Classification and measurement principles of coating thickness gauges
1、 Principle of magnetic attraction measurement and thickness gauge
The suction force between the magnet (probe) and the magnetic conducting steel is proportional to the distance between the two, which is the thickness of the coating. Using this principle to make a thickness gauge, as long as the difference in permeability between the coating and the substrate is large enough, it can be measured. Given that most industrial products are formed by stamping structural steel and hot-rolled and cold-rolled steel plates, magnetic thickness gauges are the most widely used. The basic structure of the thickness gauge consists of magnetic steel, relay spring, ruler, and self stopping mechanism. After the magnetic steel is attracted to the measured object, the measuring spring gradually elongates behind it, and the tensile force gradually increases. When the tensile force is just greater than the suction force, the coating thickness can be obtained by recording the magnitude of the tensile force at the moment when the magnetic steel detaches. The new product can automatically complete this recording process. Different models have different ranges and applications.
The characteristics of this instrument are simple operation, durability, no need for power supply, no need for calibration before measurement, and low price. It is very suitable for on-site quality control in the workshop.
2、 Principles of Magnetic Induction Measurement
When using the principle of magnetic induction, the thickness of the coating is measured by the magnitude of the magnetic flux flowing into the ferromagnetic substrate from the probe through a non ferromagnetic coating. The corresponding magnetic resistance can also be measured to represent its coating thickness. The thicker the coating, the greater the magnetic resistance and the smaller the magnetic flux. A thickness gauge using the principle of magnetic induction can, in principle, have a non magnetic coating thickness on a magnetic substrate. Generally, the permeability of the substrate is required to be above 500. If the coating material also has magnetism, it is required to have a sufficiently large difference in permeability compared to the substrate (such as nickel plating on steel). When the probe wrapped around the coil on the soft core is placed on the tested sample, the instrument automatically outputs the test current or signal. Early products used pointer type meters to measure the magnitude of induced electromotive force, and the instrument amplified the signal to indicate the thickness of the coating. In recent years, circuit design has introduced new technologies such as frequency stabilization, phase locking, and temperature compensation, utilizing magnetoresistance to modulate measurement signals. The integrated circuit designed by and the introduction of a microcomputer have greatly improved the measurement accuracy and reproducibility (almost an order of magnitude). Modern magnetic induction thickness gauge with resolution up to magnetic induction thickness gauge_ Principles of Eddy Current Measurement_ Principle of magnetic attraction measurement and thickness gauge_ The thickness gauge based on the principle of eddy current reaches 0.1um, with an allowable error of 1% and a measurement range of 10mm.
The magnetic principle thickness gauge can be used to measure the paint layer on the surface of steel, the protective layer of porcelain and enamel, the plastic and rubber coatings, various non-ferrous metal electroplating coatings including nickel chromium, as well as various anti-corrosion coatings for chemical and petroleum industries.
3、 Principles of Eddy Current Measurement
High frequency AC signals generate electromagnetic fields in the probe coil, and when the probe is close to the conductor, eddy currents are formed within it. The closer the probe is to the conductive substrate, the greater the eddy current and reflection impedance. This feedback action represents the distance between the probe and the conductive substrate, which is the thickness of the non-conductive coating on the conductive substrate. Due to the specialized measurement of the coating thickness on non ferromagnetic metal substrates, this type of probe is commonly referred to as a non magnetic probe. The non magnetic probe uses high-frequency materials as the coil core, such as platinum nickel alloy or other new materials. Compared with the principle of magnetic induction, the main difference is that the probe is different, the frequency of the signal is different, and the size and scale relationship of the signal are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge also achieves a resolution of 0.1um, an allowable error of 1%, and a high level range of 10mm.
A thickness gauge using the principle of eddy current can measure non conductive coatings on all conductive materials, such as paint, plastic coatings, and anodic oxide films on the surfaces of aerospace aircraft, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products. The coating material has a certain degree of conductivity, which can also be measured through calibration, but the ratio of conductivity between the two is required to be at least 3-5 times different (such as chromium plating on copper). Although the steel substrate is also a conductive material, magnetic principles are still more suitable for measuring such tasks
