Pros and Cons of Eddy Current Thickness Gauge and Magnetic Gauge:
The foremost benefit of using an eddy current thickness gauge is the rapid, precise non-contact measurement. It can measure the thickness of a single layer, coatings, and multi-layer composite material. Despite the physical influence, the results are stable and reliable. The negative aspect includes damage to the sample, a large measurement area, and invalidity for certain objects.
The magnetic thickness gauge is lightweight, user-friendly, highly sensitive, and has a wide measurement range. The cons include the influence of interference factors on measurement. Furthermore, the application range is below average.
Factors Affecting the Measurement Accuracy of Eddy Current Thickness Gauge
The details that can greatly affect the measurement accuracy of Eddy Current Thickness Gauge include:
- Cover thickness
Measurement uncertainty is characteristic of eddy current thickness measurement methods. Overlays less than 25μm, classified as thin, have a constant measurement uncertainty value, and no dependency is seen on overlay thickness which is approximately 0.5μm. Instruments have 0.5μm-1μm uncertainty, where the uncertainty value for this particular instrument is 2% of the overlay thickness.
The thickness value for overlays with less than or equal to 5 m shall be the average of many measurements. If the thickness of the cover layer is smaller than 3 μm, precision is questionable in measuring film thickness value.
- The conductivity of base metal
The conductivity of a material can geat;y affect the eddy current thickness measurements. Since conductivity is heavily dependent on the material composition ad treatment, the degree of influence of conductivity on the measurement fluctuates accordingly based on the model of the instrument. The conductivity of base material has little to no effect on the measurements of this instrument.
- Base metal thickness
Each instrument possesses a particular critical thickness value for the base metal. Above this value, an increase in base thickness will not hinder the measurement. The critical thickness value shows dependency on the operating frequency and conductivity of the base metal. The value of critical thickness is 0.3–0.4mm for this instrument. Superimposing an uncoated specimen of the same material whose thickness lies below the range can be risky.
- Edge effect
The eddy current thickness gauge does not like discontinuities on the sample surface. The measurements are regarded as untrustworthy if they are taken very close to the edge. When measuring a small or narrow area, it’s critical to calibrate the instrument with an uncoated substance that’s the same shape as the substrate. The instrument must be calibrated if the measuring area is less than 150mm2 or the sample width is less than 12mm.
Curvature changes can influence the measured value. An indirect relationship exists between curvature and measured value; if the curvature is small, the measured value will be greatly influenced. When measuring specimens with less than 50 mm diameter, the instrument should be recalibrated on uncoated material of the same diameter.
- Rough surface
Surface roughness and cover layers significantly affect the measured value. The effect can be willingly reduced by taking an average of various measurements are multiple locations. In the case of a rough base metal surface, instrument zero must be calibrated at various points on the corresponding metal material before coating.
- Intimate contact between the probe and the sample surface
The thickness gauge probe should be in close proximity to the sample surface. The measured value can be influenced by the dust and dirt present on the sample surface. Therefore, it’s sensational to keep the probe and sample surface neat and ready for measurement.
If two or more calibration foils, whose thickness values are known, superimposed, and measured, the resultant value will be multiple folds treated than the total of calibration foil thickness values. The deviation depends on the thickness and stiffness of the foil as the superimposition influences the close contact between the probe and the foil.
- Probe pressure
The pressure applied can also affect the measured value. The constant pressure spring provided in the probe balances the applied pressure and guarantees stable pressure during each measurement.
- The verticality of the probe
It’s vital to calibrate the instrument at a temperature identical to the operating environment as temperate changes can alter the probe parameters. The instrument exhibits good temperature compensation and a very subtle effect on the measured value due to temperature change.
Magnetic Thickness Gauge vs. Eddy Current Thickness Gauge: What’s the Difference?
The basic principle of thickness measurement of magnetic thickness gauge is identical to eddy current thickness gauge. Both are used to determine the thickness of coatings on metal substrates. However, certain elements separate the two instruments from each other.
