In addition to measuring coatings on magnetic metal substrates and non-magnetic substrates, coatings can also measure metal-plated coating thickness gauges. Therefore, coating thickness gauges are often referred to as coating thickness gauges.
Coating Thickness Gauge
Coating thickness gauges generally have the following five types according to the measurement principle:
1. Magnetic thickness measurement method: It is applicable to the measurement of the thickness of the non-magnetic permeability layer on the magnetic conductive material. The magnetic conductive material is generally: steel\iron\silver\nickel. This method has high measurement accuracy.
2. Eddy current thickness measurement method: It is applicable to the measurement of the thickness of non-conductive layer on conductive metal. This method has lower precision than magnetic thickness measurement method.
3. Ultrasonic thickness measurement method: At present, this method has not been used to measure the thickness of the coating layer. Some foreign manufacturers have such an instrument, and the measurement of the thickness of the multi-layer coating layer is not the case where the above two methods cannot be measured. But the general price is expensive and the measurement accuracy is not high.
4. Electrolytic thickness measurement method: This method is different from the above three types. It does not belong to non-destructive testing and needs to destroy the coating layer. The general precision is not high. It is more troublesome to measure.
5. Radiation thickness measurement method: This kind of instrument is very expensive (generally above 100,000 RMB), suitable for some special occasions.
The most common method currently used in China is the 1st and 2nd methods.
Principle of a conventional coating thickness gauge
Coatings for surface protection and decoration of materials, such as coatings, coatings, coatings, overlays, chemically formed films, etc., are referred to as coatings in relevant national and international standards.
Coating thickness measurement has become an important part of the processing industry and surface engineering quality testing, and is an indispensable means for products to achieve superior quality standards. In order to internationalize the products, China's export commodities and foreign-related projects have clear requirements for the thickness of the coating.
The coating thickness measurement methods are: wedge cutting method, light intercept method, electrolysis method, thickness difference measurement method, weighing method, X-ray fluorescence method, β-ray backscattering method, capacitance method, magnetic measurement method and eddy current measurement. Law and so on. The first five of these methods are lossy detection, and the measurement methods are cumbersome and slow, and are more suitable for sampling inspection.
The X-ray and beta-ray methods are non-contact non-destructive measurements, but the device is complicated and expensive, and the measurement range is small. Users must comply with radiation protection regulations due to radioactive sources. The X-ray method can measure extremely thin plating, double plating, and alloy plating. The beta ray method is suitable for coatings with coatings and substrates with an atomic number greater than 3. The capacitance method is only used when thickness measurement is applied to the insulating coating of a thin conductor.
With the advancement of technology, especially after the introduction of microcomputer technology in recent years, the thickness gauges using magnetic method and eddy current method have taken a step toward micro, intelligent, multifunctional, high-precision and practical. The resolution of the measurement has reached 0.1 micron, and the precision can reach 1%, which has been greatly improved. It has a wide range of applications, wide measuring range, easy operation and low cost. It is the most widely used thickness measuring instrument for industrial and scientific research.
Measuring principle
1. Magnetic suction measurement principle and thickness gauge
The amount of suction between the permanent magnet (probe) and the magnetically permeable steel is proportional to the distance between the two, which is the thickness of the coating. Using this principle to make a thickness gauge, measurement can be performed as long as the difference in magnetic permeability between the cladding and the substrate is sufficiently large. In view of the fact that most industrial products are formed by structural steel and hot-rolled cold-rolled steel sheets, 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 tested, 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 layer can be obtained by recording the pull-down force at the moment of the magnetic steel detachment. The new product automates this recording process. Different models have different ranges and applications.
This instrument is characterized by simple operation, ruggedness, no power supply, no calibration before measurement, and low price. It is suitable for on-site quality control in the workshop.
Eddy current measurement principle
The high frequency AC signal generates an electromagnetic field in the probe coil, and a vortex is formed therein when the probe is close to the conductor. The closer the probe is to the conductive substrate, the larger the eddy current and the greater the reflection impedance. This feedback action characterizes the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Since such probes are specifically designed to measure the thickness of a coating on a non-ferromagnetic metal substrate, they are often referred to as non-magnetic probes. Non-magnetic probes use high-frequency materials for coil cores, such as platinum-nickel alloys or other new materials. Compared with the principle of magnetic induction, the main difference is that the probes are different, the frequency of the signals is different, and the signal size and scale relationship are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge achieves a resolution of 0.1 um, a tolerance of 1%, and a high level of 10 mm.
Thickness gauges using the principle of eddy currents can in principle be measured on non-conductor coatings on all conductors, such as aerospace aircraft surfaces, vehicles, appliances, aluminum alloy doors and windows and other aluminum surface paints, plastic coatings and Anodized film. The cladding material has a certain conductivity and can also be measured by calibration, but it is required that the ratio of the conductivity of the two is at least 3-5 times (such as chrome plating on copper). Although the steel substrate is also an electrical conductor, such tasks are more suitable for measurement using magnetic principles.
