Electromagnetic eddy current thickness measurement principle and thickness.
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Electromagnetic eddy current thickness measurement principle
  
      Surface protection of materials, decorative form of cover, such as coating, plating, cladding layer, paste layer, the chemical generation film, in the national and international standards, known as the cladding (coating).
      Coating thickness measurement has become a processing industry, surface quality examination of an important part of the product necessary to achieve excellent quality standards means. To make the product internationalization, China's export and foreign projects, the thickness of the cover with a clear requirement.
      Coating thickness measurement methods are: wedge cut method, light interception, electrolysis, the thickness difference measurement, weighing, X-ray fluorescence, β-ray backscatter method, capacitance, magnetic measurements and eddy current measurement law. The first five of these methods is the damage detection measurements cumbersome, slow, multi-sample test applied.
      X-ray and β-ray method is a non-contact non-destructive measurement, but the device complex and expensive, smaller measuring range. Due to radioactive sources, users must comply with radiation protection norms. X-ray method can measure very thin coating, double coating, coating. β-ray method is suitable for coating and the substrate atoms number greater than 3, coating measurements. Capacitance conductor only in the thin coating thickness of insulation used.
      As technology advances, especially in recent years after the introduction of computer technology, using magnetic method and the eddy current method of thickness to the tiny, intelligent, multi-function, high precision, practical direction further. Reached 0.1 micron resolution measurement accuracy can reach 1%, has greatly improved. It is suitable for a wide range, wider range, simple and inexpensive, is the most widely used industrial and scientific research of thickness gages.
      Not undermine the use of non-destructive method of coating without damaging the substrate, speed, and make a lot of testing work carried out economically.

      Measurement principle and instrument
      1. Magnetic force measurement principle and the thickness
      Permanent magnet (probe) and the magnetic attraction between the size of steel is the distance between the two relations into a certain proportion, this distance is the coating thickness. Gage made use of this principle, as long as the cover layer and substrate permeability difference is large enough, can be measured. Since most of the industrial use of structural steel and hot rolled steel plate stamping molding, the most widely used magnetic thickness gauge. Basic structure of thickness from the magnet, reed relays, gauge and automatic stop agencies. Pull magnet and measured object, it will measure the gradual lengthening in the following spring, tension increased. When the tension is greater than just suction, magnetic pull from the moment of recording the size of coating thickness can be obtained. The new product can automatically complete the recording process. Different models have different ranges and for occasions.
      This instrument is characterized by simple, rugged, no power, no calibration before measurement, lower prices, it is suitable for the scene shop to do quality control.

      2. Magnetic induction measuring principle
      Using magnetic induction principle, the use of non-ferromagnetic from the probe through the coating and the magnetic flux into the ferromagnetic matrix size, to determine coating thickness. Also can determine the size of the corresponding reluctance to express its coating thickness. Thicker cladding, then the greater reluctance, the smaller the flux. Thickness using the principle of magnetic induction, in principle, can have a non-magnetic substrate magnetic coating thickness. Generally require more than 500 substrate permeability. If there are magnetic coating material, the requirements and the difference between the substrate permeability is large enough (such as steel-plated nickel). When the soft-core on the probe placed around the coil when the tested samples, the instrument output test current or test signal. Early products by the header pointer, the size of the induced electromotive force measurements, the signal amplification equipment was instructed coating thickness. Circuit design in recent years, the introduction of frequency stabilization, lock, temperature compensation and so on new technologies, reluctance to use the signal modulation measurements. Also uses patented design of integrated circuits, the introduction of the computer, so accuracy and repeatability has greatly increased (by almost an order of magnitude). Of modern magnetic induction thickness, resolution of 0.1um, allowable error of 1%, range up to 10mm.
      Magnetic theory can be used to accurately measure thickness of steel surface paint layer, porcelain enamel protective layer, plastic, rubber coating, including a variety of non-ferrous metals, including nickel-chrome plating, and chemical anti-corrosion coating of oil all unemployed layer.

      3. Eddy current measurement principle
      High-frequency ac signal generated in the probe coil electromagnetic field probe near the conductor, in which the formation of vortex. Conductivity probe from the matrix more recent, then the larger the vortex, reflecting the larger resistance. The amount of feedback and conductivity probe to characterize the distance between the size of the matrix, that is, non-conductive substrate the size of conductive coating thickness. As these specialized measurement probe of non-ferromagnetic metal coating on the substrate thickness, so often called non-magnetic probe. Non-magnetic material of high frequency probe coil iron core, such as platinum-nickel alloy or other new materials. Compared with the principle of magnetic induction, the main difference is that probe different signals of different frequencies, the signal size, scaling different. The same thickness and magnetic induction, eddy current thickness gauge has reached a resolution of 0.1um, allowable error of 1%, range 10mm high.
      Thickness by eddy current principle, in principle, all the conductors on the non-conductive body cladding can be measured, such as aerospace aircraft surfaces, vehicles, household appliances, aluminum doors, windows and other aluminum products surface coating, plastic coating and anodic oxide film. Cladding materials have some conductivity, also measurable by calibration, but requires the ratio between the conductivity difference of at least 3-5 times (such as copper on the chrome). Although the steel substrate is also a conductor, but this type of task is more appropriate measurement of the magnetic theory.

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