Dry film thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium using a digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.
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A permanent magnet is mounted on a balanced arm and the force required to pull this magnet from the surface of the coating is a measure of the thickness of the coating. Force is applied through a helical spring attached to the balanced arm at one end and to a scale wheel at the other. As the scale wheel is turned the force is progressively increased until the magnet lifts from the surface. The scale is drawn in thickness units rather than force and the thickness of the coating can be read against a pointer on the case of the instrument.
Electronic coating thickness gauges for measuring on magnetic substrate materials use the electromagnetic induction principle. A three-coil probe system is used where the central coil is powered by the instrument and the other two coils, either side of the central coil, detect the resulting magnetic field. The signal generated by the instrument is sinusoidal and therefore an alternating magnetic field is established round the central coil.
When there is no magnetic materials influencing the probe then the magnetic field cuts through the other two coils equally. As the probe is brought closer to the uncoated substrate the field becomes unbalanced with more field cutting the nearest coil and less cutting the furthest coil. This produces a net voltage between the two coils which is a measure of the distance to the substrate (the coating thickness).
In the case of the eddy current principle, a single coil probe is used with a relatively high frequency signal, several mega-hertz, to generate an alternating field in the non-ferrous metal under the coating. The field causes eddy currents to circulate in the substrate which in turn have associated magnetic fields. These fields influence the coating thickness probe and cause changes to the electrical impedance of the coil. These changes are dependant on the coating thickness.
A key decision on the overall selection of a suitable coating thickness gauge is how accurate do the readings need to be? Within the range of gauge types available, there is a progression from moderately accurate to very accurate gauges, this is mirrored by the prices of the coating thickness gauges, the more accurate, the higher the cost. In addition the coating application process and other factors affect the variability of the coating thickness on a particular surface and the skill and knowledge of the coating thickness gauge operator also influences results.
The basic measure of a coating thickness gauge's performance is the accuracy with which the gauge takes readings. That is the difference between the reading and the true coating thickness.
In order to test the accuracy of a particular gauge it is important to have traceable coating thickness standards. With the gauge adjusted to zero on an uncoated smooth substrate and set to a known thickness standard at or near to the maximum thickness, intermediate thickness standards are measured and the readings compared to the actual thickness of the standard. The errors are the differences between the values of the reading and the value of the standard. These are most conveniently expressed as a percentage of the reading.
Coating thickness gauges are non-destructive testing instruments used to quickly and accurately measure the thickness of the outer layer of a surface consisting of paint, epoxy, metal, or other materials. This is an extremely valuable tool in the quality control process in the coatings industry because it ensures that the dry film thickness (DFT) is precise and consistent across the surface of the product.
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Knowing the exact thickness of a coating can help reduce the cost of materials and guarantee finish quality. Failure to apply a coating accurately can result in a decrease in performance, appearance, and the lifetime of the finished product. In many cases, a proper coating thickness is also necessary in order to comply with a host of standards and regulations.
The operation of a coating thickness gauge may vary depending on the model that you are using but they are generally quite simple to use. For instance, a typical digital coating thickness gauge will either have a probe on a cord attached to the device or on the device itself (usually located on the bottom). Simply place the probe, or the device, against the substrate you want to test and an LCD display will give you a thickness reading. Keep in mind, some calibration and zeroing might be necessary prior to measuring in order to get the most accurate reading possible.
Some advanced models, such as the Reed Instruments R, have the ability for a user to input parameters and LED lights on the device will indicate when measurements are below (red), above (yellow), or within set parameters (green). Click here to see a quick demonstration of how a coating thickness gauge works and learn more about this product.
Selecting the right coating thickness gauge that fits your needs depends on your industry and the type of coating you are testing.
For example, if you are in the automotive industry and you wish to test the thickness of paint on vehicles, a digital thickness gauge such as the R or CM- that is capable of testing ferrous and non-ferrous substrates is the best choice. These types of coating thickness gauges are also popular in the industrial manufacturing, metal processing, scientific research, and aerospace industries.
If you are measuring the thickness of metal pipes or pressure vessels for example, you will need an ultrasonic thickness gauge such as the Reed Instruments TM-. These are great for testing the thickness of materials such as steel, wood, cast iron, aluminum, red copper, brass, zinc, quartz glass, polyethylene, PVC, grey cast iron and nodular cast iron.