Magnetic coating thickness gauge application principle

Magnetic coating thickness gauge application principle

First, the principle of magnetic attraction thickness gauge

The thickness of the coating can be measured by the relationship between the magnitude of the suction between the permanent magnet probe and the magnetically permeable steel and the distance between the two. This distance is the thickness of the coating, so as long as the coating and the substrate The difference in magnetic permeability is large enough for measurement. Since most industrial products are formed by structural steel and hot-rolled cold-rolled steel sheets, magnetic thickness gauges are the most widely used. The basic structure of the measuring instrument is magnetic steel, tension spring, scale and self-stop mechanism. When the magnetic steel is attracted to the measured object, a spring is gradually elongated and the tensile force is gradually increased. When the tensile steel is larger than the moment when the suction magnetic steel is separated, the thickness of the coating can be obtained. Generally speaking, depending on the model, different ranges and adaptations.

In a range of about 350°, the scale can be represented by 0 to 100 μm; 0 to 1000 μm; 0 to 5 mm, and the precision can reach 5% or more, which can meet the general requirements of industrial applications. The instrument is characterized by simple operation, strong and durable, no power supply and calibration before measurement, and the price is also low, which is very suitable for on-site quality control in the workshop.

Second, the magnetic induction principle thickness gauge

The principle of magnetic induction is to measure the thickness of the cladding by using the magnetic flux of the probe flowing through the non-ferromagnetic coating into the iron substrate. The thicker the coating, the smaller the magnetic flux. Because it is an electronic instrument, it is easy to calibrate, can perform a variety of functions, expand the range, improve the accuracy, and because the test conditions can be reduced a lot, it is more extensive than the magnetic suction application field. When the probe around the coil on the soft iron core is placed on the object to be tested, 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 early products were indicated by the meter, and the accuracy and repeatability were not good. Later, the digital display was developed and the circuit design was becoming more and more perfect. In recent years, the introduction of microprocessor technology and electronic switches, frequency stabilization and other latest technologies, a variety of patented products have come out one after another, the accuracy has been greatly improved, reaching 1%, the resolution reaches 0.1μm, the measurement of magnetic induction thickness gauge The head is mostly made of mild steel as the magnetic core. The frequency of the coil current is not high, so as to reduce the influence of the eddy current effect, the probe has a temperature compensation function. Since the instrument is intelligent, different probes can be identified, with different software and automatically changing the probe current and frequency.

One instrument can be used with a variety of probes, or the same instrument can be used. It can be said that instruments suitable for industrial production and scientific research have reached a very practical stage. The thickness gauge developed by the electromagnetic principle is applicable to all non-magnetic permeability coatings in principle, and generally requires a basic magnetic permeability of more than 500. If the cladding material is also magnetic, it is required to have a sufficiently large difference from the magnetic permeability of the substrate (such as nickel plating on steel). The magnetic principle thickness gauge can be applied to paint coatings, porcelain, enamel protective layers, plastics, rubber coatings, various non-ferrous metal plating layers including nickel-chromium, and various anti-corrosion coatings for the chemical petroleum industry. Floor. For film production industries such as photographic film, capacitor paper, plastics, and polyester, measurement platforms or rolls (manufactured by steel) can also be used to achieve measurement at any point on a large area.

Third, the eddy current principle thickness gauge

The eddy current thickness measurement method is mainly applied to the measurement of various non-metal coating layers on a metal substrate.

Using a high-frequency alternating current to generate an electromagnetic field in the coil as a probe, when the probe is brought close to the conductive metal body, a eddy current is formed in the metal material and increases as the distance from the metal body decreases, and the eddy current affects the probe coil The magnetic flux, so the amount of feedback is a measure of the distance between the probe and the base metal. Because the probe is used to measure the thickness of the coating on a non-ferromagnetic metal substrate, we usually call the probe non-magnetic. Probe. Non-magnetic probes generally use high-frequency and high-magnetic materials for the coil core, which is usually made of platinum-nickel alloy and other new materials.

Compared with the principle of magnetic measurement, their electrical principles are basically the same. The main difference is that the probes are different, the frequency of the test current is different, and the signal size and scale relationship are different. In the latest thickness gauge, by continuously improving the structure of the probe, in conjunction with the microcomputer technology, different control programs are automatically called to call different control programs, respectively output different test currents and change the scale conversion software, finally making two kinds of Different types of probes are connected to the same thickness gauge, which reduces the burden on the user. Based on the same idea, the thickness gauges that can be matched with up to 10 side heads greatly expand the thickness measurement range (up to 100,000 times). The non-magnetic coating on the surface of the magnetically permeable material, the non-conductive coating on the conductive material and the conductive layer on the non-conductive material can be measured, which basically meets the needs of most industries in industrial production. Thickness gauges using the principle of eddy currents, in principle, all non-conductor coatings on electrical conductors can be measured, such as aerospace aircraft surfaces, vehicles, home appliances, aluminum alloy doors and windows and other aluminum surface paints, plastic coatings and anodes. Oxide film.

Some special applications are diamond coatings on certain metals and other non-conductive layers. The cladding material can also have a certain conductivity, and can also be measured by calibration, but the ratio of the conductivity of the two is required to differ by at least 3 to 5 times (for example, chrome plating on copper). The principle of calibration is that the calibration sample without coating and the substrate of the test object should have the same composition and the same thickness (mainly when the thickness is less than 0.5mm below the minimum specified by the instrument), and have the same radius of curvature, such as The measured area is smaller than the technical parameters of the instrument (about 20mm in diameter), and should have the same measured area. If the coating contains conductive components, the coating of the calibration sample should also have the same conductivity as the coating of the analyte.

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