1) Magnetic thickness measureming method: the suction force between the permanent magnet (probe) and the magnetically conductive steel is proportional to the distance between the two, which is the thickness of the coating.
The magnetic coating thickness gauge is composed of magnetic steel, relay spring, ruler, and automatic stop mechanism. After the magnet and the object to be measured are attracted, the measuring spring is gradually elongated, 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 when the magnetic steel is detached.
2) Eddy current thickness measuring method: the high-frequency AC signal generates an electromagnetic field in the coil of the thickness measuring gauge probe. When the probe is close to the conductor, an eddy current is formed in it.
The closer the probe is to the conductive substrate, the greater the eddy current and the reflection impedance are. Based on this principle, the feedback action of the thickness measuring gauge characterizes the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate.
Because these probes are designed to measure the thickness of coatings on non-ferromagnetic metal substrates, they are often referred to as non-magnetic probes. Non-magnetic probes use high-frequency materials as coil cores, such as platinum-nickel alloys or other new materials.
The magnetic thickness measuring method is suitable for the thickness measurement of non-magnetic conductive layers on magnetic conductive materials. Magnetic materials are generally: steel, iron, silver, and nickel. This method has high measurement accuracy.
The main difference between the eddy current thickness measuring method and the magnetic induction principle is different probes, different frequencies of the signal, and different magnitude and scale relationships of the signal. Like the digital thickness gauge meter using magnetic induction, the eddy current thickness measuing gauge also achieves a high resolution of 0.1um, an allowable error of 1%, and a range of 10mm.
The thickness measuring gauge using the eddy current principle can measure the non-conductive coating on all conductors in principle, such as paint, plastic coating, and anodic oxide film on the surface of aerospace aircraft, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products.
The cladding material has a certain conductivity, which can also be measured by calibration, but the ratio of the conductivity of the two is required to be at least 3-5 times different (such as chrome plating on copper). Although the steel substrate is also an electrical conductor, it is more suitable for such tasks to be measured by a magnetic thickness measuring gauge.
The eddy current thickness measuring gauge is suitable for measuring the thickness of non-conductive layers on conductive metals, but this method is less accurate than the magnetic thickness measuring method.
With the increasing advancement of technology, especially after the introduction of microcomputer technology in recent years, the thickness measuring gauge using magnetic method and eddy current method has taken a step forward in the direction of miniaturization, intelligence, multi-function, high precision, and practicality.
The resolution of the measurement has reached 0.1 micron, and the accuracy can reach 1%, which has been greatly improved. It has a wide range of applications, wide measuring range, easy operation, and low price, and is the most widely used thickness measuring gauge in industry and scientific research.