Principle of magnetic suction measurement
The amount of suction between the magnet (probe) and the magnetic steel is proportional to the distance between the two, which is the thickness of the coating.Using this principle to make a thickness gauge, as long as the difference between the magnetic conductivity of the coating and the substrate is large enough, it can be measured.Since most industrial products are made of structural steel and hot and cold rolled steel sheets, magnetic thickness gauges are widely used.The basic structure of the thickness gauge is composed of magnetic steel, relay spring, ruler and self-stop mechanism.Magnetic steel and the object to be measured after the suction, the measuring spring in the rear gradually elongated, the tension gradually increased.When the tensile force is just greater than the suction force, the thickness of the coating can be obtained by recording the tensile force at the moment when the magnetic steel is detached.Newer products can automate this recording process.Different models have different ranges and applicable occasions.
This instrument is characterized by simple operation, strong and durable, no power supply, no calibration before measurement, and low price, which is very suitable for on-site quality control in workshops.
Principle of magnetic induction measurement
Using the principle of magnetic induction, the size of magnetic flux flowing into the ferromagnetic matrix from the probe through the non-ferromagnetic coating can be used to measure the coating thickness.The corresponding magnetoresistance can also be measured to indicate the thickness of the coating.The thicker the coating, the magnetoresistance
The larger the flux, the smaller the flux.The thickness gauge using the principle of magnetic induction can in principle have the thickness of the non-conductive coating on the magnetically conductive substrate.Generally, the magnetic conductivity of the substrate is more than 500.If the coating material is also magnetic, the difference between the magnetic permeability and the substrate is required to be large enough (e.g., nickel plating on steel).When the probe with the coil on the soft core is placed on the sample to be measured, the instrument automatically outputs the test current or test signal.Early products used pointer heads to measure the size of the induced electromotive force, which the instrument amplified to indicate the thickness of the cladding.In recent years, new techniques such as frequency stabilization, phase lock and temperature compensation have been introduced into circuit design, and the measurement signal is modulated by magnetoresistance.In addition, the design of integrated circuit and the introduction of microcomputer have greatly improved the measurement accuracy and reproducibility (almost an order of magnitude).Modern magnetic induction thickness gauge, resolution up to 0.1um, allowable error up to 1%, range up to 10mm.
Magnetic principle thickness gauge can be applied to accurately measure the paint layer on the surface of steel, porcelain, enamel protective layer, plastic, rubber coating, including nickel chromium and other non-ferrous metal electroplating, as well as a variety of chemical and petroleum industry anti-corrosion coating.
Principle of eddy current measurement
The high frequency AC signal generates an electromagnetic field in the probe coil, which, when the probe is close to the conductor, forms an eddy current in it.The closer the probe is to the conductive substrate, the greater the eddy current and the greater the reflection impedance.
This feedback measure measures the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate.This type of probe is often referred to as a non-magnetic probe because it is designed to measure the thickness of the coating on a non-ferromagnetic metal substrate.The non-magnetic probe uses a high frequency material 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, the size of the signal, the scale relationship is different.Like the magnetic induction thickness gauge, the eddy current thickness gauge achieves a resolution of 0.1um, a tolerance of 1%, and a measuring range of 10mm.
The thickness gauge based on eddy current principle can in principle measure the non-conductive coating on all conductive materials, such as paint on the surface of aerospace aircraft, vehicles, home appliances, aluminum doors, Windows and other aluminum products, plastic coating and anodized film.The cladding material has a certain electrical 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 different (such as chromium plating on copper).Although the steel matrix is also a conductive material, it is more appropriate to use magnetic principle to measure this kind of tasks