The principle of magnetic attraction measurement The attraction between the permanent magnet (probe) and the magnetically permeable steel is directly proportional to the distance between them, that is, the thickness of the coating. As long as the difference between the permeability of the coating and the substrate is large enough, the thickness gauge made by this principle can be measured. In view of the fact that most industrial products are stamped from structural steel and hot-rolled and cold-rolled steel plates, magnetic thickness gauges are most widely used. The basic structure of the thickness gauge consists of magnetic steel, relay spring, scale and self-stop mechanism. After the magnetic steel is attracted to the measured object, the measuring spring is gradually elongated and the tension is gradually increased. When the tensile force is just greater than the suction force, the coating thickness can be obtained by recording the tensile force at the moment of magnetic steel separation. New products can automatically complete this recording process. Different models have different measuring ranges and applicable occasions.
The instrument is simple to operate, durable, without power supply and calibration before measurement, and its price is low, which is very suitable for on-site quality control in the workshop.
When the principle of magnetic induction measurement is adopted, the thickness of the coating is measured by the amount of magnetic flux that the probe flows into the ferromagnetic matrix through the non-ferromagnetic coating. The corresponding magnetoresistance can also be measured to indicate the coating thickness. The thicker the coating, the greater the magnetic resistance and the smaller the magnetic flux. In principle, the thickness gauge based on magnetic induction principle can have the thickness of non-magnetic coating on magnetic substrate. Generally speaking, the magnetic permeability of the substrate is required to be above 500. If the coating material is also magnetic, it is required that the difference between the permeability and the substrate is large enough (such as nickel plating on steel). When the probe wound on the coil on the soft core is placed on the tested sample, the instrument automatically outputs the test current or test signal. Early products used a pointer instrument to measure the induced electromotive force, and the instrument amplified the signal to indicate the coating thickness. In recent years, new technologies such as frequency stabilization, phase locking and temperature compensation have been introduced into the circuit design, and the signal is measured by reluctance modulation. Patented integrated circuits and microcomputers are also adopted, which greatly improves the measurement accuracy and reproducibility (almost an order of magnitude). The resolution of modern magnetic induction thickness gauge is 0.65438 0 micron, the allowable error is 65438 0%, and the measuring range is 65438 0 mm.
Magnetic principle thickness gauge can be used to accurately measure paint layer, enamel protective layer, plastic and rubber coating, electroplating layer of various non-ferrous metals including nickel and chromium, and various anti-corrosion coatings in chemical and petroleum industries.
Eddy current measurement principle High-frequency AC signals generate electromagnetic fields in the probe coil, and when the probe is close to the conductor, eddy currents are formed in it. The closer the probe is to the conductive substrate, the greater the eddy current and the greater the reflection impedance. This feedback represents the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. This probe is usually called a nonmagnetic probe because it measures the thickness of the coating on a nonmagnetic metal substrate. The nonmagnetic 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 signal frequency is different, and the signal size and scale relationship are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge has reached a high resolution of 0. 1um, with an allowable error of 1% and a measuring range of10 mm.
The thickness gauge based on eddy current principle can, in principle, measure all non-conductive coatings on electrical conductors, such as paints, plastic coatings and anodic oxide films on the surfaces of aluminum products such as aerospace vehicles, vehicles, household appliances and aluminum alloy doors and windows. The coating material has a certain conductivity, which can also be measured by calibration, but the ratio of conductivity between the two is required to be at least 3-5 times (such as chromium plating on copper). Although the steel matrix is also a conductor, it is more suitable for this kind of task to be measured by magnetic principle.