Several Measuring Principles of Coating Thickness Gauge
1. Magnetic force measurement principle and coating thickness gauge
The suction force between the probe and the magnetic steel is proportional to the distance between the two. This distance is the thickness of the coating. According to this principle, a coating thickness gauge is made, as long as the magnetic permeability between the coating and the substrate is large enough, the coating can be measured. In view of the fact that most industrial products are stamped and formed by structural steel and hot-rolled cold-rolled steel plates, magnetic thickness gauges are the most widely used. The basic structure of the coating thickness gauge is composed of magnetic steel, relay spring, scale and automatic stop mechanism. After the magnetic steel is attracted to the measured object, the measuring spring is gradually elongated thereafter, and the pulling force is gradually increased. When the pulling force is just greater than the attractive force, the coating thickness can be obtained by recording the pulling force at the moment when the magnetic steel is detached. This instrument is characterized by easy operation, durability, no power supply, no calibration before measurement, relatively low price, and is very suitable for on-site quality control in workshops.
2. The principle of magnetic induction method to measure coating
When the coating is measured by the principle of magnetic induction, the thickness of the coating is measured by the magnitude of the magnetic flux that flows into the ferromagnetic substrate through the non-ferromagnetic coating. The corresponding magnetoresistance can also be measured to indicate the coating thickness. The thicker the coating, the greater the reluctance and the smaller the flux. The coating thickness gauge using magnetic induction can in principle have the thickness of the non-magnetic coating on the magnetic substrate. Generally, the magnetic permeability of the substrate is required to be above 500. If the coating material is also magnetic, the difference between the magnetic permeability of the substrate and the substrate is required to be sufficiently large (such as nickel plating on steel). When the probe with the coil wound on the soft core is placed on the sample to be tested, the coating thickness gauge will automatically output the test current or test signal. The early products used a pointer gauge to measure the magnitude of the induced electromotive force, and the coating thickness gauge amplified the signal to indicate the coating thickness. In recent years, the use of professionally designed integrated circuits, the introduction of single-chip microcomputers, and the addition of advanced tools have greatly improved the measurement accuracy and reproducibility. The magnetic principle coating thickness gauge can be used to measure the paint layer on the steel surface, porcelain, enamel protective layer, plastic, rubber coating, and the thickness of various anti-corrosion coatings in the chemical petroleum industry.
3. Principle of 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 reflection impedance. This feedback action characterizes the size of the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Because this type of coating thickness gauge probe is 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 coil cores. Compared with the principle of magnetic induction, the main difference is that the probe of the coating thickness gauge is different, the signal frequency is different, and the signal size and scale relationship are different. The coating thickness gauge using the principle of eddy current can measure non-conductive coatings on all conductive substrates in principle, such as paint and plastic coatings on the surface of aerospace vehicles, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products. and anodized film. The coating material has a certain conductivity, which can also be measured through calibration, but the ratio of the two conductivity is required to be at least 3-5 times different. Although steel substrates are also conductors, the magnetic principle of measuring coating thickness is more suitable for this type of task.