The magnetic thickness gauge utilizes the magnetic method for evaluating the thickness of the coating. The principle is based on:
- Magnetic Attraction Coating Thickness Gauge
- Magnetic Induction Coating Thickness Gauge.
In the magnetic attraction coating thickness gauge, the thickness can be evaluated with the help of magnetic force between the probe’s permanent magnet and the metal substrate magnet. Once both the magnets (magnetic metal base material and magnet steal) develop strong magnetic attraction, elongation of the relay spring takes place. When the pulling force exceeds the suction force, detachment of magnetic steel from the magnetic metal base material will be carried out.
The magnetic induction coating thickness is based on magnetic induction. It calculates the coating thickness by evaluating the magnetic flux of the probe’s permanent magnetic, which flows into the magnetic metal substrate with the help of a non-ferromagnetic coating. A direct relationship is visualized between the coating and magnetic flux; thicker coating equates to high magnetic flux.
The magnetic induction method is more extensively used than magnetic coating thickness gauges because it has fewer test criteria.
Working Principle of Eddy Current Thickness Gauge
The measuring principle of Eddy Current Thickness Gauge is based on Eddy current. Once the probe’s coil core is energized, a high-frequency magnetic field can be induced. The interaction between the probe and the metal substrate of the measured coating mirrors the probe and the substrate. The distance decides the extent of the eddy current. If the between the probe and metal substrate is less, the eddy current will be high and vice versa. The thickness of the coating can be measured by evaluating the proportional relationship between eddy current and distance.
Application of magnetic thickness gauges
Based on magnetic attraction and induction, it’s established that the magnetic thickness gauge is ideal for determining the thickness of the non-magnetic layer of the magnetic metal substrate. Additionally, it can also be used to measure the thickness of non-ferrous metal coatings. The thickness of plastics, polyester films, and other materials can also be determined.
Application of eddy current thickness gauges
Eddy Current Thickness Gauges are preferably used for determining the thickness of non-conductive overlays. The primary use is the thickness evaluation of coatings, ceramics, etc.
Eddy Current Thickness Gauge and Magnetic Thickness gauge are extremely different from each other in their principle and application. Yet the majority of the manufacturers utilize both for better results.
Eddy current thickness gauge shows e6; how to solve it?
If the Eddy Current Thickness Gauge displays E6 error along with varying and incorrect results at normal calibration, the device must be reset. The reset operation is relatively simple, push the clear and statistic button simultaneously until three small horizontal bars are displayed on the screen.
How is an Eddy Current Coating Thickness Gauge calibrated?
- Push the power button and hold it until it shows “CAL.” The display will present “F:H.” Press zero to confirm.
- Push the power button and hold it until the display shows “Ln.” The display will present “0.8XX’’. If the data obtained is too big, around 127um, and the diaphragm shows 420um, which is also large, either increase or decrease the number. It will decrease based on the increase of 0.001.
- It should be recalibrated when the operation is completed, whether after the operation step or the second step.
What are the applications of eddy current thickness gauges?
Eddy Current Thickness Gauges are useful for measuring the thickness of non-conductive coatings such as aluminum-based samples. It’s ideal for rapid and non-destructive film thickness examination of substances.
How to calibrate magnetic thickness gauges?
Instrument zero adjustments: An uncoated substrate with identical composition and thickness to the sample should be used for the zero adjustment plate. When using a galvanized sheet as a substrate, adjustments must be made to zero on the substrate with the zinc layer eliminated. The zero position error should not exceed 1M.
Instrument calibration: A standard sheet with identical thickness to the measured coating should be placed to make adjustments such that the thickness of the standard sheet is precisely evaluated. Repetition of zero and calibration operations are made until standard readings are received.
What are the factors that affect the measurement accuracy of magnetic thickness gauges?
The primary factors influencing the measurement accuracy include base thickness, edge effect, probe position, probe pressure, sample deformation, metal magnetism, etc.