DRF series thickness gauge features:
There are two measurement methods: continuous measurement (CONTINUE) and single measurement (SINGLE);
There are two ways of working: direct mode (DIRECT) and group mode (APPL);
There are five statistics: mean (MEAN), maximum (MAX), minimum (MIN), number of tests (NO.), standard deviation (S.DEV)
Zero calibration and two-point calibration are available, and the system error of the probe can be corrected by the basic calibration method;
With storage function: can store 300 measured values;
With delete function: delete single suspicious data appearing in the measurement, or delete all data in the storage area of ​​the coating thickness gauge for new measurement;
The limit can be set: the measured value outside the limit can be automatically alarmed;
It has the function of communicating with the PC: the measured value and the statistical value can be transmitted to the PC, so that the coating thickness gauge can further process the data; and the power supply undervoltage indication function;
There is a beeping prompt during the operation;
With error prompt function;
With automatic shutdown function.
Factors affecting the accuracy of the measured value of the coating thickness gauge
Relevant explanation of the influencing factors
a base metal magnetic
The magnetic thickness measurement is affected by the magnetic change of the base metal (in practical applications, the change of the magnetic properties of the low carbon steel can be considered as slight). In order to avoid the influence of heat treatment and cold working factors, the same properties as the test piece base metal should be used. The standard piece calibrates the instrument; it can also be calibrated with the test piece to be coated.
b matrix metal electrical properties
The conductivity of the base metal has an effect on the measurement, and the electrical conductivity of the base metal is related to its material composition and heat treatment method. The instrument is calibrated using a standard sheet having the same properties as the specimen base metal.
c base metal thickness
Each instrument has a critical thickness of the base metal. Above this thickness, the measurement is not affected by the thickness of the base metal. The critical thickness values ​​of this instrument are shown in Table 1.
d edge effect
This instrument is sensitive to the steepness of the surface shape of the test piece. Therefore, it is unreliable to measure near the edge of the test piece or at the inner corner.
e curvature
The curvature of the specimen has an effect on the measurement. This effect always increases significantly as the radius of curvature decreases. Therefore, measurement on the surface of the curved test piece is unreliable.
f Deformation of the test piece
The probes deform the soft cover specimens, so reliable data is measured on these specimens.
g surface roughness
The surface roughness of the base metal and the cover layer has an effect on the measurement. The roughness is increased and the influence is increased. Rough surfaces can cause systematic and accidental errors, and the number of measurements should be increased at different locations for each measurement to overcome this accidental error. If the base metal is rough, it is necessary to check the zero point of the instrument on the uncoated roughness of the base metal test piece with a similar position; or dissolve the cover layer with a solution that does not corrode the base metal, and then proofread the instrument. Zero point.
g magnetic field
The strong magnetic field generated by various electrical equipment around it will seriously interfere with the magnetic method of thickness measurement.
h attached substance
The instrument is sensitive to adhering substances that prevent the probe from coming into close contact with the surface of the cover. Therefore, the attached material must be removed to ensure direct contact between the instrument probe and the surface of the test piece.
i Probe pressure
The amount of pressure applied by the probe on the test piece affects the measured reading, so keep the pressure constant.
j orientation of the probe
The way the probe is placed has an effect on the measurement. In the measurement, the probe should be kept perpendicular to the surface of the specimen.
Rules to be observed when using the instrument
a base metal properties
For the magnetic method, the magnetic properties and surface roughness of the base metal of the standard sheet should be similar to the magnetic properties and surface roughness of the base metal of the test piece. For the eddy current method, the electrical properties of the standard sheet base metal should be similar to the electrical properties of the test piece base metal.
b base metal thickness
Check if the thickness of the base metal exceeds the critical thickness. If not, use one of the methods in 3.3 to calibrate.
c edge effect
Measurements should not be made immediately adjacent to the specimen's sudden changes, such as edges, holes, and inner corners.
d curvature
It should not be measured on the curved surface of the test piece.
e number of readings
Usually because each reading of the instrument is not exactly the same, several readings must be taken within each measurement area. The local difference in the thickness of the cover layer also requires multiple measurements in any given area, especially when the surface is rough.
Surface cleanliness
Before the measurement, remove any adhering substances on the surface, such as dust, grease and corrosion products, but do not remove any covering materials.
The difference between F, N and FN in the coating thickness gauge:
F stands for ferrous ferromagnetic matrix. F-type coating thickness gauge uses electromagnetic induction principle to measure non-ferromagnetic coatings and coatings on ferromagnetic metal substrates such as steel, iron, such as paint, powder, plastic and rubber. , synthetic materials, phosphating layers, chromium, zinc, lead, aluminum, tin, cadmium, porcelain, tantalum, oxide layers, etc.
N stands for Non-ferrous non-ferromagnetic substrate, N-type coating thickness gauge adopts eddy current principle; to measure enthalpy, rubber, paint, plastic layer, etc. on copper, aluminum, zinc, tin and other substrates by eddy current sensor.
Application field
Application: Magnetic sensors are used to measure non-ferromagnetic coatings and coatings on ferromagnetic metal substrates such as steel, iron, etc., such as: paint, powder, plastic, rubber, synthetic materials, phosphating layer, chromium, zinc, lead, aluminum, Tin, cadmium, porcelain, bismuth, oxide layer, etc. The eddy current sensor is used to measure tantalum, rubber, paint, plastic layers, etc. on substrates such as copper, aluminum, zinc, and tin. Widely used in manufacturing, metal processing, chemical industry, commodity inspection and other testing areas. When measuring the object, in addition to the measurement method, there are other factors that will cause the measurement results to deviate. For the specific influence factor, please refer to the following table. Measurement method Magnetic measurement eddy current measurement Matrix metal magnetic properties
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