Magnetic force measurement principle and thickness gauge
The attraction force between the magnet (probe) and the magnetically permeable steel is proportional to the distance between the two. This distance is the thickness of the coating. 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.
Magnetic induction measurement
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 conductive coatings on magnetically permeable substrates. 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, circuit design has introduced new technologies such as frequency stabilization, phase locking, and temperature compensation, and magnetoresistance is used to modulate measurement signals. It also uses designed 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 measure the paint layer on the surface of steel, the protective layer of porcelain and enamel, the plastic and rubber coating, various non-ferrous metal plating layers including nickel and chromium, and various anti-corrosion coatings in the chemical and petroleum industries. .
Eddy current measurement
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 paint, plastic coatings on the surface of aerospace aircraft, vehicles, home appliances, aluminum alloy doors and windows, and other aluminum products. Anodized film. 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.
