Magnetic induction thickness gauge

1. Magnetic attraction principle thickness gauge
   The thickness of the coating can be measured by using the suction force between the permanent magnet probe and the magnetic steel in a certain proportional relationship with the distance between the two. This distance is the thickness of the coating, so as long as the difference in magnetic permeability between the coating and the substrate is large enough, it can be measured. In view of the fact that most industrial products are formed by stamping of structural steel and hot-rolled and cold-rolled steel plates, solid phase extraction , magnetic thickness gauges are the most widely used. The basic structure of the measuring instrument is magnetic steel, tension spring, scale and self-stop mechanism. When the magnetic steel is attracted to the measured object, a spring gradually stretches thereafter, and the tension gradually increases. When the tension steel is greater than the suction force, the magnetic steel is separated at the moment of recording the tension to obtain the coating thickness. Generally speaking, different models have different ranges and applicable occasions. Within an angle of about 350º, the coating thickness of 0~100µm; 0~1000µm; 0~5mm can be indicated by scales, and the accuracy can reach more than 5%, which can meet the general requirements of industrial applications. This instrument is characterized by simple operation, ruggedness, no need for power supply and calibration before measurement, and low price, making it very suitable for on-site quality control in the workshop.

2. Magnetic induction principle thickness gauge
    The magnetic induction principle is to use the magnetic flux of the probe flowing into the iron substrate through the non-ferromagnetic coating to determine the coating thickness. The thicker the coating, the smaller the magnetic flux. Because it is an electronic instrument, it is easy to calibrate, can realize multiple functions, expand the range, and improve the accuracy. Because the test conditions can be reduced a lot, it has a wider application field than the magnetic attraction type.
    When the probe with the coil wound on the soft iron core is placed on the object to be measured, the servo labeling machine instrument automatically outputs the test current. The size of the magnetic flux affects the size of the induced electromotive force. The instrument amplifies the signal and then indicates the coating thickness. Early products used the meter to indicate, and the accuracy and repeatability were not good. Later, the digital display type was developed, and the circuit design was also improved. In recent years, the latest technologies such as microprocessor technology, electronic switches, and frequency stabilization have been introduced. A variety of patented products have been launched one after another, and the accuracy has been greatly improved, reaching 1%, and the resolution has reached 0.1µm. The probe of the magnetic induction thickness gauge mostly uses soft steel as the magnetic core, and the frequency of the coil current is not high to reduce the influence of the eddy current effect. The probe has a temperature compensation function. Since the instrument is intelligent, it can identify different probes, cooperate with different software and automatically change the probe current and frequency. One instrument can cooperate with multiple probes, or the same instrument can be used. It can be said that the instruments suitable for industrial production and scientific research have reached a very practical stage. The
   thickness gauge developed using the electromagnetic principle is in principle suitable for all non-magnetic coating measurements, and generally requires a basic magnetic permeability of more than 500. If the coating material is also magnetic, it is required to have a sufficiently large difference in magnetic permeability with the substrate (such as nickel plating on steel). The magnetic principle thickness gauge can be used to accurately measure the paint coating on the surface of steel, porcelain and enamel protective layers, plastics, rubber coatings of thin layer chromatography plates, various non-ferrous metal electroplating layers including nickel and chromium, and various anti-corrosion coatings in the chemical and petroleum industries. For the film production industries such as photosensitive film, capacitor paper, plastic, polyester, etc., the use of measuring platforms or rollers (steel manufacturing) can also be used to achieve measurement of any point on a large area.

