimen surface;
3. Before measuring, pay attention to whether other electrical equipment around will generate magnetic field, if it will interfere with the magnetic thickness measurement method.