Coating thickness or dry film thickness (DFT) is an important variable that plays a role in product quality, process control, and cost control. Measurement of film thickness can be accomplished by selecting the best mil gage for the particular application.
Dry film thickness (DFT) or coating thickness is arguably the single most important measurement made during the application and inspection of protective coatings. Coatings are designed to perform their intended function when applied within a tight DFT range as specified by the manufacturer. Correct thickness ensures optimum product performance. Even the most basic specification will require DFT to be measured.
Regular film thickness measurement helps control material costs, manage application efficiency, maintain finish quality and ensure compliance with contract specifications. Paint manufacturers recommend target ranges to achieve optimum performance characteristics and clients expect these parameters to be met.
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Stated accuracy of Td is ±3C for Td from -40 to 80C.
The following charts illustrate the maximum error of Td with respect to Ta and RH.
View PDF of Accuracy Statement here.
The innovative PosiPatch uses a magnetic ring to hold it against the surface, so no adhesive is required. This means that the PosiPatch isn’t destroyed when removed from the surface, unlike conventional patches. After rinsing with deionized water, the PosiPatch can be used again.
The PosiPatch can be reused multiple times until water begins to leak through the air-permeable membrane or the seal against the substrate.
In our tests on freshly blasted steel with a typical 50-100 micron (2-4 mil) profile, PosiPatches were reused dozens of times with no noticeable wear or leakage. Patch life will vary depending on use- if dragged along the substrate, lifespan will be reduced.
We believe that 10 uses is a very conservative estimate of lifespan and still yields the lowest per-test cost of any commercially available Bresle Method Patch. The below tables compare the per-test costs of various options, using competitive prices found online.
If performing 10 tests, and reusing the PosiPatch:
Replica Tape provides a simple way to obtain an impression of a surface for analysis. It consists of a layer of crushable plastic micro foam affixed to a 50.8 μm (2 mil) incompressible polyester film.
When compressed against a roughened surface, the foam collapses and acquires an impression, or reverse replica, of the surface. It is available in a number of grades to accommodate measurements in different profile ranges.
Placing the compressed tape (replica) into the PosiTector RTR gives a measure of the average maximum peak-to-valley height of the surface roughness profile.
Annotate images using drawing tools—ideal for identifying a specific location or area within an image
Unequal pulling force during testing caused by uneven adhesive bond lines and coating surfaces can result in random, unexplainable readings. To obtain more repeatable and meaningful adhesion measurements, it is imperative that the pulling force applied to the test dolly is uniformly distributed over the surface being tested.
Both the PosiTest AT-M manual and PosiTest AT-A automatic models compensate for misalignment. The self-aligning, quick-coupling actuator and spherical articulating dolly head enable uniform distribution of the pulling force over the surface being tested, preventing a one-sided pull-off.
Two grades of Testex™ Press-O-Film™ replica tape, “Coarse” and "X-Coarse", are available to span the primary range of surface profiles for the coatings and linings industry –– 20 to 115 µm / 0.8 to 4.5 mils.
An unfortunate characteristic of replica tape is that conventional spring micrometer measurements are most accurate near the middle of each grade's range and least accurate at the outer ends of each grade's range. That is why two other grades, Coarse Minus (< 20 µm / 0.8 mils) and X-Coarse Plus (> 115 µm / 4.5 mils), are used to check and, if necessary, adjust measurements at the upper and lower ends of the primary range.
Inside the primary range, Coarse and X-Coarse tape share a 38 - 64 μm (1.5 - 2.5 mils) "overlap" region. Measurements with conventional micrometers require a complicated and time consuming procedure of averaging one reading using Coarse grade and one reading using X-Coarse grade to achieve reasonable accuracy.
With a single measurement, the PosiTector RTR produces a more accurate peak-to-valley height measurement HL from Coarse or X-Coarse tapes that has been adjusted for their non-linearity. There is no need to average two or more replicas from different grades of tape AND there is no need to subtract the 50.8 μm / 2 mils of incompressible polyester film. The advantages are a reduction in measurement uncertainty, inspector workload, likelihood of error, and the number of replicas needed by inspectors to assure accuracy.
The PosiTector RTR can also display a height value (H) that is comparable to what conventional analog spring micrometers would display after the 50.8 μm / 2 mils of incompressible polyester film has been subtracted.
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. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. Coating thickness is determined by measuring this pull-off force. Thinner coatings will have stronger magnetic attraction while thicker films will have comparatively less magnetic attraction. Testing with magnetic gages is sensitive to surface roughness, curvature, substrate thickness, and the make up of the metal alloy.
Magnetic pull-off gages are rugged, simple, inexpensive, portable, and usually do not require any calibration adjustment. They are a good, low-cost alternative in situations where quality goals require only a few readings during production.
Pull-off gages are typically pencil-type or rollback dial models. Pencil-type models (PosiPen shown in Fig 1) use a magnet that is mounted to a helical spring that works perpendicularly to the coated surface. Most pencil-type pull-off gages have large magnets and are designed to work in only one or two positions, which partially compensate for gravity. A more accurate version is available, which has a tiny, precise magnet to measure on small, hot, or hard-to-reach surfaces. A triple indicator ensures accurate measurements when the gage is pointed down, up, or horizontally with a tolerance of ±10%.
Rollback dial models (PosiTest shown in Fig 2) are the most common form of magnetic pull-off gage. A magnet is attached to one end of a pivoting balanced arm and connected to a calibrated hairspring. By rotating the dial with a finger, the spring increases the force on the magnet and pulls it from the surface. These gages are easy to use and have a balanced arm that allows them to work in any position, independent of gravity. They are safe in explosive environments and are commonly used by painting contractors and small powder coating operations. Typical tolerance is ±5%.
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 coating 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 by a second, adjacent coil.
Magnetic film gages are used to non-destructively measure the thickness of a nonmagnetic coating on ferrous substrates. Most coatings on steel and iron are measured this way. Magnetic 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. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. Coating thickness is determined by measuring this pull-off force. Thinner coatings will have stronger magnetic attraction while thicker films will have comparatively less magnetic attraction. Testing with magnetic gages is sensitive to surface roughness, curvature, substrate thickness, and the make up of the metal alloy.
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 coating thickness can be determined.
PosiTector users can capture and save an image copy of the current gage display by simultaneously pressing both the (-) and (+) buttons. The last 10 screen captures are stored in memory and can be accessed within the PosiSoft USB Drive.
Statistics mode continually displays/updates average, standard deviation, min/max thickness and number of readings while measuring.
Display Languages: English, French, German, Spanish, Chinese, Japanese, Portuguese, Italian, Norwegian, Russian, Czech, Polish and Korean.
All PosiTector 6000 regular separate probes are suitable for underwater measurement and are available with extended cable lengths up to 250 feet / 75 meters.
Ideal for measuring coating thickness on underwater pipes, ships, bulkheads, offshore oil rigs or anywhere extended reach is required.
Maximum cable lengths vary depending on probe type...
Extended cable lengths are also available for ferrous micro probes (F0S, F45S, F90S) and FKS probes (thick coatings) – up to 50 ft (15 m).
Note: Micro probes and FKS probes do not support underwater usage
Contact us for additional information including lead time.