3. Eddy current thickness gauge
   Eddy current thickness measurement is mainly used to measure various non-metallic coatings on metal substrates. High-frequency alternating current is used to generate an electromagnetic field in the coil as a probe. When the probe is close to a conductive metal body, an eddy current is formed in the metal material, and it increases as the distance from the metal body decreases. The eddy current will affect the magnetic flux of the probe coil, so the feedback action is a value indicating the distance between the probe and the base metal. Because the probe is used to measure the coating thickness on a non-ferromagnetic metal substrate, we usually call the probe a non-magnetic probe. Non-magnetic probes generally use high-frequency and high-magnetic permeability materials as coil cores, and are often made of platinum-nickel alloys and other new materials. Compared with the magnetic measurement principle, their electrical principles are basically the same. The main difference is that the probes are different, the frequency of the flow and level instrument test current is different, and the signal size and scale relationship are different. In the latest thickness gauge, by continuously improving the probe structure, in conjunction with microcomputer technology, different control programs are called by automatically identifying different probes, and different test currents are output and scale conversion software is changed. Finally, two different types of probes are connected to the same thickness gauge, reducing the burden on users. Based on the same idea, the thickness gauge that can be equipped with up to 10 side heads has greatly expanded the thickness measurement range (up to more than 100,000 times). The needle filter can measure non-magnetic coatings on the surface of magnetic materials, non-conductive coatings on conductive materials, and conductive layers on non-conductive materials, basically meeting the needs of most industries in industrial production.
   Thickness gauges using the eddy current principle can, in principle, measure non-conductive coatings on all conductors, such as paint, plastic coatings and anodized films on the surfaces of aerospace and aircraft, vehicles, home appliances, aluminum alloy doors and windows, and other aluminum products. Some special uses such as diamond coatings on certain metals and other sprayed non-conductive layers. The coating material can also have a certain conductivity, and can also be measured through calibration, but the conductivity ratio of the two is required to be at least 3 to 5 times different (such as chrome plating on copper).
   The principle of calibration is that the calibration sample without coating and the substrate of the object to be measured should have: the same composition, the same thickness (mainly when the thickness is less than the minimum value of about 0.5mm specified by the instrument), the same radius of curvature, and if the measured area is less than the requirements of the instrument technical parameters (diameter is about 20mm or less), it should also have the same measured area. If the coating contains conductive components, the coating of the calibration sample should also have the same conductive properties as the coating of the object to be measured. After the coating of the calibration sample is calibrated by other tests (including destructive testing methods) or coated with a calibrated calibration sheet, the thickness gauge can be calibrated on it according to the method in the manual. After calibration, rapid non-destructive testing can be performed on the product to be measured. The calibration sheet is generally made of triacetate film or hard paper impregnated with phenol resin.
   Microcomputer thickness measurement generally has multiple calibration values ​​stored. With the different positions of the tested products, material changes, replacement of probes, etc., they can be calibrated and stored separately. In actual use, the calibration values ​​can be directly called without re-adjustment. This is the so-called "quick change benchmark". It greatly improves the detection efficiency.
   Test data can generally be stored, printed, and calculated for analysis in intelligent instruments. There is also a function to print histograms to make the coating thickness distribution clear at a glance. If the upper and lower limits are set, the statistical data can be more accurate. During measurement, all points that exceed the limit will have a sound reminder and will not be taken into statistical calculations.
Factors affecting the measured value and solutions
   When using a thickness gauge, just like using other instruments, the coating thickness gauge must master the instrument performance and understand the test conditions. Coating thickness gauges using magnetic principles and eddy current principles measure the coating thickness based on the electrical and magnetic properties of the substrate being measured and the distance from the probe. Therefore, the electromagnetic physical properties and physical dimensions of the substrate being measured will affect the size of the magnetic flux and eddy current. That is, it affects the reliability of the measured value. The following is an introduction to this issue.
1. Boundary spacing
If the distance between the probe and the measured body boundary, hole, cavity, or other cross-sectional changes is less than the specified boundary spacing, measurement errors will occur due to insufficient cross-sectional area of ​​the magnetic flux or eddy current carrier. If the coating thickness at this point must be measured, it can only be measured after calibration on a non-coated surface under the same conditions. (Note: The latest products have a unique function of through-coating calibration that can achieve an accuracy of 3-10%)
2. Substrate surface curvature
Calibrate an initial value on a flat comparison sample, and then subtract this initial value after measuring the coating thickness. Or refer to the next item.
3. Minimum thickness of base metal
The base metal must have a given minimum thickness. The coating thickness gauge allows the probe's electromagnetic field to be completely contained in the base metal. The minimum thickness is related to the performance of the measuring instrument and the properties of the metal base. Above this thickness, measurements can be made without correcting the measured value. The influence caused by insufficient base thickness can be eliminated by placing a piece of the same material close to the base material. If it is difficult to decide, or it is impossible to add a substrate, the difference from the rated value can be determined by comparing it with a sample with a known coating thickness. And take this into account in the measurement and make corresponding corrections to the measured value or refer to the correction in Article 2. And those instruments that can be calibrated can get accurate direct reading thickness values ​​by adjusting the knobs or buttons. Vertical labeling machines can, on the contrary, use the influence of too small thickness to develop a thickness gauge that directly measures the thickness of copper foil, as mentioned above.
4. Surface roughness and surface cleanliness
In order to obtain a representative average measurement value on a rough surface, multiple measurements must be made. Obviously, whether it is a substrate or a coating, the rougher it is, the less reliable the measurement value. In order to obtain reliable data, the average roughness Ra of the substrate should be less than 5% of the coating thickness. And for surface impurities, they should be removed. Some instruments have upper and lower limits to eliminate those "flying points".
5. The force of the probe measuring plate
The force of the probe during measurement should be constant. And it should be as small as possible. It will not cause the soft coating to deform, resulting in a decrease in the measurement value. If the detector is not able to produce large fluctuations, a hard, non-conductive, and hard film of a certain thickness can be placed between the two if necessary. In this way, the residual magnetism can be appropriately obtained by subtracting the thickness of the film.
6. External constant magnetic field, electromagnetic field and substrate residual magnetism
Measurements should be avoided near external magnetic fields with interfering effects. The remaining residual magnetism may cause more or less measurement errors depending on the performance of the detector, but structural steel, deep-drawn steel plates, etc. generally do not have the above phenomenon.
7. Ferromagnetic and conductive components in coating materials
When there are certain ferromagnetic components in the coating, such as certain pigments, it will affect the measured value. In this case, the coating of the comparison sample used for calibration should have the same electromagnetic properties as the coating of the object being measured and be used after calibration. The method used can be to apply the same coating on aluminum or copper plate samples, and obtain a comparison standard sample after testing with the eddy current method. It can also be purchased from the relevant metrology and testing department.