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Dry film thickness gauges and meters are used to measure the thickness of dry films on a variety of surfaces. This is important for a variety of reasons, such as ensuring that the correct amount of coating has been applied, detecting corrosion, and determining the condition of a surface.
See our range of Dry Film Thickness (DFT) Gauges and instruments. Metals, non-metals, ferrous/non-ferrous/combo, concrete, fiberglass, wood, and more.
WHY USE A DFT GAUGE?
An integral component of coating longevity is getting the correct film build. Too low a film build and you risk delamination or insufficient protection from the environment. Too high a film can compromise the bond between coating and substrate. An essential part of a coating inspectors kit is a coating thickness gauge. There is a gauge and probe combination for almost any coating over almost any substrate. Modern dry film thickness gauges not only read the coating thickness, but the more advanced models stores the test readings, analyze and interpret them and allow them to be downloaded.
COMMON ISSUES WITH COATING THICKNESS INSTRUMENTS
Coating thickness gauges range from the very simple touch and go types to sophisticated data collection tools that measure, store and analyze the readings. The advantage of the more simple units is their low cost and simple operation. Their downside is a lesser accuracy than the more sophisticated units and the lack of compliance to the full range of corrosion control standards. Conversely, the more sophisticated units provide accurate data that is in compliance with these standards and feeds into quality assurance systems. Their downside is more complicated operation which requires a higher level of proficiency.
IMPORTANT CONSIDERATIONS FOR CHOOSING THE RIGHT DFT INSTRUMENT:
Two key questions to be asked when considering purchasing a coating thickness gauge – What level of accuracy and standards compliance do I require? and How competent is the operator who will be using the gauge? Compliance to an international coating thickness standard requested by plant owner and being used by a competent and experienced operator? The DeFelsko Positest 6000 is your gauge. Just doing some quick checks for your own information or bit intimidated by anything too geeky or techish? Select the PosiTest. Another application for the PosiTest is in Hazardous Environments. As the PosiTest is an electronic device it cannot be used in a flammable or explosive atmosphere as there always is the potential for a spark.
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PosiTector 6000 | Yes |
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PosiTector 200 | No |
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PosiTest DFT F/NF | No |
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PosiTest | No |
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| PosiTector 6000 | PosiTector 200 | PosiTest DFT | PosiTest | |
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PosiTest PC | No |
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PosiPen | No |
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| PosiTest PC | PosiPen | Powder Comb | Wet Film Thickness Gage | |
| Available For Rent | No | No | No | No |
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Magnetic film gages (paint depth gauges) are used to nondestructively measure the thickness of a nonmagnetic coating on ferrous substrates. Most coatings on steel and iron are measured this way. Magnetic paint mil gages use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction.
Magnetic pull-off gages use a permanent magnet, a calibrated spring, and a graduated scale. The attraction between the magnet and magnetic steel pulls the two together.
In Type 1 pull-off (PosiTest or PosiPen) gages, a permanent magnet is brought into direct contact with the coated surface. The force necessary to pull the magnet from the surface is measured and interpreted as the coating thickness value on a scale or display on the gage. The magnetic force holding the magnet to the surface varies inversely as a non-linear function of the distance between magnet and steel, i.e., the thickness of the dry coating. Less force is required to remove the magnet from a thick coating
Magnetic induction instruments use a permanent magnet as the source of the magnetic field. A Hall-effect generator or magneto-resistor is used to sense the magnetic flux density at a pole of the magnet. Electromagnetic induction instruments use an alternating magnetic field. A soft, ferromagnetic rod wound with a coil of fine wire is used to produce a magnetic field. A second coil of wire is used to detect changes in magnetic flux.
These electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it nears a steel surface. The magnitude of the flux density at the probe surface is directly related to the distance from the steel substrate. By measuring flux density the paint thickness can be determined.
