The covering layer formed to protect and decorate the surface of the material, such as coating, plating, coating, lamination, chemically formed film, etc., is called coating in relevant national and international standards.
Coating thickness measurement has become an important part of quality inspection in the processing industry and surface engineering, and is an essential means for products to meet excellent quality standards. In order to internationalize products, my country's export commodities and foreign-related projects have clear requirements for the thickness of the coating.
The main methods for measuring coating thickness are: wedge cutting method, light interception method, electrolysis method, thickness difference measurement method, weighing method, X-ray fluorescence method, β-ray backscattering method, capacitance method , magnetic measurement method and eddy current measurement method, etc. The first five of these methods are destructive testing. The measurement methods are cumbersome and slow, and they are mostly suitable for sampling inspection.
X-ray and β-ray methods are non-contact and non-destructive measurements, but the devices are complex and expensive and the measurement range is small. Due to the presence of radioactive sources, users must comply with radiation protection regulations. The X-ray method can measure extremely thin coatings, double coatings, and alloy coatings. The β-ray method is suitable for measurement of coatings and coatings with substrate atomic numbers greater than 3. The capacitance method is only used when measuring the thickness of insulating coatings on thin conductors.
With the increasing advancement of technology, especially after the introduction of microcomputer technology in recent years, thickness gauges using magnetic methods and eddy current methods have developed in the direction of miniaturization, intelligence, multi-function, high precision and practicality. step. The measurement resolution has reached 0.1 micron, and the accuracy can reach 1, which has been greatly improved. It has wide application range, wide measuring range, easy operation and low price. It is the most widely used thickness measuring instrument in industry and scientific research.
The non-destructive method neither destroys the coating nor the base material, and the detection speed is fast, allowing a large number of inspections to be carried out economically. one. Magnetic force measurement principle and thickness gauge
The magnitude of the suction force between the permanent magnet (probe) and the magnetically permeable steel is proportional to the distance between the two. This distance is the thickness of the coating. thickness. This principle is used to make a thickness gauge, which can be measured as long as the difference in magnetic permeability between the coating and the base material is large enough. Since most industrial products are stamped from 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, scale and self-stop mechanism. After the magnetic steel is attracted to the object to be measured, the measuring spring is gradually stretched and the pulling force is gradually increased. When the pulling force is just greater than the suction force, the thickness of the coating can be obtained by recording the pulling force at the moment the magnet is detached. Newer products can automate this recording process. Different models have different measuring ranges and applicable occasions.
The characteristics of this instrument are that it is easy to operate, sturdy and durable, does not require power supply, does not require calibration before measurement, and is relatively low-priced. It is very suitable for on-site quality control in workshops. When using the principle of magnetic induction, the thickness of the coating is measured by using the magnitude of the magnetic flux flowing from the probe through the non-ferromagnetic coating and into the ferromagnetic substrate. The corresponding magnetic resistance can also be measured to indicate the coating thickness. The thicker the coating, the greater the magnetic resistance and the smaller the magnetic flux. Thickness gauges that use the principle of magnetic induction can in principle have the thickness of non-magnetic coating on a magnetically permeable substrate. Generally, the magnetic permeability of the base material is required to be above 500. If the coating material is also magnetic, the difference in magnetic permeability from the base material is required to be large enough (such as nickel plating on steel). When the probe with the coil around the soft core is placed on the sample being tested, the instrument automatically outputs the test current or test signal. Early products used a pointer-type meter to measure the magnitude of the induced electromotive force. The instrument amplified the signal and then indicated the coating thickness. In recent years, new technologies such as frequency stabilization, phase locking, and temperature compensation have been introduced into circuit design, and magnetoresistance is used to modulate measurement signals. It also uses patented integrated circuits and introduces microcomputers, which greatly improves the measurement accuracy and reproducibility (almost by an order of magnitude). Modern magnetic induction thickness gauges have a resolution of 0.1um, an allowable error of 1, and a measuring range of 10mm.
The magnetic principle thickness gauge can be used to accurately measure the paint layer on the steel surface, porcelain and enamel protective layers, plastic and rubber coatings, various non-ferrous metal plating layers including nickel and chromium, and chemical industry Various anti-corrosion coatings for the petroleum industry. The high-frequency AC signal generates an electromagnetic field 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 reflected impedance. This feedback action represents the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Since this type of probe is specifically designed to measure the thickness of coatings on non-ferromagnetic metal substrates, it is often called a non-magnetic probe. Non-magnetic probes use 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 scaling relationship of the signal are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge also reaches a high level of resolution of 0.1um, allowable error of 1, and measuring range of 10mm.
Thickness gauges that use the eddy current principle can, in principle, measure non-conductive coatings on all conductive bodies, such as the surfaces of aerospace aircraft, vehicles, home appliances, aluminum alloy doors and windows, and other aluminum products. paint, plastic coatings and anodized films. The cladding material has a certain conductivity, which can also be measured through calibration, but the ratio of the conductivities between the two is required to be at least 3-5 times different (such as chromium plating on copper). Although the steel matrix is ??also a conductor, it is more appropriate to use magnetic principle measurement for this type of task
Features of the DRF series thickness gauge:
It has two measurement methods: continuous measurement method ( CONTINUE) and single measurement mode (SINGLE);
Has two working modes: direct mode (DIRECT) and group mode (APPL);
Five statistics : Average value (MEAN), maximum value (MAX), minimum value (MIN), number of tests (NO.), standard deviation (S.DEV)
Zero-point calibration and two-point calibration can be performed, and The basic calibration method can be used to correct the system error of the probe;
It has a storage function: it can store 300 measurement values;
It has a deletion function: it can delete a single value that appears in the measurement. Delete suspicious data, or delete all data in the storage area of ??the coating thickness gauge in order to conduct new measurements;
Limits can be set: measurement values ??outside the limits can automatically alarm;
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It has the function of communicating with PC: it can transmit measured values ??and statistical values ??to PC so that the coating thickness gauge can further process the data; it has the function of power undervoltage indication;
There is a beep sound prompt during the operation;
It has an error prompt function;
It has an automatic shutdown function.