Thickness gauge calibration
The coating of the calibration specimen can be calibrated to the thickness gauge by calibration of other (including lossy test methods) or by coating with a calibrated calibration sheet. After calibration, you can perform fast nondestructive testing on the product under test. The calibration sheet is typically a triacetate film or a hard paper impregnated with a phenol resin. Microcomputer thickness measurement generally has multiple calibration values ​​stored. The different positions of the product to be tested, material changes, replacement of the probe, etc. can be separately calibrated and stored. In the actual use, the calibration values ​​are called directly, and there is no need to recalibrate. This is the so-called "speed change benchmark." Greatly improved detection efficiency. Test data can be stored, printed, calculated, and analyzed in intelligent instruments, as well as the ability to print histograms to make the thickness of the overlay visible at a glance.
If the upper and lower limits are set, the statistical data can be made more accurate. When measuring, all the points that exceed the limit have an audible reminder and are not taken into account for statistical calculation. Factors Affecting Measurements and Solutions Using a gage is the same as using other instruments, both in terms of instrument performance and test conditions. The coating thickness gauge using the magnetic principle and the eddy current principle is based on the electrical and magnetic properties of the measured substrate and the distance from the probe to measure the thickness of the coating. Therefore, the electromagnetic physical properties and physical dimensions of the tested substrate are affected. The size of the flux and eddy currents. That is to affect the reliability of the measured values, the following is a description of this issue.
1. Boundary spacing If the distance between the probe and the boundary of the measured object, the hole, the cavity, and other cross-section changes is less than the specified boundary spacing, measurement errors may result due to insufficient cross-section of the magnetic flux or eddy current carrier. If it is necessary to measure the thickness of the coating at this point, it can only be measured if it is calibrated in advance on the uncoated surface of the same condition. (Note: The latest products have a unique function through cladding calibration up to 3 to 10% accuracy)
2. The surface curvature of the substrate is calibrated to an initial value on a flat contrast sample and then subtracted from this initial value after measuring the thickness of the coating. Or refer to the next article.
3. The minimum thickness of the base metal The base metal must have a given minimum thickness so that the electromagnetic field of the probe can be completely contained in the base metal. The minimum thickness is related to the performance of the measuring device and the nature of the metal matrix. Make measurements without correcting the measured values. The effect of insufficient substrate thickness can be eliminated by placing a piece of the same material under the substrate. If it is difficult to decide, or if the substrate cannot be added, the difference from the nominal value can be determined by comparison with a sample of known cladding thickness. And take this into account in the measurement and make corresponding corrections to the measured values ​​or refer to the second amendment. And those instruments that can be calibrated can get accurate direct reading thickness values ​​by adjusting knobs or buttons. On the contrary, by using the influence of the thickness too small, it is possible to develop a thickness gauge for directly measuring the thickness of the copper foil, as described above.
4. Surface Roughness and Surface Cleanliness A number of measurements must be taken on the roughness surface to obtain a representative average measurement. Obviously, the rougher the substrate or the cladding, the less reliable the measurements. In order to obtain reliable data, the average roughness Ra of the substrate should be less than 5% of the thickness of the coating. For surface impurities, it 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 measured by the probe should be constant. And should be as small as possible. It does not cause the soft coating to deform, so that the measured value decreases. There is a large fluctuation in the activity. If necessary, a hard, non-conductive, hard film with a certain thickness can be placed between the two. Thus, the residual magnetism can be appropriately obtained by subtracting the film thickness.
6. The external constant magnetic field, electromagnetic field and matrix remanence should be avoided in the vicinity of the external magnetic field with interference. The residual remanence may cause more or less measurement errors depending on the performance of the detector, but such as structural steel, deep-drawn steel sheets, etc., generally do not occur.
7. There are some ferromagnetic components in the ferromagnetic component and the conductive component coating in the coating material, such as a certain pigment, which will affect the measured value. In this case, the comparative sample used for calibration The coating shall have the same electromagnetic properties as the coating of the test object and shall be used after calibration. The method used may be to apply the same coating to an aluminum or copper plate sample, and to obtain a comparative standard sample by an eddy current test. It can also be purchased from the relevant metrology testing department.
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