Other ferrous electronic paint thickness gages operate on the principle of electromagnetic induction. A coil containing a soft iron rod is energized with an AC current thereby producing a changing magnetic field at the probe. As with a permanent magnet, the magnetic flux density within the rod increases when the probe is brought near the steel substrate. This change is detected by a second coil. The output of the second coil is related to the coating thickness. These paint meters also need temperature compensation due to the temperature dependence of the coil parameters.
Eddy current techniques are used to nondestructively measure the thickness of nonconductive coatings on nonferrous metal substrates. A coil of fine wire conducting a high-frequency alternating current (above 1 MHz) is used to set up an alternating magnetic field at the surface of the instrument’s probe. When the probe is brought near a conductive surface, the alternating magnetic field will set up eddy currents on the surface. The substrate characteristics and the distance of the probe from the substrate (the thickness) affect the magnitude of the eddy currents. The eddy currents create their own opposing electromagnetic field that can be sensed by the exciting coil or a second, adjacent coil.
The ultrasonic pulse-echo technique is used to measure the thickness of paint and other coatings on non-metal substrates (plastic, wood, concrete, and composites) without damaging the coating.
Ultrasonic coating thickness gages (e.g. PosiTector 200) emit a high frequency sound pulse that travels into the coating via a coupling gel and reflects from ANY surface that is different in density. Paint thickness readings are obtained by measuring the time taken for the ultrasonic signal to propagate from the probe to the coating/substrate interface and back. The travel time is divided by two and multiplied by the velocity of sound in the coating to obtain the thickness of the coating.
The PosiTector 200 interprets the largest “echo” within the selected range as the coating/coating or coating/substrate echo.
For more information about the PosiTector 200 and paint thickness measurement on non-metal substrates:
Thickness measurements of powder can be taken before and after curing. Substrate type, thickness range, part shape, and economics determine the best method to be employed.
For uncured applied powders, height measurement can be performed with DeFelsko Powder Combs. This technique is destructive and may require recoating the part. Coating powders generally diminish in thickness during the curing process so these procedures require a reduction factor be determined to predict cured film thickness.
Non-contact ultrasonic instruments, such as the PosiTest PC Powder Checker also measure uncured powder, but do so without touching the surface. Instead of measuring powder height, they automatically display a predicted cured thickness result. Accurate measurement at the time of application: eliminates over usage, improves quality, controls set-up, reduces rework & waste — saving money.
For after-cure measurement, a variety of hand held paint meters are available. These non-destructive instruments employ either magnetic, eddy current, or ultrasonic principles depending on the substrate.
No, the DeFelsko PosiTector 6000 system has 1 body but accepts a full range of probes. Not only coating thickness (magnetic, eddy current and ultrasonic measurement), but also surface profile, climatic monitoring, soluble chlorides contamination, infra-red temperature,ultrasonic wall thickness, hardness and gloss measurement.
The DeFelsko PosiTector 6000 advances is compliant with the widest range of standards -ISO 2178/2360/2808, ISO 19840, ASTM B244/B499/D1186/D1400/D7091/E376/G12, BS3900-C5, SSPC-PA2, US Navy NAVSEA PPI 63101-000, US Navy NAVSEA 009-32, AS 2331.1.4, AS3894.3-B, and others. The PosiTest, ASTM D7091/B499, ISO 2808 and others.
Most commonly available coating systems can be measured with a coating thickness gauge using one of the three commonly available technologies discussed above. Metallic filled coatings are not as common as previously, however should you be required to measure the thickness of one of these, a destructive test may be required. These place an angled cut in the cured coating and use trigonometry calculate the coating thickness.
Ferrous probes can range up to 80 mils/2,000 um. Non-ferrous probes 25 mils/ 625 um. Ultrasonic probes up to 300 mils /7,600 um. Two provisos – when measuring thinner coatings use the probe with the smallest range suitable, it will be more accurate at a lower range; when measuring a thicker coating (over 60 mils/1,500 um) make sure that the probe has an upper limit greater than the target thickness on the specification. It is possible when using the wrong probe to get a measurement and not realize that it is inaccurate.
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