Detection principle and classification of coating thickness gauge
Magnetic measurement
It is suitable for measuring the thickness of the non-magnetic layer on the magnetic material. Magnetic materials are generally: steel, iron, silver, nickel. This method has high measurement accuracy.
Eddy current measurement
Suitable for thickness measurement of non-conductive layers on conductive metals. This method is less accurate than the magnetic thickness measurement method.
ultrasonic measurement
It is suitable for accurate measurement of various plates and various processed parts, and can also monitor the degree of thinning of various pipes and pressure vessels in production equipment after being corroded during use.
Magnetic measurement principle thickness gauges can be divided into two types: magnetic suction principle thickness gauges and magnetic induction principle thickness gauges, and eddy current measurement principle thickness gauges only have one type of eddy current thickness gauges.
Magnetic suction principle The thickness gauge measures the thickness of the coating by using the suction force between the permanent magnet measuring head and the magnetically conductive steel in a certain proportion to the distance between the two. This distance is the thickness of the coating. , so as long as the difference between the magnetic permeability of the coating and the substrate is large enough, it can be measured.
The magnetic induction principle thickness gauge uses the magnetic flux flowing into the iron base material through the non-ferromagnetic coating to measure the thickness of the coating. The thicker the coating, the smaller the magnetic flux. When the probe with the coil around the soft iron core is placed on the object to be tested, the instrument will automatically output the test current, the magnitude of the magnetic flux will affect the magnitude of the induced electromotive force, and the instrument will amplify the signal to indicate the coating thickness.
The eddy current thickness gauge uses high-frequency alternating current to generate an electromagnetic field in the coil of the measuring head. When the probe is brought close to the conductive metal body, an eddy current is formed in the metal material. The eddy current decreases with the distance from the metal body. Increase, and will affect the magnetic flux of the probe coil, this feedback action is a value that represents the distance between the probe and the base metal.
The eddy current probe is used to measure the thickness of the coating on the non-ferromagnetic metal substrate, so we usually call the probe a non-magnetic probe. Compared with the magnetic measurement principle, their electrical principle is basically the same, the main difference is that the probe is different, the frequency of the test current is different, and the signal size and scale relationship are different. In the past two years of thickness gauges, through continuous improvement of the probe structure, combined with microcomputer technology, different control programs are called by automatic identification of different probes, different test currents are output respectively, and scale conversion software is changed. Two different types of measuring heads are connected to the same thickness gauge. Based on the same idea, a thickness gauge that can be connected with up to 10 types of measuring heads has also emerged as the times require.
The ultrasonic thickness gauge measures the thickness according to the principle of ultrasonic pulse reflection. When the ultrasonic pulse emitted by the probe passes through the object to be measured and reaches the interface of the material, the pulse is reflected back to the probe, which is determined by accurately measuring the propagation time of the ultrasonic wave in the material. The thickness of the material to be measured.
Although there are many differences in the selection of measuring points and standard materials in the calibration of several thickness gauges, there are some points that need to be paid attention to in operation, such as the surface curvature and minimum thickness of the substrate for each thickness gauge. There is a lower limit. In the actual calibration, a substrate with a reasonable size should be selected for operation; the orientation and pressure of the probe during measurement will also affect the results. Keep the probe perpendicular to the substrate, and the pressure should be constant and as small as possible; In addition, when calibrating the coating thickness gauge, attention should be paid to the interference of the external magnetic field and the residual magnetism of the matrix. When calibrating the ultrasonic thickness gauge, attention should be paid to the influence of temperature changes and the viscosity of the coupling agent.
